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HOST PLANT RESISTANCE: CONCEPT AND SIGNIFICANCE

Dr. Muhammad Salim M.Sc. (Hons.)PPR(UAE); M.Sc. ENT

(UPLB/IRRI, Philippines & Ph.D. ENT (UPLB/IRRI, Philippines

Ex-CSO/ Sr. Director,

Plant Sciences Division (PARC) Islamabad

HIGHER EDUCATION COMMISSION ISLAMABAD

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CONTENTS

ABOUT THE AUTHOR……………………………………………………………...…..i

FOREWORD…………………………………………………………………..……...…iii

PREFACE………………………………………………………….............................vii

CHAPTER-1

INTRODUCTION.……….………………………………………...…….1

1.1. PLANT –INSECT INTERACTIONS CONCEPT…… ……………………...1

1.1.1. Defence Strategies of Plants………………………………………..1

1.1.2. Insect Strategies to Counter Plant Defence……………………….2

1.2. HISTORY OF HOST PLANT RESISTANCE DEVELOPMENT…………..4

1.3. DEFINITIONS…………………………………………………………….……7

1.4. FUNCTIONAL CATEGORIES………………………………………………..8

1.5. ADVANTAGES…………………………………………………………………9

1.6. LIMITATIONS…………………………………………………………………10

1.7. HOST PLANT RESISTANCE PROGRAMME IMPLEMENTATION…....12

1.7.1. Scientific Manpower………………………………………………..12

1.7.2. Scientific Knowledge……………………………………………….12

1.7.3. Availability of Insect Culture……………………………………….12

1.7.4. Genetic Resources………………………………………………....13

1.7.5. Genetic Aspects of Resistance…………………………………...13

1.7.6. Selection of Breeding Methods……………………………………13

1.8. REFERENCES……………………………………………………………….14

CHAPTER-2

MECHANISM OF HOST PLANT RESISTANCE ………………....25

2.1. ECOLOGICAL RESISTANCE………………………………………………25

2.1.1. Host Evasion/Escape/ Phenological Asynchrony……………….25

2.1.2. Induced Resistance………………………………………….……..27

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2.2. EVOLUTIONARY RESISTANCE…………………………………………..28

2.2.1. Sympatric Resistance………………………………………….…..28

2.2.2. Allopatric Resistance……………………………………………....28

2.3. TROPHIC LEVEL RESISTANCE…………………………………………..29

2.3.1. Intrinsic Resistance………………………………………………...29

2.3.2. Extrinsic Resistance………………………………………………..29

2.4. MECHANISM BASED ON INHERITANCE OF RESISTANCE………….30

2.5. MECHANISM BASED ON CROP STAGE………………………..……….30

2.6. MECHANISM BASED ON SCREENING CONDITIONS………………...30

2.7. MECHANISM BASED ON BIOTYPE REACTION………………………..31

2.8. MECHANISM BASED ON POPULATION………………………..……….33

2.9. MECHANISM BASED ON NATURE OF GENES………………….……..33

2.10. MECHANISM BASED ON FUNCTIONAL CATEGORIES……….…..….33

2.10.1. Non-preference/Antixenosis……………………………………....33

2.10.2. Antibiosis……………………………………………………….…....36

2.10.3. Tolerance……………………………………………………….…...40

2.11. INTENSITY OF RESISTANCE…………………………………..………....41

2.12. REFERENCES…………………………………………………………........45

CHAPTER-3

BEHAVIOURAL AND PHYSIOLOGICAL

COMPONENTS OF HOST PLANT SELECTION………….………55

3.1. HOST HABITAT FINDING…………………………………………………..56

3.2. HOST FINDING……………………………………………………………....56

3.3. HOST RECOGNITION……………………………………………………....57

3.4. HOST ACCEPTANCE…………………………………………………….…58

3.5. HOST SUITABILITY…………………………………………………………59

3.6. REFERENCES…………………………………………………………….…61

CHAPTER-4

HOST-INSECT INTERACTIONS THEORIES……………………...67

4.1. THEORIES OF HOST-PLANT SELECTION……………………………...68

4.1.1. Botanical Instinct Theory…………………………………………..68

4.1.2. Token Stimuli Theory………………………………………….…...69

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4.1.3. Dual Discrimination Theory………………………………………..70

4.1.4. Non-preference and Antibiosis Theory…………………………...71

4.1.5. Chemosensory Theory……………………………………………..71

4.1.6 Inhibitory Chemical Theory or Negative Stimuli Theory………..73

4.1.7. Thorsteinson’s Theory……………………………………………..74

4.1.8. Nutrients Imbalance Theory……………………………………….75

4.2. THEORIES OF HOST PLANT DEFENCE………………………………...76

4.3. THEORIES OF CO-EVOLUTION………………………………................77

4.3.1. Ehrlich and Raven (1964) Theory……………………………...…77

4.3.2. Sequential Co-evolution…………………………………………...80

4.3.3. Difuse Co-evolution………………………………………………...80

4.3.4. Geographic Mosaic Theory………………………………………..81

4.4. REFERENCES……………………………………………………………….82

CHAPTER-5

MORPHOLOGICAL BASIS OF RESISTANCE IN PLANTS TO INSECTS…………………………………..………..87

5.1 COLOUR AND SHAPE……………………………………………………...87

5.2. THICKENING OF CELL WALLS…………………………………………...88

5.3. STRUCTURE AND TEXTURE OF TISSUES……………………………..89

5.4. INCRUSTATION OF MINERALS IN CUTICLE…………………………...90

5.5 SURFACE WAXES…………………………………………………………..91

5.6. ANATOMICAL ADOPTION OF ORGANS…………………………………92

5.7. TRICHOMES………………………………………………………………....94

5.7.1. Locomotion, Attachment and Related Behaviour……………….94

5.7.2. Insect Feeding and Digestion……………………………………..95

5.7.3. Insect Development and Population……………………………...96

5.7.4. Insect Mortality………………………………………………….…..96

5.7.5. Oviposition…………………………………………………………..96

5.7.6. Insect Infestation or Plant Damage……………………………....97

5.7.7. Pubescence Associated with Allelochemical Factors…………..97

5.8. TRICHOMES AND RESISTANCE………………………………………....98

5.9. REFERENCES……………………………………………………………….99

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CHAPTER-6

BIOCHEMICAL BASIS OF RESISTANCE

IN PLANTS.TO INSECTS………………………………………......115

6.1. NON-PROTEIN AMINO ACIDS…………………………………………...117

6.2. LECTINS…………………………………………………………………….118

6.3. CYANOGENIC GLYCOSIDES (CGs) …………………………………...119

6.4. GLUCOSINULATES……………………………………………………..…119

6.5. ALKALOIDS…………………………………………………………….…...120

6.6. PHENOLICS……………………………………………………..………….122

6.7. TERPENOIDS/ISOPRENOIDS……………………………………………123

6.8. EPICUTICULAR LIPIDS……………………………………………...……125

6.9. TANNINS……………………………………………...…………………….126

6.10. PROLINE……………………………………………...…………………….126

6.11. SUGARS……………………………………………...………………….….126

6.12. BIOCHEMICAL BASIS OF RESISTANCE OF MAJOR CROPS………126

6.12.1. Cotton……………………………………………...……………….126

6.12.2. Sugarcane……………………………………………...………….127

6.12.3. Maize……………………………………………...………………..127

6.12.4. Oilseed Crops……………………………………………...……...128

6.12.5. Rice……………………………………………...………………….128

6.13. REFERENCES……………………………………………...……………...139

CHAPTER-7

FACTORS AFFECTING HOST PLANT RESISTANCE…………153

7.1. TEMPERATURE STRESS……………………………………………...…154

7.2. SALINITY STRESS……………………………………………..............…163

7.3. ALUMINUM STRESS……………………………………………...........…176

7.4. NUTRIENTS…………………………………………….........................…181

7.4.1. Nitrogen (N).……………………………………………...………..182

7.4.2. Phosphorus (P) ……………………………………………...……190

7.4.3. Potassium (K) …………………………………………………..…193

7.4.4. Iron (Fe) ………………………………………………..……….…199

7.4.5. Silica (Si) ……………………………………………................…205

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7.4. 6. Boron (B).……………………………………………...…………..209

7.4.7. Metal Chelates………………………………………………….…209

7.5. GROWTH REGULATORS……………………………………………...…210

7.6. HERBICIDES……………………………………………...………………..210

7.7. PLANT SPACING………………………………………………………..…210

7.8. ATTACK OF INSECTS AND DISEASES………………………………...211

7.9. LIGHT……………………………………………......................................212

7.10. pH……………………………………………........................................….212

7.11. WATER……………………………………………………………….…...…213

7.12. RELATIVE HUMIDITY (RH) ……………………………………………....213

7.13. INSECTICIDES…………………………………………….............………213

7.14. MISCELLANEOUS……………………………………………...............…218

7.15. REFERENCES…………………………………………………..……….…221

CHAPTER-8

DEVELOPMENT OF INSECT RESISTANT VARIETIES………..235

8.1. NEED FOR RESISTANT VARIETIES……………………………...…….235

8.2. SOURCES OF RESISTANCE……………………………...……………..236

8.3. MULTIDISCIPLINARY TEAM APPROACH……………………………...237

8.4. SETTING PRIORITIES FOR THE DEVELOPMENT OF INSECT

RESISTANT VARIETIES…………………………………………………..241

8.5. SCREENING AND EVALUATION OF GERMPLASM

FOR INSECT RESISTANCE………………………………………………241

8.5.1. Whitebacked Planthopper, Sogatella furcifera……………..….244

8.5.2. Yellow stem borer, Scirpophaga incertulas………………….....245

and white stem borer,Scirpophaga innotata.

8.5.3. Leaffolder, Cnaphalocrocis medinalis…………………………..247

8.6. MAINTENANCE OF INSECT POPULATION……………….…………...249

8.6.1. Increasing Infestation of Sorghum Shoot Fly..………………....250

Atherigona soccata

8.6.2. Increasing Field Infestation of Rice Gall Midge………….........250

Pachydiplosis oryzae

8.6.3. Increasing Population of Rice Brown Planthopper..………..…250

Nilaparvata lugens

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8.7. BREEDING METHODS FOR HOST PLANT RESISTANCE…………..251

8.7.1. Introduction of Varieties .…………………………………………252

8.7.2. Selection.………………………………...……………..………….252

8.7.3. Hybridization.………………………………………..…………….252

8.7.3.1. Pedigree selection.………………………………………253

8.7.3.2. Bulk selection.………………………………………..….253

8.7.3.3. Backcross method.……………………………………...254

8.7.4. Mutation Breeding.…………………………………………….….254

8.7.5. Methods of Producing Transgenic Plants.……………………..255

8.7.5.1. Agrobacterium mediated gene transfer.………………255

8.7.5.2. Direct gene transfer.…………………………………….256

8.8. LEVEL OF RESISTANCE: EXAMPLES FRO PAKISTAN……………..257

8.8.1. Cotton.………………………………………..…………...……….257

8.8.2. Sugarcane.………………………………………..……………….260

8.8.3. Pulses.………………………………………..…………...……….261

8.8.4. Rice.………………………………………..…………...………….263

8.8.5. Wheat.………………………………………..…………...……….274

8.8.6. Sunflower.………………………………………..………….....….274

8.8.7. Maize.………………………………………..…………...………..275

8.8.8. Soybean.………………………………………..…………...…….278

8.8.9. Brassica.………………………………………..…………...…….279

8.8.10. Mulberry.………………………………………..…………...…….282

8.8.11. Barley.………………………………………..…………...……….282

8.8.12. Cucumber.………………………………………..…………....….282

8.8.13. Okra.………………………………………..…………...……..….282

8.9. REFERENCES.………………………………………..………………....282

CHAPTER-9

GENETICS OF INSECT RESISTANCE

IN CROP PLANTS…………………………………………………...290

9.1. INSECT RESISTANCE IN MAJOR CROPS…………………………….290

9.1.1. Rice………………………………………………………………...290

9.1.1.1 Brown plant hopper (BPH), Nilaparvata lugens……...290

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9.1.1.2. Whitebacked planthopper, Sogatella furcifera……….291

9.1.2. Wheat……………………………………………………………....292

9.1.3. Maize……………………………………………………………....294

9.1.3.1 European corn borer, Ostrinia nubilalis……………....294

9.1.3.2 Corn earworms, Helicoverpa zea and H. armigera….295

9.1.3.3. Stem borer, Chilo partellus……………………………..295

9.1.3.4 Shoot fly,Atherigona soccata…………………………..295

9.1.4 Sorghum………………………………………………………..….296

9.1.5. Cotton………………………………………………………………296

9.2. STRATEGIES TO PROLONG THE UTILITY OF MAJOR GENES…...298

9.3. REFERENCES……………………………………………………………...299

CHAPTER-10

ROLE OF HOST PLANT RESISTANCE IN INTEGRATED PEST MANAGEMENT………………………………………………..…….305

10.1. SIGNIFICANCE OF HOST PLANT RESISTANCE……………………..306

10.2. COMPATIBILITY WITH OTHER COMPONENTS OF IPM…………….306

10.2.1. Chemical…………………………………………………………...307

10.2.2. Cultural Control…………………………………………….……...307

10.2.3 Biological Control………………………………………………….308

10.3. CONCLUSION………………………………………………………………316

10.4. REFERENCES……………………………………………………………...317

GLOSSARY…………………………………………………………………..321

INDEX………………………………………………………………...............337

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PREFACE Shrinking area under cultivation mainly due to urbanization and industrialization, declining natural resource base, increase in yield losses incurred by insect pests, and rapidly increasing human population necessitate enhancing productivity of crops to achieve the objectives of food security and poverty alleviation of farming community in Pakistan and other developing countries.

New human interventions have in most of the cases been successful to achieve such objectives but only temporarily. With the passage of time, these interventions resulted in to a number of new problems. For example the discovery of insecticidal activity of DDT and its sister compounds in1940s was a major break through in the history of pest control and it was believed that these pesticides would solve the problems of insect pests. Initially pesticides were so powerful tool and man used this tool so massively that the term pest control became synonymous to the chemical control. However, due to indiscriminate use or miss-use of pesticides, multifarious and multi-facet complex problems emerged such as resurgence of target pests, outbreak of secondary pests, environmental pollution, pesticide residues in the produce, bio-magnification of pesticides in food chains and food webs, disturbance in the natural balance, and destruction of bio-control agents and other beneficial organisms. The solution of these problems became very difficult. Likewise, a quantum jump in crop yields, especially those of rice and wheat was obtained through Green Revolution in 1960s due to the cultivation of high yielding crop varieties and increase in input use. In short span of time it was realized that Green Revolution gave birth to certain serious problems including increase in the in the population of insect pests in different crop production systems especially in cotton. As per estimates, environmental and social cost to the country was in the tune of US dollar 206 million per year.

Some efforts have been made to develop and implement integrated pest management (IPM) in Pakistan to minimize the losses incurred by insect pests and to solve pesticide- induced problems. This approach was unbalanced because very less emphasis was laid on the development and cultivation of resistance varieties as compared to other components of IPM. It is well established fact that the use of resistant varieties represents one of the simplest and most convenient tactics of insect pest management. In general, the pest management offered by resistant varieties is sustainable and without any extra cost to the growers, is compatible with other components of IPM and helpful to improve the natural balance not only in agro-ecosystems systems but also in whole agricultural matrix, minimizes and in certain cases eliminates the use of pesticides, and proves helpful to achieve stability and sustainability in crop production systems. It is, therefore, imperative to give pivotal position to host plant resistance in IPM.

Insect resistant varieties provide substantial economic returns on investment on research and development and give much greater returns on investment than pesticides. It has been estimated that the multiple insect resistant rice variety IR36 has provided US

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dollars one billion additional annual incomes to rice producers and processors in South and South Asia. Likewise, resistant cultivars of alfalfa, corn, beet and cotton gave lot of benefits to growers. The usefulness of host plant resistance is increasing with the passage of time due to advancement in science and technology. Initially, resistant varieties were developed through Mendelian and biometrician approaches, where the gene pool was very narrow due to the ability of manipulating only primary and secondary gene pools of cultivated species for crop improvement. In certain cases, genes for resistance were not available in the primary and secondary gene pools. Moreover, development of resistant variety took very long time. Real breakthrough in broadening the gene pool, reducing the time of incorporation of resistant genes into target material and to develop varieties with high level of resistance was obtained through biotechnological approaches especially that of genetic engineering. Now scientists are well equipped to identify, conserve, preserve and incorporate resistant genes from diverse sources such as viruses, bacteria, animals and unrelated plant specie into the commercial varieties or promising candidate varieties to develop cultivars with high level of resistance against major insect pests of different crops.

Due to rapid pace of research, augmented sophisticated instrumentation and techniques, there has been a tremendous progress in basic and applied aspects of host plants resistance in recent years. It is the need of hour to bring Gene Revolution in Pakistan for the improvement of crops in general and to develop crop varieties with high level of resistance against insects and other pests in particular. I do hope, the book will be helpful to improve the level of knowledge of readers on host plant resistance and would facilitate the teachers and scientists in the preparation and implementation of projects on the subject.

The book consists of ten chapters, covering basic and applied aspects of host- plant resistance. First chapter deals with plant-insect interactions concepts, history and definitions of host plant resistance and pre-requisites for the implementation of programme on host plant resistance. In chapter two, mechanism of resistance in plants against insect pests has been explained keeping in view the ecological basis, evolutionary process, trophic levels, mode of inheritance, crop stage, screening conditions, biotype reactions, nature of genes, functional categories of resistance. Chapter three, deals with behavioural and physiological basis of host plant selection. In chapter four, information has been given relating to theories of host plant selection by insects, host plant defence theories and theories of co-evolution of plants and insects. Chapter five deals with important morphological or bio-physical traits of plants which impart resistance against insect pests. Chapter six gives information on biochemical basis of resistance in plants against insect pests of major crops with specific examples from Pakistan. Chapter seven deals with factors/stresses such as salinity, temperature, relative humidity, light, hormones, nutrient deficiencies, nutrient toxicity or excessive amounts of nutrients which affect the level of resistance/susceptibility in crop plants against insect pests. In chapter eight, approaches/techniques for the development of resistant varieties to insect pests have been briefly described. It also contains information on the level of resistance of crop varieties against major insect pests in Pakistan. Information on the genetics of insect resistance in crop plants is given in chapter nine. Strategies to prolong the utility of major geneses are also discussed in this chapter. Chapter ten provides information on the role of host plant resistance in pest management

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and its compatibility with other components of IPM. In addition, the book contains glossary of important terms and technical index for the benefit of readers.

It is not possible to mention all the names that have been helpful for the completion of this book; therefore, I intend to thank all of them. I am thankful to Mr. Saleem Akhtar for doing tiresome work of typing the manuscript and that of Mr. F.D. Shahzad for assisting in search of literature and critically reading the manuscript. My special thanks are due to Mr. Ashfaq, KPO, NARC for his help at different stages in the preparation of book. I am thankful to Mr. M. Younis, PSO and Mr. M. Ayoob, KPO,

NSCRI, Thatta and Mr. Zhou Xu Sheng for extending help in the preparation of

technical index. I express my gratitude to Dr. H.I. Javed, NARC, Islamabad for his valuable suggestions for the improvement of manuscript and for providing a list of maize varieties resistant to insect pests alongwith some pictures. My thanks are due to Dr. Amjid, Oilseed Research Programme; Dr. A. Rehman, Rice Research Programme; Dr. M. Zubair, Sugar Crops Research Programme, NARC, Islamabad for providing list of resistant varieties to insect pests of their respective crop. Dr. Amjid also offered some useful pictures to be included in the book. The cooperation of PSD, PARC scientists especially Dr. Muhammad Munir is highly appreciated. I express my gratitude and sincere thanks to Dr. M.E .Tusneem, ex-Chairman PARC for his encouragement, valuable guidance and for according permission to write this book. The technical review and suggestions for the improvement of book by both the experts are gratefully acknowledged. Higher Education Commission (HEC), Islamabad is gratefully acknowledged for providing incentive and making arrangements for the evaluation and publication of book. Finally heart-full gratitude to my loving wife Dr. Kausar Parveen; daughters- Dr. Fatimah Zahra, Dr. Amina Mobeen and sons- Dr. Muhammad Bilal and Dr. Abdul Shakoor for their help and encouragement in the completion of book.

I thank Allah (SWT) from the core of my heart for providing me inspiration for this endeavour and enabled me to complete this piece of work. This is entirely Allah’s favour and beneficence that He blessed me with the grace to render this service for the benefit of students, teachers and scientific community. In the end, I pray to Allah (SWT) to accept this humble effort of mine and enable the readers to get full benefit from this book.

Dr. Muhammad Salim

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FOREWORD The Natural System has been developed and stabilized in billions of years through evolution, co-evolution, actions and reactions of innumerable organisms at different trophic levels and by the actions of environmental factors. The system is very complex and complicated (consisting of well connected large number of food webs and food chains), dynamic, uniform, harmonious, beautiful, diverse, balanced, self-sustained and self-supportive.

God has given whatever He created, the right proportion and balance, provided every creature the best conceivable form and shape, blessed every organism with the ability to find its suitable habitat, adopt a way of life that is needed for its nourishment, survival and reproduction and defend itself from enemies, unfavourable environmental conditions and natural hazards. Every thing in the World is bound to perform its prescribed role with out any deviation. Every creature that exists has some role to play and nothing has been created in vain. In reality, we cannot create something from nothing, nor can we totally annihilate something to nothing, but, can only change forms of things or manage the populations of different organisms. Elimination of any organism in the World is, therefore, neither feasible nor desirable.

The Natural System was operating as per requirements of all organisms on a sustainable basis, before the creation of humankind on the planet Earth. Humankind has been created in the most excellent mould, with finest erect body, most appropriate organs and a complete set of viable senses such as sight, hearing, touch, taste, and smell and well developed brain. God has endowed man with faculties of thinking and reasoning, imagination, discrimination, and will-power. The guidance provided to him is the most comprehensive one and of higher order, than all the created beings in the World. Guidance to other creatures on the Earth is either through instinct or both with instinct and senses. But the guidance provided to humankind consists of instinct, senses, intellect or power of reasoning and Revealed Knowledge. God created man as His vicegerent/representative on the planet Earth with special authority and heavy responsibilities, which other creatures could not dare to accept. Humankind has been given the power to subjugate the natural forces and resources, but keeping in view the guidance provided by the Creator.

Humankind is considered the greatest manipulator of the Natural System due to his inherent strength and capabilities. It is his obligation to ponder over the Natural System and make knowledge-based discoveries and inventions for the benefit of his fellow beings and other creature, but without damaging the System. The Holy Quran lays repeated emphasis on the search for Truth and on the need of exercising one’s reason and insight and reflecting over the inward and outward experience of life and drawing valid conclusions therein. In general, scientists has been doing wonderful job through new inventions and discoveries. However, in some cases, due to narrow focus, non-seriousness, vested interests, lack of knowledge, deviation from the guidance provided by the Supreme Creator, scientists are creating a number of serious problems and

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ecological catastrophe in the World at micro and macro levels. For example plant breeders and geneticists have been rendering a great service of humanity through enhancing crop productivity by developing varieties with high yield potential. But, while developing varieties they did not take into consideration the plant-insect interactions which were developed and stabilized in millions of years in the Natural System. As a result, due to narrow focus, they developed varieties with high yield potential but susceptible to insect pests. Cultivation of such varieties encouraged the population build up of insect pests and as a result, massive amounts of insecticides were used to control them. Both the interventions temporarily proved useful but in the long run, a number of problems emerged including- disturbance in the natural system, destruction of bio control agents and other beneficial organisms, resurgence of target pests, out-break of secondary pests, pesticide residues in the produce, pesticide resistance, environmental pollution, health hazards, bio magnification of pesticides in the food chains and food webs, and substantial increase in the cost of production of different crops. These problems occurred due to man-made interventions without studying their effects on different components of agro-ecosystems.

The solution of these problems lies in the development and cultivation of varieties with high level of resistance against insect pests with the help of conventional and innovative approaches. Even varieties with moderate level of resistance in plants can minimize insecticide-induced problems, if utilized in a sensible manner. Use of resistant varieties is compatible with other components of IPM. In general, the pest management offered by resistant varieties is environment friendly and with out any cost to the farming community. Insect resistant varieties provide substantial economic returns on investment in research and development and cost: benefit ratio in most of the cases is better than investment on pesticides. Host plant resistance is a precious natural resource. There is need to conserve it properly and exploit its potential with the help of modern tools and in the light of new scientific discoveries, but with extra care in order to avoid adverse effects on the Natural System in general and to resistant sources in particular.

Development of resistant varieties requires joint and collaborative efforts of well trained plant breeders, geneticists, entomologists, biochemists and biotechnologists. Universities can produce such breed of scientists by offering courses on host plant resistance at graduate and post-graduate levels, to students of relevant disciplines. The capability of scientists working on host plant resistance can further be improved by in-service trainings. Gene revolution in general and in the development of resistant varieties in particular can be helpful to minimize pesticide-induced problems, reduce losses incurred by insect pests, improve quality of produce, enhance productivity and achieve sustainability in production systems.

Availability of relevant local literature is pre-requisite to develop scientific manpower. As I know, no serious effort has been made for the collection of literature on host plant resistance in Pakistan. The book “Host Plant Resistance: Concepts and Significance” covers both basic and applied aspects of host plant resistance, including host-insect interaction concepts, mechanism of host plant resistance, biophysical and biochemical basis of resistance, factors affecting level of resistance in plants against insect pests, procedures for the development of resistant varieties, and role of host plant resistance in IPM.

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As I understand, the textbook has basically been written for post-graduate students, but it seems equally useful to university teachers, and all those scientists and researchers engaged in studies on plant-insect interactions and for the development of insect resistant crop varieties. I, therefore, express my deep appreciation for the endeavour of author and congratulate him for bringing out an excellent publication in the form a textbook, first of its nature in Pakistan.

Dr. Zafar Altaf, Chairman, PARC, Islamabad

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ABOUT THE AUTHOR Dr. Muhammad Salim was born on May 8, 1948 at chak no. 394 J.B. Jaja, district Toba Tek Singh. He received elementary education in his native village and obtained first position in the school in Middle Examination in 1962; passed Secondary School Certificate Examination (Matric) in 1964 from D.C. High School, Toba Tek Singh. In the same year, he got admission in the University of Agriculture, Faisalabad, (then West Pakistan Agricultural University, Lyallpur) and passed F.Sc. Agri.; B.Sc. (Hons.) Agri.; and M.Sc. (Hons.) Agri. in1966, 1969 and 1971, respectively. He obtained first divisions and got merit scholarships throughout his educational career from Matric onwards. He got the opportunity of higher education from abroad on scholarship/ fellowship and got his M.Sc and PhD, degrees in Entomology from International Rice Research Institute (IRRI) / University of Philippines at Los Banõs (UPLB), Philippines in 1985 and 1988, respectively.

He served in different departments from 1971 to 1977 including Pakistan Central Cotton Committee (PCCC) as Research Assistant; Commonwealth Institute of Biological Control (CIBC), Rawalpindi as an Assistant Entomologist/ Junior Entomologist and Ministry of Social Welfare, Local Government and Rural Development, Islamabad as Deputy Director. He joined Pakistan Agricultural Research Council (PARC), Islamabad as an Entomologist/ Senior Scientific Officer in April 4, 1977 and retired from the same department from the post of Chief Scientific Officer/ Senior Director on May 8, 2008.

After retirement, he served as rice expert in Rice Exporters Association of Pakistan (REAP) for about six months. He interacted with National Logistics Cell (NLC), Lahore through a small consultancy in connection with the preparation of initial proposal for the establishment of Rice Export Centre in Pakistan. He joined PARC as rice consultant in 2009 and continuing his services to-date. His major emphasis as rice specialist is to work with scientists and farming community to enhance rice productivity and water use efficiency through a comprehensive approach such as need-based use of pesticides on the basis of ecosystem analysis, popularization of water saving technologies such as alternate wetting and drying (AWD), maintaining thin layer of water in paddy fields through light irrigations or draining of water from paddy fields two to three times and timely and balanced use of fertilizers etc. The goal is to produce more rice with less water.

Dr. Salim is the author of 180 national and international publications including books, chapter in books, scientific research papers, proceedings, popular articles, brochures and technical reports. Recently he has written two books-“Host Plant Resistance: Concepts and Significance.”(Textbook for post-graduate students) and “Water for All: An Islamic Perspective”- Significance, Subjugation and Utilization (a reference book).

As a scientist, Dr. Salim generated lot of new knowledge on biological control and plant-insect interactions in connection with physiochemical plant stresses, developed techniques for the conservation of bio-control agents in agro-ecosystems through

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manipulation of agricultural matrix and developed pesticide saving and environment friendly pest management tactics for cotton and rice crops.

Dr. Salim made significant contribution in the development of agriculture in Pakistan. Under his leadership, a team comprising of scientists belonging to different disciplines and institutions and agriculture extension personnel was able to successfully launch rice productivity enhancement campaign on large area in Sindh province. Likewise, he supervised a campaign on rice IPM in the province of Punjab. He also carried out research and development work on cotton and rice crops with the active participation of growers. The purpose of these campaigns and on-farm studies was to share knowledge and improved production technologies with growers to enable them to make knowledge based decisions and interventions in their farms to enhance productivity, reduce cost of production and conserve natural resource base.

As a PARC scientist, Dr. Salim represented Pakistan at various International and Regional forums in different countries such as Thailand, Philippines, Nepal, Indonesia, India, China and Bangladesh. He participated and presented papers in large number of National and International conferences, workshops and seminars.

In addition to scientific achievements, Dr Salim worked as Vice President and President of International Muslims Students Association (IMSA) during his higher studies at International Rice Research Institute (IRRI)/ University of Philippines at Los Banõs (UPLB), Philippines. In this capacity he worked for the welfare of Muslim community and played a vital role for the establishment of an Islamic Centre at Los Banõs. He made lot of efforts to make IMSA a dynamic and viable Association.

Dr. Salim got best paper awards five times in competitive essay writing on different topics in National Seerat Conferences organized by Ministry of Religious Affairs at Islamabad. These awards were conferred by General Pervez Musharraf, Chief Executive/ President, Islamic Republic of Pakistan (2002 and 2000); Mr. M. Nawaz Sharif, Prime Minister, Islamic Republic of Pakistan (1997); Mr. Farooq A. Khan Leghari, President, Islamic Republic of Pakistan (1996) and Mr. Moeen A. Qureshi, care taker Prime Minister, Islamic Republic of Pakistan(1993).

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CHAPTER-1 INTRODUCTION

1.1. PLANT-INSECT INTERACTIONS CONCEPT

The adaptation of insects to the host plants involved behavioural and metabolic changes which enabled insects to cope with the physical and chemical defence systems of plants during evolution (Benson 1976). Appearance and evolution of flowering plants accelerated the evolution of insects. Two living systems, comprising of plants and insects have evolved together over hundreds of millions of years and then coexisted depicting their intricate interdependence and interactions in contrasting ways. Herbivory is an important aspect of plant-insect interactions. Nearly half of the insect species feed on plants. Most important herbivorous insects belong to Lepidotera, Hemiptera, Orthopetra, Thysanoptera and Phasmida orders (Shoonhoven et al., 1998).

Different factors such as host plant chemistry, environmental conditions and insect adaptations involving feeding specialization tend to influence pest growth performance. Basic knowledge of evolution of insects and plants, factors that affect the feeding behaviour of insects and defence mechanisms of plants are helpful to understand host plant-insect interactions. The interactions between plants and insects are dynamic and intricate. To understand the dynamic equilibrium existing between insects and their host plants, role of orientational, feeding and ovipositional factors, allomonal factors and plant nutrients must be understood properly. Phytophagous insects live in an environment which is full of plant chemicals, which are released in the air and may affect insects before or just after their contact with the plant surface. Phytochemicals present in plant tissues act upon the insects when they start feeding on plants. The responses may be behavioural, physiological or ecological (Scriber, 1983). Understanding the mechanism of host-insect interactions is very important to develop resistant varieties against insect pests. Under natural environment, plants develop defence against insect pests and in response insect pests use certain tactics to counter defence and get food and shelter from plants.

1.1.1. DEFENCE STRATEGIES OF PLANTS

Plants have developed certain ways and means to defend themselves from insect attacks. These include tolerance, avoidance, physical or chemical defences (Kennedy and Barbour, 1992). Defence may be passive/constitutive or dynamic/induced (Horsfall and Cowling, 1980). Defence has also been divided into three categories: (i) constitutive defence, (ii) induced defence and (iii) cross-talk between plant defence signaling pathways. Induced defence may be due to (a) nutrient removal, (b) cell lignification, (c) controlled chemical biosynthesis and (d) uncontrolled chemical biosynthesis (Singh and Dhaliwal, 2005). Constitutive defences can deter, repel, intoxicate or interfere with the development or

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reproduction of herbivores (Singh, 1988). Such resistance may be attributed to texture and composition of the plant surface (Johnson, 1975), presence of anatomical structures (Singh, 2002), absence of nutrients required by the insects (House, 1961), and presence of allelochemics (Duffey and Stout, 1996). Plants may tie up nitrogen in forms unavailable to herbivores (White, 1978)

Induced defence involves the production of chemicals or physical structures or removal of nutrients essential to insects, in response to attack by insects, diseases or other herbivores (Wold and Marquis, 1997). After attack, the plant produces tough cell wall and less digestible food (Baltensweiler et al., 1977) or produce certain chemicals (Mc Cloud and Baldwin, 1997). In certain cases, plants defend themselves by promoting the effectiveness of natural enemies of insect pests through the provision of shelter, alternate food or the production of signals that enable natural enemies to locate insect pests (Kessler and Baldwin, 2001a and 2001b).

Plants have established association with certain organisms which play an important role in defending plants from insects. Several endophytic fungi in plants enhance resistance. Resistance to more than 23 species of insects has been found associated with endophyte infection of perennial ryegrass (Breen, 1994). Some species of Acacia are protected from the attack of insects by ants living on them (Janzen, 1975).

1.1.2. INSECT STRATEGIES TO COUNTER PLANT DEFENCE

Insects have adapted a number of strategies to counter the plant defences for successful survival and reproduction. These strategies include: (i) detoxification of defence chemicals, (ii) sequestration of defensive substances from plants, (iii) use of allomones as kairomones or as nutrients, (iv) improvement in the nutritive quality of host plants, (v) suppressing pheromonal communication between plants, (vi) avoiding plant allelochemics, (vii) avoiding physical defence, (viii) utilizing alternate diets, (ix) increase in consumption rate and (x) nutrition enhancement through association with microorganisms (Singh and Dhaliwal, 2005).

Certain species of insects have detoxifying enzymes which are considered as powerful defence against plant toxicants (Brattsten, 1983). For example detoxification of glycoalkaloids by potato aphid, Microsiphum euphorbiae(Soule et al., 1999); phenylpropanoid compounds by sweet bay silk moth, Callosamia securifera (Johnson, 1999) and phenolic compounds by gypsy moth, Lymantria dispar (Henn, 1999). Feeding behaviour of certain insect species avoid the toxicity of some of the allomones. For example, Epilachna tredecimnotata and E. borealis chew a circular trench through lower epidermal leaf tissue effectively isolating the leaf area on which they are about to feed. Such behaviour may prevent the rapid translocation of cucurbitacins and other antifeedants to the damaged sites, thus preserving the palatability and quality of leaf for about 40 minutes, during which the insect can feed safely on the plant.

Several species of insects sequester and deploy the toxin for their own advantage (Duffey, 1980). Various species of butterflies and moths sequester

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unpalatable or toxic substances from their host plants to protect themselves from their predators (Nishida, 2002). Some chrysomelids store high concentration of cucurbitacin in their bodies. Such selectively sequestered compounds strongly deter feeding by some predators (Nishida and Fukami, 1990). Many insect species sequester certain allomones for their defence (Boppre, 1990). Insects have evolved certain mechanisms to tolerate host plant toxins (Hoy et al., 1998), while other insect species can utilize allomones as nutrients (Rosenthal, 1983). In certain cases insect attack improves the nutrient flow or quality of food in infested plant parts (Way and Cammell, 1970). Some insects evolved adaptations to suppress pheromonal communication between damaged plant parts (Panda and Khush, 1995).

Insects can avoid plant toxins or allomones by avoiding feeding on those plant parts where these toxins are localized (Zangeril, 1990). The neonate larvae of Heliothis virescens avoid feeding on gossypol glands during first 48 hours after hatching (Hedin et al., 1992). Certain insect species avoid young leaves that are high in toxin (Cates, 1980; Barros and Zucoloto, 1999) or other such parts (Schafellner et al., 1999).

Insects have the ability to avoid certain biophysical or morphological defences (Hulley, 1998). Insects can differentiate between more suitable and less suitable host plants (Zucoloto, 1991). Many insect species increase their consumption rate when nutrient concentration in the host plants declines to fulfill their nutritional needs (Brodbeck et al., 1996). Utilization of microorganisms by insects helps in overcoming their nutritional constraints (Douglas, 1998).

Some phytophagous insects can use plant allomones as kairomones. This happens through manipulating allelochemics in food plants. Such manipulations become complex in plant-insect interactions. A good example of such interactions is that of cucumber beetle and cucumber plant (Da Costa and Jones, 1971). The bitter gene, Bi was dominant and responsible for the production of cucurbitacins (Tetracyclic triterpenoids). These chemicals are bitter in taste and are, responsible for antibiosis in plants against insect pests. Due to co-evolution, the antibiotic chemicals in cucurbit plants stimulated feeding by the specific cucumber beetle, so plants homozygous for recessive gene, bi, lacking cucurbitacins, would offer a means of beetle control. However, bibi gene plants were susceptible to general pests such as two spotted mites. The step wise co-evolutionary process is as follows:

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Cucumber progenitor Heavy herbivore Mutation in cucumber

Bibi pressure by progenitor occurred

general feeders Bibi

such as mites

Mutation occurred General Strong selection Bi plants resistant

in progenitor of herbivore for Bi plants and to general feeders

cucumber beetle numbers Bi gene spreaded

that permitted reduced through population

breakdown of

cucurbitacins

Heavy herbivore Strong selection

Pressure by pressure for any

cucumber beetles defence mechanism that

reduced beetle pressure

1.2. HISTORY OF HOST PLANT RESISTANCE DEVELOPMENT

The concept of host plant resistance is very old. Unintentional selection for plant resistance against insect pests probably occurred in the very early stage of agriculture. The process of natural selection for resistant plants continued until man started to interfere actively in the process by favouring certain plants of desired qualities. Improvement of the desired products, frequently involved the deterministic reduction of factors that coincidently were involved in resistance mechanisms. A wheat variety resistant to the Hessian fly, Mayetiola destructor was first reported in 1782 when Havens published a paper regarding Hessian fly resistance in the ‘underhill’ variety of wheat in USA. The earliest instance of the use of plant resistance to insects was the ‘Winter Majetin’ variety of apple in England resistant against Woolly aphid, Eriosoma lanigerm (Lindley, 1831). The apple variety ‘Northern Spy’ grown for its fruit in eastern Canada, USA, Australia, parts of New zealand and to some extent in South Africa never suffered damage by this pest. One of the most outstanding early successes in utilizing plant resistance in pest management was the control of grape phyloxera, Phylloxera vitifoliae in Europe, utilizing highly resistant grape vines from USA. The entire French wine industry was on the brink of collapse by 1880. French vineyards were reconstituted by

?

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grafting the susceptible European grape vine scions on resistant North American root stocks.

In South Africa, the selection of a single hairy plant resistant to leaf hopper belonging to genus Amrasca enabled to grow cotton over extensive areas. In the Subcontinent (Indo-Pak), the earlier work due to Lal (1937) and Hussian and Lal (1940) led to evolving hairy cotton varieties resistant to cotton jassid, Amrasca devastans. By 1943, other resistant varieties including Punjab 4F, LSS and 289 F/43 covered extensive areas in localities where A. devastans posed a serious threat to cotton production (Afzal et al.,1944).

Research in the field of host-plant resistance was inaugurated in the late 1920s by Painter at Kansas State University, USA. Since then, lot of work has been done on resistance in plants against insect pests. Some results of productive research in this direction have been published. Painter (1951) reviewed more than 2100 papers that were published prior to 1949. Painter (1958) revised his original list by reviewing 178 articles and discussing 52 insect-host relationships for 20 crops. Beck (1965) carried out an extensive review of investigations made during 1956-1963. Cheskonov (1962) published a review of Russian research relating to pest resistance in agricultural crops. Maxwell (1972) attempted comprehensive review of studies conducted during 1958-1972. A short review of host-plant resistance to selected insect pests was published by Sprague and Dahms (1972). A number of review articles have been published by Gallun et al. (1975) on cereal crops resistant to insects, Pathak (1975) on rice crop and Russel (1975) on corn. Hanover (1975) reviewed the work on resistance of trees to insects.

The first extensive research for resistant plant sources to Hessian fly, M. destructor was made in California over a period of ten years, beginning in 1881, but success could not be achieved. It was during 1920s and 1930s, that the development of plant breeding techniques made it possible to apply this information fruitfully by incorporating genetic resistant factors into a promising wheat variety. Field and green house studies of, M. destructor resistant to wheat were commenced by the Kansas State Agricultural Station in 1914 and continued without interruption to the present time. Various resistant wheat varieties against the pest have been released. First resistant wheat variety, ‘Kawvale’ was developed in 1931. The first resistant variety was ‘Pawnee’ which was released by the Nebraska station in 1942 and Kansas and Oklahoma station in 1943. Another resistant variety in eastern Kansas was released in 1951. Due to cultivation of resistant varieties, the population of M. destructor reduced drastically and it became almost impossible to find the pest in any form in the central Kansas. The use of resistant varieties such as,Big Club, 43 and ‘Posa, 42’ in California virtually eliminated the fly problem for a long span of time. Release of new resistance varieties are keeping the population of the pest at low levels. The major resistant varieties included ‘Dual’ and ‘Benhur’ (Caldwell et al.,1966 a, b ), Arthur (Schafer et al.,1968), Arthur 71, Abe, and Oasis (Patterson et al., 1975 a,b). Most of these varieties are resistant to multi-races of the insect. The knowledge gained from the studies of genetic interrelationship between the fly races and wheat plant has helped in combating the biotypes of the pest by utilizing different genetic resistant factors.

Wheat stem saw fly, Cephus cinctus, was a serious pest of wheat in Canada. Scientists observed that solid-stemmed wheat suffered less damage than hollow-

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stemmed wheat. This observation became the basis for the development of the C. cinctus resistant varieties. Selection of solid-stemmed wheat from Portugal, originally developed to resist lodging was also resistant to the C. cinctus. Rescue was the first high yielding wheat variety resistant to the insect. Cultivation of resistant variety reduced the population of the insect in Montana from an epidemic to a minimum level. After the release of Rescue, American and Canadian scientists coordinated their research to develop resistant wheat varieties for all areas infested with C. cinctus. Number of resistant varieties were developed which are being extensively grown. This is a good example of success story of managing insect pests with the help of host plant resistance.

The spread of cotton boll weevil, Anthonomus grandis, throughout cotton belt brought marked changes in the type of cotton grown. The late, vigorous, long-staple, upland cotton varieties were rapidly replaced by early-maturing and short-staple lines for reducing the losses caused by weevil because of their shorter period of susceptibility and thicker carpel walls. Extensive studies on host plant resistance in cotton have demonstrated that several factors contribute significantly to the difference in relative resistance and susceptibility against the insect (Newson and Brazzel, 1968). A. grandis showed non-preference for hairy, glandless leaf and red plants (Bailey et al., 1967). The insect showed non-preference to frego bract cotton (Hunter et al., 1965). The frego bract cotton M 64 had a tremendous impact on A. grandis population suppression under field conditions (Jenkins and Parrott, 1971).

Varieties with higher level of resistance to insect pests have been evolved in number of crops and against many insect species. The strategy for producing insect resistant varieties is to identify resistance gene sources against a particular pest and incorporate them into improved plant type/varieties using a number of techniques (Sadasivam and Thayumanavan, 2003). Through conventional breeding techniques a insect resistant crop varieties have been developed and are grown extensively, representing the greatest achievements of modern agriculture in increasing and stabilizing the crop productivity (Singh and Verulkar, 2005).

Development of resistant varieties passed through different stages and at each stage there was some improvement. The conventional (Mendelian) approach utilizes the inheritance of characteristics from plant germplasm that are qualitatively variable and can be transferred from a source plant(s) to the recipient plant(s) by a process of breeding and selection. It deals with single gene characters that are readily identifiable and easily transferred during breeding. Biometricians utilize the inheritance of characters that are quantitatively variable and controlled by many genes (polygenic characters). Biometricians developed methods of plant breeding that involve changes in gene frequency for a particular character. Differences tend to be statistical, hence the name biometricians. The approach of biotechnologists is based on techniques to transfer genes from unrelated sources in crop plants. Both the Mendelians and the biometricians manipulate only the primary and secondary gene pools of the cultivated species for crop improvement, while through biotechnological techniques and genetic engineering, resistant genes have been incorporated in promising germplasm from distant sources. Use of such tools has become necessary in certain cases. For example, more than 30000 rice varieties have been screened for stem borer resistance and none of the rice varieties developed so far have high level of resistance. Similar situation exists in case of various insect pests of different crops. It is therefore, imperative to assist conventional

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breeder with certain biotechnological tools. It has been investigated by the scientists that Bt genes encode delta-endotoxins, are a highly effective means of defending the plants against certain insect pests. A variety of gene encoding delta-endotoxins have been transferred to higher plants including tobacco (Vaeck et al., 1987), cotton (Peerlak et al., 1990), maize (Koziel et al., 1993), rice (Tu et al., 2000) and potato (Perlak et al., 1993), resulting in built-in insect resistance. There is shift from one Bt genes to two or more resistant Bt genes (Cohen et al., 2000) for getting more durable and stable resistance.

Traditional plant breeding techniques are limited by sexual incompatibility barriers. In addition, valuable traits such as tolerance to specific herbicides do not exist in plants. Biotechnological methods allow to overcome the genetic incompatibility barriers. Genes can be isolated from bacteria, viruses, fungi, plants or animals and are made to express in easy and quick growing organisms. Genetically modified organisms (transgenic plants, animals and microorganisms) have been produced and used commercially (GOP, 2005). Development of resistant varieties to insect pests have passed through different stages. Through biotechnological approaches, lot of advancement has been made and resistance is being sought from distant sources and incorporated in the promising germplasm. Area under transgenic crops is increasing rapidly almost throughout the world.

1.3. DEFINITIONS

Plant resistance represents the inherent ability of a certain crop variety or cultivar to resist, retard, or overcome pest infestations. Host plant resistance has been defined by a number of scientists with some similarities and differences as well. Few of the definitions are given.

Host plant resistance includes those characteristics that enable a plant to avoid, tolerate, or recover from attacks of insects under conditions that would cause serious injury to other plants of the same species (Snelling, 1941). It is the relative amount of heritable qualities possessed by the plant which influence the ultimate degree of damage done by the insect. In practical agriculture, it represents the ability of a certain variety to produce a larger crop of good quality than do ordinary varieties at the same level of insect populations (Painter, 1951).

Beck (1965) emphasized the process of establishment of insects on plants and considered resistance as the collective heritable characteristics by which a plant species, race, clone or individual may reduce the probability that an insect species, race, biotype, or individual successfully uses the plant as a host. This definition restricts the spectrum of plant-insect interactions to the extent of successful use by the insect of a plant as a host, but excludes the possible ability of plant to recover or repair the loss after injury occurs.

Resistance is the ability of the host to hinder a pest, pathogen or disease causing agent (Robinson, 1969). Maxwell et al., (1972) extended the definition of Painter (1951) by considering level of insect infestation and environmental conditions. According to them, resistance includes “those heritable characteristics possessed by the plant which influence the ultimate degree of damage done by the insect”. From a practical point of view, resistance is the ability of certain variety to produce larger yield of good quality than

24

other varieties at the same initial level of infestation and under similar environmental conditions.

Resistance is any inherited characteristic of host plant which lessens the effects of parasitism. In other words, resistant plants are less damaged by parasites than are susceptible plants. This definition of resistance is concerned with reduction of the severity of a pest attack and does not necessarily imply any direct inhibitory effect of the host plant on the development or activities of the pest concerned. All forms of escape or avoidance of pest or disease attack that renders a plant less likely to be attacked, is included in this definition (Russell, 1978).

Host plant resistance is expressed as the relative success or failure of an insect species to survive, develop and reproduce on a plant species and may also describe the relative damage to the host plants in qualitative or quantitative terms (Horber, 1980)

Resistance to insects is the “inheritable property that enables a plant to inhibit the growth of insect populations or to recover from injury caused by populations that were not inhibited to grow. Inhibition of population growth generally derives from the biochemical and morphological characteristics of a plant which affect the behaviour or the metabolism of insects so as to reduce the relative degree of damage which these insects can potentially cause (Kogan, 1982).

Host plant resistance refers to the heritable qualities of a cultivar to counteract the activities of insects so as to cause minimum per cent reduction in yield as compared to other cultivars of the same species under similar conditions (Dhaliwal et al., 1993). The emphasis in this definition is not on the absolute yield obtained from the so called resistant variety as compared to the susceptible one, but on the per cent decrease in yield vis-à-vis the yield obtained without the attack of the insect. It means that a cultivar may yield poor but carries the genes for resistance and on the contrary a cultivar may yield good without having any genes for resistance.

1.4. FUNCTIONAL CATEGORIES

Resistance has been categorized in a number of ways. Painter (1951) has established three broad categories (non-preference, antibiosis and tolerance) of true resistance based on inherited characters. Painter’s categorization of resistance was based mainly on field observations. Though widely adopted by plant breeders and entomologists working on insect resistance, these terms are arbitrary and not clearly separated. These categories of resistance often show a high degree of interdependence and may be shared by common plant characters. Non-preference has usually been projected as a property of the plant rather than the insect’s negative response or avoidance of the plant. To remove this confusion Kogan and Ortman (1978) proposed the term antixenosis to replace non-preference. This term indicates that the plant is unsuitable as a host. The term non-acceptance has been suggested by Van Marrewijk and De Ponti (1975) instead of non-preference. Russell (1978) considered non-acceptance a more accurate term, because in most of the known examples of this type of resistance, insects will not accept a resistant host plant even if there is no alternate source. In plants, resistance may be caused by one (non-preference/antixenosis or antibiosis or tolerance) or more of these factors. Some of the scientists excluded

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tolerance from resistance (Kogan, 1976). Tolerance in host plant may be responsible for the build up of combination population than even on susceptible varieties (Pathak, 1970). Tolerant plants may be helpful for the conservation of population of natural enemies by providing food to them. Non-preference/antixenosis may be attributable to morphological or biochemical factors in the host plants. In general, size, shape and colour of a plant can contribute to non-preference shown by an insect pest. Some biochemical plant compounds also act as repellents to pests. Other scientists consider tolerance as an important component of resistance. It would be wrong to exclude tolerance from any consideration of resistance. A plant which is attacked to the same degree as other plants, but which suffers less damage (in terms of yield or quality) as a result of the attack is said to be tolerant (Robinson, 1969).

1.5. ADVANTAGES

o Resistant varieties provide an inherent control which involves no extra expense to the farmers.

o Host plant resistance poses no problem of environmental pollution.

o It is safe against natural enemies (parasites, parasitoids, predators of insect pests) and in certain cases enhances their efficiency.

o Host plant resistance is compatible with other components of IPM such as biological control, chemical control etc.

o The cultivation of resistant plants is not subject to vagaries of weather, as are chemical and biological measures, and in certain circumstances it is the only effective means of control.

o Resistance is especially valuable in low value crops, particularly in developing countries where crops are planted on small holdings and economic constraints coupled with lack of technical knowledge limit the use of other components of IPM.

o Resistant varieties, manage pests even at low infestation or populations of insect pests, while it is not feasible to use insecticides below economic threshold levels.

o In some cases, resistance developed in plants for one pest species may also be effective against other pest species. Now varieties with multiple resistance are being developed successfully.

o Depending on its level, plant resistance can be used as the principal control method or as a supplement to other measures in insect pest management.

o Use of resistant varieties appears to be the most practical method of pest management in that; it requires less technological inputs.

o Resistant varieties cause cumulative reductions in pest populations. The magnitude of reduction in pest population due to resistance increases with increase in the number of generations. For example the population of stripe stem borer, Chilo suppresselis within 120 days (four generations) became 30 fold

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larger on susceptible rice varieties than on resistant rice variety (Pathak et al., 1971).

o Varieties possessing an antibiosis type of resistance cause high mortality, reduce number of eggs, and have slow rates of growth and in general reduce body size of insect pests in comparison to those reared on susceptible varieties. Even normal adults from susceptible hosts when reared on non-preferred plants, frequently lay a smaller number of eggs than they do on preferred hosts or susceptible hosts.

1.6. LIMITATIONS

One of the major limitations for the use of host plant resistance in the management of insect pests on sustainable basis is the development of virulent biotypes of pest, capable of overcoming plant resistance. It is essential to discuss here the mechanism of biotype development and give some important examples of biotypes of some major insect pests of important crops.

The term biotype is used for insects which are primarily distinguishable on the basis of their interaction with relatively genetically stable varieties or clones of host plants. The presence of insect biotypes constitutes an important environmental feature affecting the resistance. The development of biotypes in insect pests is obviously traced to the severe selection pressure exerted by the resistant crop variety. The hardy surviving remnants may interbreed to form a new biotype population that is able to feed on the same host plant which proves as disaster to their sensitive ancestors. The resistant plant variety is then turned into a susceptible one (Panda, 1979).

Insect biotypes are the populations which are capable of thriving on and damaging plants which are resistant to other populations of the same species. They represent dynamic biological and taxonomic units which exist in nature as products of evolution. They are often selected out of a general population by a cultivar developed for resistance and grown in areas where exposure to the insect is common. Recognition of pest biotypes has strengthened the co-evolutionary concept of host plant-insect interactions (Kogan, 1975).

Painter (1966) classified the insect biotypes into two broad categories. The first represents the least common biotypes that thwart the host resistance by dint of their sheer large size and greater virility than those possessed by other members of their species. Second category of biotypes is related to specific genes for resistance. In this kind of biotypes, there appears to exist a “lock and key” relationship between a particular insect biotype and a genetic factor for resistance in the host.

When a resistant variety carrying a major gene for resistance to a specific insect pest is planted continuously over a large area, then most of the insect population is nearly decimated. However, a small insect population with a virulent gene at the locus corresponding to the host’s resistance gene is able to thwart the resistance and gets a chance to multiply rapidly. Thus, a new biotype is developed which renders the resistant cultivar to susceptible cultivar. Chances of development of new biotypes of insects in varieties with monogenic resistance are more than those of polygenic resistant varieties. New biotypes can also develop through mutation or by immigration of new individuals in

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an area. Mechanism of biotype development can be explained by the gene for gene theory (Person, 1959).

Resistance-breaking biotypes of a pest circumvent or neutralize resistance mechanism. They do not overcome the resistance genes themselves, but affect the mechanisms that the genes control, directly or indirectly. Biotypes of various insect species are known to exist, but very few of them are adapted to specific varieties of host plants. The other biotypes differ from one another, either in preferring particular species of food plants, in some morphological features, or in degree of general vigour or size. Such biotypes are not, therefore, true resistance-breaking biotypes. Those types of resistance which have been overcome by resistance-breaking pest biotypes have, almost invariably, been those which are controlled by major genes. However, some types of monogenic resistance to crop pests have not been overcome by resistance-breaking biotypes, even when these resistant varieties have been exposed extensively to pest attack for many years (Russell, 1978).

A knowledge of insect biotypes can help plant breeders to identify diverse sources of resistance and to breed for multigenic resistance (Pathak and Saxena, 1981). Failure to recognize insect biotypes can have costly and frustrating consequences in pest management (Diehl and Bush, 1984). Biotypes are generally detected by differential host varieties. In certain cases biotypes can be detected or discriminated morphologically using characters of rostrum, legs and antennae (Saxena et al., 1983). Salient variations in nuclear and chromosomal measurements were also detected in these biotypes (Saxena and Barrion, 1983, 1984). Using horizontal starch gel electrophoresis showed that brown palnthopper, Nilaparvata lugens biotypes differed in the extent of enzyme polymorphism (Saxena and Mujer, 1984). Morphological differences and the associated behavioural and physiological characteristics of N. lugens biotypes represent primordial steps which may ultimately lead to speciation by eventually acquiring a certain degree of genetic isolation (Saxena and Rueda, 1982).

Understanding the genetic basis of interactions between plant varieties and insect biotypes is critical in development of durable resistance. In Hessian fly, Mayetiola destructor, gene for gene association has been demonstrated (Hatchett and Gallun, 1970). Twenty seven resistance genes have been identified and incorporated into the wheat cultivars and 14 biotypes of the insect have been recorded. Virulence in the insect is inherited as recessive trait conferred by a single gene in the plant. H3 gene either in homozygous or heterozygous state conferred high level of resistance against insect carrying v H3 gene in homozygous or heterozygous state. However, H7 and H8 genes were highly effective in the homozygous condition but displayed a reduced level of resistance in the heterozygous condition. H6 and H9 genes showed different levels of resistance against the reciprocal heterozygous larvae. Avirulence in the insect to resistant genes is incompletely dominant (Bohssini, et al., 2001).

Six biotypes of gall midge, Orseolia oryzae have been characterized in India (Katiyar et al., 2000). Reaction of at least eight resistance genes against Indian and Chinese biotypes of O. oryzae have been studied. Virulence in Indian biotypes 3 and 4 against Gm2 gene is conferred by a single recessive gene (Bentur et al., 1992).

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Planting rice varieties resistant to N. lugens was considered a solution of this major pest of rice. IR 26 developed by IRRI, Philippines was found to be totally damaged in India in 1975. Investigations revealed that biotype of N. lugens in South Asia was quite different from other countries (Seshu and Kauffman, 1980). Biotypes 1 and 2 of the insect are widely distributed in South-East Asia, biotype 3 is a laboratory biotype produced in the Philippines and biotype 4 is used to represent undefined population and spread across South Asia (Khush and Brar, 1991). Six biotypes of O. oryzae on the basis of reaction to four groups of differentials have been identified. Studies through DNA fingerprinting involving AFLP techniques revealed that new O. oryzae biotype in Manipur state of India was more likely through migration of the pest from China rather than through selection pressure of the resistant varieties (Katiyar et al., 2000).

1.7. HOST PLANT RESISTANCE PROGRAMMEME IMPLEMENTATION

Implementation of host plant resistance programme has various basic requirements such as trained scientific manpower, scientific knowledge about insect and plant and their interactions, availability of insect culture at the time of need, availability of genetic resources, information on the genetic aspects of resistance and knowledge about suitable selection and breeding methods. Brief description of each pre-requisite component for the implementation of host plant resistance programmeme successfully is as follows:

1.7.1. SCIENTIFIC MANPOWER

To carry out research on host plant resistance, well qualified multidisciplinary team is required. The purpose of multidisciplinary team is to establish cooperative and interactive relationship between the entomologists and plant breeders as an initial step. Other scientists may be involved in the later stages according to the requirements.

1.7.2. SCIENTIFIC KNOWLEDGE

To initiate programmeme on host plant resistance, availability of information on the influence of biotic and abiotic factors on the biology of pest including behaviour especially food habits, oviposition and movements; parameters of growth and fecundity; and effects of the environment on pest populations is of paramount importance (Ortman and Peters, 1980).

1.7.3. AVAILABILITY OF INSECT CULTURE

The continuous availability of uniform population of target insect is of paramount importance for the screening and evaluation of germplasm for insect resistance. A primary function of the entomologist is to establish the culture of insect and maintain its population so that the proper infestation level may be obtained. It is very important to infest proper stage of the plant with suitable age or instar of insect based on well established screening and evaluation techniques. Studies carried out with natural infestation alone may be very slow and results could be misleading. Artificial infestation gives fast and accurate

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results. For screening and evaluation of germplasm for insect resistance natural infestation may not give proper results (Galt, 1977) and it is unreasonably slow process (Mihm, 1985). Most of the entomologists are convinced that artificial infestation is superior and more efficient (Mihm, 1983 a, b; Javed, 2005).

1.7.4. GENETIC RESOURCES

Success in identifying sources of resistance is directly related to the diversity of germplasm available and the probability of resistance occurring in the host populations. The search for sources of resistance is carried out in a logical sequence: (i) adapted cultivars, (ii) plant introductions, (iii) exotic germplasm, (iv) near relatives of cultivar and (v) from wide sources.

1.7.5. GENETIC ASPECTS OF RESISTANCE

Information on the mode of inheritance and the number of genes involved in the resistance of plants to a pest species, although not essential for developing resistant varieties, but has great practical significance in identifying donors of resistance, developing isogenic lines and breeding broad-based resistant varieties. Resistance may be monogenic, oligogenic or polygenic. Monogenic resistance is easy to incorporate in promising germplasm but it is often short-lived. In single gene conditioning, only a single gene mutation in the insect is enough to overcome resistance. This is true in most of the cases. In certain cases monogenic resistance may remain effective for long time. For example cotton jassid, Amrasca devastans resistant cotton varieties having a major gene or single gene resistance remained resistant for more than 50 years in Africa (Gallun and Khush, 1980) and rice varieties resistant to green leafhopper, Nephotettix spp. remained resistant for more than 40 years (Khush and Beachell, 1972). In general, polygenic types of resistance are likely to involve more complex resistance mechanisms than mongenic resistance. The presence of several mechanisms in a variety may delay or stop the development of new biotypes. Polygenic resistance is considered as biotype nonspecific due to involvement of several minor genes. Efforts should be made to develop all types of varieties based on local conditions, the availability of genetic resources and trained manpower.

1.7.6. SELECTION OF BREEDING METHODS

Appropriate testing and selection procedures in breeding for resistance to insect pests depend on number of factors such as breeding system of the host plant, the type and genetics of resistance involved and the biology of the pest. In cross-pollinated crops individual plants in a population usually have different genotypes and there is considerable plant to plant variation in resistance to a pest. It is imperative to select for improved resistance on an individual plant basis in cross-pollinated crops. It is desirable to self-pollinate selected plants or cross them in pairs or in small groups. The progenies of selections are then tested for resistance and further selections are made, then re-selections can be inter-pollinated in larger groups to produce breeding lines or crossed with progenies of genetic variability. In self-pollinating crops, the progeny of a plant is selected for

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resistance to a pest, consists of genetically identical individuals, but segregation for resistance occurs in later generations. True breeding lines can be selected for pest resistance in the later generations. Screening for resistance to a pest in self-pollinated crops usually involves testing a number of individuals, genetically stable genotypes and retaining the more promising genotypes for retesting and multiplication. Testing for resistance to pests can be carried out in the field or in the glasshouse. When field tests are conducted under natural environmental conditions, experimental errors may be very high. However, if natural infestation levels are high enough, field testing may be reliable. Experiments under glasshouse are more precise and reliable. In most of the cases standard procedures for screening and evaluation and incorporation of resistance genes in desired genotypes are available. Certain techniques are also available to get desired levels of infestation levels under field conditions.

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Sprague, G. F., and R. G. Dahms. 1972. Development of crop resistance to insects. J. Envir. Qual. 1: 28-34.

Stephens, S. G., and H. S. Lee. 1961. Further studies on the feeding and oviposition preferences of the boll weevil (Anthonomus grandis). J. Econ. Ent. 54: 1085-1090.

Stout, M. J., J. Workman, S. S. Duffey. 1994. Differential induction of tomato foliar proteins by arthropod herbivores. J. Chem. Ecol. 20: 2575-2594.

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CHAPTER-2 MECHANISM OF HOST PLANT RESISTANCE

No precise definition of the concept of plant resistance has been advanced. Different ideas have been presented by different scientists, which at the best describe variously the characteristics of the complex phenomenon of resistance (Panda 1979). Mechanism of resistance has been explained on ecological, evolutionary, trophic basis, crop stage, screening conditions, number and nature of genes, biotype reaction, population concept and genetic basis etc. Brief description of each base of resistance mechanism is given:

2.1. ECOLOGICAL RESISTANCE

The ecological resistance is also called pseudoresistance or apparent resistance. Such rersistance may result from transitory characters in potentially susceptible host plants. Cultivars or varietirs showing ecological resistance are important in economic entomology but should be distinguished from cultivars that show resistance throughout a wide range of environments (Horber, 1979). Such resistance does not involve any heritable trait. Painter (1951) classified ecological resistance into host evasion, induced resistance and escape. Panda (1979) divided ecological resistance into two broad categories viz. host evasion and induced resistance. Kogan (1982) classified ecological resistance into phenological asynchrony and induced resistance. Induced resistance has been mentioned by almost all the scientists while differences exist in the terminology in case of other categories as mentioned above.

The terms host evasion and host escape seem to be synonymous but critical analysis reveals that host evasion pertains to whole population of the host and insect is absent or insignificant while escape pertains to one or a few individuals in the presence of insects causing damage to other plants (Singh and Dhaliwal, 2005).

2.1.1. HOST EVASION/ESCAPE/PHENOLOGICAL ASYNCHRONY

Inspite of some differences, host evasion, host escape and phenological asynchrony can be discussed together. The host plant passes through the susceptible stage quickly or passes when insect population is low. A crop which is in the field when insect populations are low or matures before the insect population reaches damaging levels shows host evasion (Heinrichs et al., 1985).

The host evasion takes place when asynchronies in the phenologies of host and insects are found. The level of resistance at different growth stages of the plant may vary in certain cases. For example the resistance of European corn borer, Ostrinia nubilalis depends on the presence of an effective concentration of resistance factor in the right tissues at proper stage of plant growth (Klun and Robinson, 1969). Pea aphid, Acyrthosiphon pisum has shown a clear effect on the stage of host plant development upon differentiation of offspring. The larvae

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of European winter moth, Operophtera brumata normally fed in early spring when the leaf tannin content of Oak was low (0.19% fresh weight). As the season advanced, the tannin content increased to 5% and simultaneously substantial reduction in larval and pupal weight was recorded (Feeny, 1968). Asynchrony between food availability and larval emergence as an effective mechanism has been observed in balsam fir trees (Eidt and Mac Gillivray, 1972). Such phenomena can play an important role in field crops by adjusting proper sowing time. The author has observed that in Pakistan the infestation of whitebacked planthopper, Sogatella furcifera and leaf folder, Cnaphalocrocis medinalis can drastically be reduced by sowing rice crop at proper time. Likewise, in India, it has been proved experimentally that planting time of rice has direct influence on the prevalence and incidence of gall midge, Pachydiplosis oryzae on rice (Prakasa Rao et al., 1971) and shoot fly, Atterigona varia soccata on sorghum (Vidyabhushanam, 1972).

The phenologies of the plant and insect must be synchronized for a plant structure to exist when a certain stage needs it. Alterations in plant growth patterns that result in asynchronies of insect-host phenologies constitute the modality of resistance called host evasion (Painter, 1951). Asynchronies may be induced by the early or late planting, depending on the situation. The early varieties of soybean planted in Illinois mature in the beginning of September, when the second generation of the bean leaf beetle, Cerotoma trifurcata emerges in soybean fields, most plants are already mature and almost ready to harvest. Thus the insect is not in a position to attack the crop, since the mature pods are not a suitable food for the beetle. Thus short life cycle of the crop in central Illinois allows it to escape attacks by the beetle in the latter part of the growing season (Kogan, 1982).

Insect resistance and short duration can be incorporated into a variety. Very early maturing rice varieties which have yields equal to those of intermediate maturing varieties can escape the third generation of brown plant hopper, Nilaparvata lugens (Heinrichs et al., 1985).

Plants that evade insect attack by this mechanism are in reality susceptible, provided the pest incidence occurs at the right time. As the length of the life cycle is under genetic control, the technique of breeding for early or late maturing crops to enhance asynchrony may be used as a component of the resistance programmeme.

Escape refers to the absence of infestation or injury to the host plant because of transitory circumstances such as incomplete infestation. An uninfested plant located in a susceptible population does not necessarily mean that it is resistant. Even under very heavy infestation, susceptible plants will occasionally escape. The level of resistance can be determined correctly through certain tests under laboratory or greenhouse conditions. Under natural conditions escape of susceptible plants may be found.

As a result of the change in cultural practices certain varieties which are otherwise susceptible sometimes escape from the attack of insects. These

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varieties under a particular set of conditions behave in a resistant manner. The planting dates, crop maturity, fertilizer and irrigation play an important role in imparting resistance in plants to insect pests. Planting of crop at optimum time is important to avoid condition that synchronizes crop pest association. Careful selection of planting date enables the plants to escape damage during susceptible growth stage.

Early maturing varieties normally suffer less by pink bollworm, Pectinophora gossypiella in North India. Population of the insect increases by 20-folds from generation to generation. One-two generations of the pest can be avoided by timely sowing of cotton. High incidence of P. gossypiella was observed in early and late sown cotton crop (Dhawan and Sidhu, 1985; Sundramurthy and Chitra, 1992; Janeja and Dhindwal, 1985). Late sown cotton crop was conducive for build up of Helichoverpa armigera population (Raja Sekhar et al., 1996). The incidence of P. gossypiella was low on early maturing cotton variety (F 414) (Dhawan and Sidhu, 1985). The incidence of spotted bollworms Earias spp. was low on early flowering/boll setting varieties due to escape mechanism (Katiyar, 1978). Late sown crop was attacked more heavily by the cotton jassid as compared to the early sown crop in Punjab (Butter et al., 1992). In Andhra Pradish, higher cotton yield was obtained by early sowing of susceptible cotton varieties, because the crop escapes from the virulent brood of cotton whitefly, Bemisia tabaci during the period of outbreak (Rao and Reddy, 1987).

2.1.2. INDUCED RESISTANCE

Induced resistance means increase in resistance temporarily as a result of some changed conditions in plants or in environment (Singh and Agarwal, 1983). Certain environmental changes or conditions and disease infections may alter the physiology of a plant to the extent that it becomes unsuitable as a host of an insect pest.

It has been proved experimentally that optimum temperature induces resistance in rice plant against whitebacked planthopper, Sogatella furcifera (Salim and Saxena, 1991). Optimal use of Nitrogen and application of higher amounts of Potassium and Silicon induced resistance in rice plants (Salim and Saxena, 1992). Application of Boron induced resistance against mites in the seedlings of oil palm (Rajaratnam and Hock, 1975). Application of chelated metal complexes of Iron and Boron induced resistance in brinjal plants to brinjal shoot borer, Leucinodes orbonalis (Panda et al; 1975).

Growth regulators promote, inhibit, or otherwise modify physiological processes in plants (van Overbeck et al., 1954). Application of growth regulator, Maleic hydrazide (Robinson, 1961) and Chlormequat (van Emden, 1969) to host plants reduced the fecundity and growth of aphids. Growth regulators may have direct effect on the insects or indirectly by causing certain physical or biochemical changes in host plants. Growth regulators can also alter the timings of plant development in a way that susceptible material is not available for infestation at the time of pest attack (Smith, 1967).

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Application of weedicides generally induced susceptibility in crop plants to insect pests. The susceptibility of crop to insects significantly increased on the application of weedicide 2, 4-D (2, 4-dichloro-phenoxyacetic acid). With the application of 2, 4-D, infestation of leaf corn aphid, Rhopalosiphum maidis (Oka and Pimental, 1974) and that of Asiatic rice borer, Chilo suppressalis (Hirano, 1964) increased. Higher infestation due to application of weedicide may be attributed to the improved nutritional quality of host plants. There are various other approaches by which resistance in crop plants may be induced such as balanced use of fertilizers, optimal use of water, proper sowing times etc. (Salim 1988).

2.2. EVOLUTIONARY RESISTANCE

Insects and plants are coevolved organisms. Plants during establishment are subjected to multiple pressures, including the pressure from insects. As a sequence, Insects attack plants for food and shelter and plants used to defend from the attack of insects. In this competition, plants evolved resistance against insects. The adaptation of the insects to the host plants involves behavioural as well as metabolic changes which enable insects to cope with the physical and chemical defence systems of plants during evolution. Development of plants provided valuable resource for the herbivore insects (Panda and Khush, 1995). The extensive spread of several herbivorous insect orders like Lepidoptera, Coleoptera, Diptera and Hymenoptera occurred after the appearance of angiosperms (Strong et al., 1984). The appearance and evolution of flowering plants accelerated the evolution of insects (Schoonhoven et al., 1998). Plants and insects have evolved together over hundreds of millions of years and now coexist depicting their intricate interdependence and interaction in contrasting ways. On the basis of such evolutionary relationships resistance mechanism has been classified as sympatric resistance and allopatric resistance.

2.2.1. SYMPATRIC RESISTANCE

Sympatric resistance is defined as those heritable qualities possessed by the plant or other organisms which influence the ultimate degree of damage done by a parasite having a prior continuous, co-evolutionary history with that species of plant or other organisms. This type of resistance evolves at original home of plants and insects. Association at the gene centres results in natural selection for resistance in plants. The resistance is evolved as a result of gene-for-gene nature of co-evolution of plants and herbivores. Sympatric resistance is governed by major genes (Harris, 1975).

2.2.2. ALLOPATRIC RESISTANCE

Allopatric resistance is defined as those heritable qualities possessed by plant or an organism which influence the ultimate degree of damage done by a parasite or an insect species having no prior continuous co-evolutionary history with that species of organism. The resistance to insects in plants is evolved in the absence of insect to which the host is resistant. It is not the result of coevolution, but due to fortuitous, pleiotropic effects of gene which are present as a result of

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selective forces unrelated to the pest insect. In general, allopatric resistance is polygenic in nature (Harris, 1975).

2.3. TROPHIC LEVEL RESISTANCE

The set of plants with which an insect species is trophically associated is called its host range. Plants fall in the first trophic level and herbivorous insects are in the second trophic level. Therefore, herbivorous insects and plants have intricate relationships. Herbivorous insects get food and shelter from the plants. Generally, plants provide food to the insects through different ways. Such resistance mechanism consists of intrinsic resistance and extrinsic resistance.

2.3.1. INTRINSIC RESISTANCE

In intrinsic resistance, plant produces defence either through physical means or through production of allelochemicals or by both means. Intrinsic resistance of host plant may affect positively or negatively the third trophic level such as parasites and predators (Shepard and Dahlman, 1988). Trichomes are one of the important physical means with plants for defence against herbivorous insects. Plants with more trichomes showed more resistance against maize stem borer, Chilo partellus than plants with lower number of trichomes (Javed, 2005). Trichomes restrict insect mobility on plant surface by entrapping, immobilizing, impaling, bleeding, starvation, decreased longevity and fecundity, increased mortality and time of larval development, decreased size. In short, plant exhibits a variety of physical traits that usually reduce their susceptibility to insect herbivores. However, chemical bases of plant resistance affect adversely in a number of ways. These effects are due to absence of some nutritional factors, the presence of toxic metabolites or anti-enzymes, the presence of allomones or the absence of kairmones etc. Also, presence of enzymes play a major role in plant defence against pests. It has been reported that protease type enzyme play an effective role in resistance against herbicides and insects in weeds, Chenopodium album, Amaranthus retroflexus, and Lolium rigidum (Ahmad et al., 2005).

2.3.2. EXTRINSIC RESISTANCE

In this case, natural enemies including pathogens, parasitoids/parasites and predators (third trophic level) of insect pests (second trophic level) benefit the host plants (first trophic level). Several endophytic fungi have been associated with plant defence against insect herbivores. Certain fungal toxins such as that of Rhabdocline parkeri kill different species of midges, Contarinia spp. Likewise, some species of Acacia are protected by ants living on them. The ants are usually hostile to intruders and protect their hosts from herbivores including insects.

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2.4. MECHANISM BASED ON INHERITANCE OF RESISTANCE

On the basis of inheritance, resistance has been categorized as monogenic, oligogenic and polygenic. Resistance controlled by one gene or single gene is called as monogenic resistance. Oligogenic resistance is controlled by a few genes. When resistance is governed by many genes it is referred to as polygenic resistance. It is also called as horizontal resistance.

Most of the known high level of resistance is either monogenic or oligogenic. Monogenic resistance is easy to incorporate in promising germplasm, but it is often short-lived because with a single gene conditioning, only a single gene mutation in the insect is enough to overcome resistance. For example erosion of monogenic resistance, conferred by Bph1 gene in rice variety IR 26 within two years of its cultivation, occurred by biotype 2 of the brown planthopper, Nilaparvata lugens (Saxena and Barrion, 1983). Polygenic resistance is moderate but is more stable and longer lasting than monogenic or oligogenic (major gene) resistance. Polygenic resistance is biotype-nonspecific due to involvement of several minor genes. There are less chances of the development of new biotypes of insects in case of polygenic resistance (Gallun and Khush, 1980).

2.5. MECHANISM BASED ON CROP STAGE

Based on crop stage, resistance is divided into (i) seedling resistance or juvenile resistance and (ii) adult plant resistance or old age resistance or mature plant resistance. Seedling resistance/juvenile resistance is measured at the seedling stage of the crop. Adult plant resistance/old age resistance or mature plant resistance, is mainly found in maturing plants and is less apparent in the seedling stage. Generally older plants are less preferred by the insect and more difficult to damage or to kill. Adult plant resistance may involve horizontal resistance but not always horizontal resistance is concerned with the adult plant. Adult plant resistance can be demonstrated by different planting dates. For example, maize plants in whorl stage are more resistant than in later stages to the European corn borer, Ostrinia nubilalis (Horber, 1979). Seedling resistance can be identified in very young plants although the resistance may persist and be displayed by older plants as well. Adult or mature plant resistance is difficult or impossible to identify in the seedling stage. Seedling resistance is often used as a synonym of major gene, race-specific resistance, but this is not justified because adult plant resistance can also be controlled by major genes and is often highly race-specific (Russell, 1978). There are some differences relating to the concept of seedling resistance and adult plant resistance. For the sake of simplicity it is concluded that in certain cases resistance against a particular species of insect pest is found at the seedling stage but not at the adult stage of the plant and vice versa.

2.6. MECHANISM BASED ON SCREENING CONDITIONS

On the basis of screening conditions, resistance has been classified as green house resistance and field resistance. The term field resistance commonly applied to resistance which gives an effective control of pest under natural conditions in the field. Greenhouse resistance is detected under greenhouse conditions by exposing plants to insect populations. This may involve seedling resistance as well as mature plant resistance. While field resistance is usually observed under field conditions due to the

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exposure of plants to natural populations of insects. It may involve seedling resistance and adult plant resistance. Field resistance is also called moderate resistance.

Field resistance is difficult to detect or to characterize in laboratory or glasshouse tests. Although field resistance is often polygenically controlled and has often been durable. Where field conditions can be satisfactorily simulated in the glasshouse or laboratory, field resistance can often be detected under artificial conditions; hence, in this respect, the use of the word ‘field’ is not strictly accurate. However, the term field resistance is very widely used and it is a valuable general term to describe complex kinds of resistance that give a partial control of a pest under natural field conditions (Russell, 1978).

2.7. MECHANISM BASED ON BIOTYPE REACTION

On the basis of biotype reaction, resistance is divided into vertical resistance and horizontal resistance. Van der Plank (1963) introduced the terms vertical resistance and horizontal resistance to categorize these two functional types of resistance. These terms were derived from diagrams in which histograms showed the degree of resistance of different host varieties to different races of a pathogen. A variety with race specific (vertical resistance) had columns that were tall (resistant) and short (susceptible); on the other hand, a variety with non-specific (horizontal resistance) had columns with the same height whose tops made a horizontal line somewhere between the extremes of susceptibility and resistance.

Based on the description of Vander plank, horizontal resistance is an inherent quality of the host. It always operates against pathogen attack and influences infection rate (r). On the other hand, vertical resistance is not an inherent quality of the host. The same genotype may have great vertical resistance, or it may have none. Vertical resistance depends on what races of the pathogen come into contact with it. In turn, the capability of the race of the pathogen to become pestiferous depends on the genotypes in their host plants. The vertical resistance in a cultivar is effective if the vertical genes in the host plant do not match with those in the parasite population but it fails to operate if these genes match. This host-parasite vertical relationship is called the matching gene theory. Vertical resistance complicates the comparison, unless all varieties being compared have same vertical resistance. Vertical resistance occurs in plants against some specific races of the pathogen and leaves them susceptible to other races of the pathogen. The reliance on vertical resistance alone to protect a crop against disease may leave the crop vulnerable to pests. Horizontal resistance operates against all races of pathogen. It occurs in all plants against all pests though its degree may vary between a wide range. It does not breakdown due to change in pathogencity. It persists in plants even after vertical resistance is broken down. Thus, if the old cultivar is replaced by a new one with better horizontal resistance and the average rate of infection if reduced, then the pathogen population will have to multiply many fold to produce an epidemic of the earlier intensity. It would adversely affect the offensive capability of pests. The use of horizontal resistant varieties is more effective against a severe general epidemic than a small local one. If the horizontal resistance is high enough, the epidemic of disease would not spread beyond the limited area before the fields are ready for harvest and are out of harm. Generally the higher the horizontal resistance, the more limited the area would be.

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A good example of the success of horizontal resistance is the suppression of stem rust in wheat. It has been observed that the more widespread the epidemic, the more effectively it protects, which as a spectacular defence strategy can be pitted by a host cultivar against its pathogens. The effect of the horizontal resistance in wheat against stem rust is cumulative. It means, when horizontal resistant varieties are introduced in an area, the initial performances are usually less pronounced in their resistance potential. When such varieties cover majority of fields, their impact becomes more evident mainly due to reduction in r (average infection rate). To bring down the average value of r there is need to plant such varieties over the whole tract.

In short, the mechanisms of horizontal and vertical resistance have their distinct traits and roles in alleviation or at least reduction of pest damage. Horizontal resistance is all pervasive and essential in nature and does not breakdown. Vertical resistance usually provides a complete protection against non-matching vertical pathogens, but a change in the pathogen population may cause its breakdown. It occurs only in certain heterogeneous varieties and acts as supplementary resistance mechanism to horizontal resistance.

The terms vertical resistance and horizontal resistance were initially used by plant pathologists but later on, entomologists also adopted this terminology. It is imperative to discuss shortly the view point of entomologists relating to the concept of these categories of resistance. Vertical or specific resistance is expressed against only some biotypes of an insect pest species. While, horizontal or general resistance, expresses equally against all biotypes of same pest species. Vertical resistance is qualitative as the frequency distribution of resistance and susceptible plants is discontinuous. While horizontal resistance is effective against all the known biotypes of the insect. It is also called non-specific resistance. Horizontal resistance is quantitative as the degree of resistance depends on the number of minor genes each contributing a small effect. Horizontal resistance is generally considered as durable (Yencho et al., 2000). Since it is quantitative in nature, therefore, strong environmental effects and genetic variability of insect often complicate the identification, transfer and selection of this type of resistance (Smith, et al., 1994).

Vertical resistance is controlled by major genes or oligogenes. It is also referred as qualitative or biotype-specific resistance and usually exhibits high level of resistance. The term horizontal and non-race specific resistance are synonymous but must be used carefully because it is not possible to test varieties or breeding material against all possible genetic variations of a pest. The term durable is more appropriate than either non-race-specific or horizontal to describe examples of long-lasting resistance. Many single genes were incorporated into cultivars to provide resistance against various insects, especially wheat varieties resistant to Hessian fly in USA; rice varieties resistant to N. lugens in Asia (Khush, 1992). It has been observed that in most of the cases, major genes exert strong selection pressure on insect population, therefore, new biotypes develop and resistance breaks down. However, various stratgegies can be employed to prolong the life of resistant varieties with major genes (Hu et al., 1997).

Each type of resistance has certain advantages and limitations because of such inherent limitations in both these resistant types and for the complementarity of vertical

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resistance to horizontal resistance, it is appropriate that the reinforcement of horizontal resistance with vertical resistance in a plant breeding programmeme be done.

2.8. MECHANISM BASED ON POPULATION

Based on population, resistance is divided into pure line resistance and multiline resistance. The resistance exhibited by a line which is phenotypically and genotypically similar is called pure line resistance. When a number of pure lines phenotypically similar and genotypically dissimilar are mixed, the resistance conveyed is called multiline resistance. The component lines differing genotypically usually involve vertical resistance. Multiline resistance is successful against the insects with limited movement.

2.9. MECHANISM BASED ON NATURE OF GENES

On the basis of nature of genes, resistance has been divided into major genes and minor gene resistance. The resistance controlled by one (monogenic) or a few (oligogenic) major genes is called major gene resistance (vertical resistance). When resistance is controlled by a number of minor genes, each contributing a small effect is called minor gene resistance (horizontal resistance).

2.10. MECHANISM BASED ON FUNCTIONAL CATEGORIES

The functional categories of resistance include: Non-preference/antixenosis, antibiosis and tolerance. These categories are briefly discussed in this section.

2.10.1. NON-PREFERENCE/ANTIXENOSIS

Painter (1951) used the word non-preference while Kogan and ortman (1978) proposed the term antixenosis to describe the host plant properties responsible for non- preference. Antixenosis signifies that the plant is considered as undesirable or a bad host. Non-preference becomes evident when a plant possesses characteristics that make it unattractive to insect pests for feeding, oviposition and shelter.

Non-preference or antixenosis is the first stage in the host-insect interactions. Ovipositional responses are elicited by characters of the plants that are perceivable before the adult arrive as well as those perceivable after arrival on the plant (Kumar and Saxena 1985). Antixenosis in a cultivar may be due to morphological characters or the presence or absence of allelochemicals. Morphological antixenosis results from plant structural characteristics that interfere with normal behaviour of insects (Kogan, 1982). Under certain circumstances, the non-preference response of the insect can be quite damaging, such as infestation by insect vectors of plant diseases or insects which cause severe damage including borer infestation resulting in white heads (Khan and Saxena, 1998). Under field conditions non-preferred varieties usually escape infestation (Pathak and Dhaliwal, 1986).

In studies on feeding reaction of N. lugens, it was found that resistant varieties with high level of antixenosis for feeding generally had more feeding probes because the insect did not find suitable food substrate and moved around

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making numerous probes in search of preferred site (Nanda et al., 1999). A higher number of leaf and planthoppers adults/nymphs settled on susceptible varieties than on resistance varieties. A higher number of adults/nymphs of S. furcifera settled on susceptible varieties than on resistant varieties (Mishra and Misra, 1991). Both nymphs and adults of S. furcifera had a tendency to move away from resistant plants after some time. More feeding marks were made by S. furcifera females on resistant varieties while fewer feeding marks were made on susceptible TN1 (Singh and Pathak, 1997). The fecundity of the insect was significantly reduced on resistant cultivar (Bing 93-93) exhibiting stronger antixenosis to its adults (Liu et al., 1998).

Antixenosis mechanism for feeding and oviposition has been observed in Mudgo rice variety.(Kalode,1984). Feeding rates varied significantly between resistant and susceptible rice variety. Feeding was less in case of resistant varieties (Rath et al., 1999).

Rehman and Salim (2003) carried out studies on the mechanism of resistance in rice to leaffolder, Cnaphalocrocis madinalis: a major pest of rice in Pakistan. In a test comparising 67 entries the number of adults alighted varied from 2-5 per plant. In another test the number of adults per plant varied from 3-5 (Table-1).

Table 1: Preference of adult leaffolder to rice varieties/lines

2001-02 2002-03

No. of No. of adults No. of No. of adults

lines/varieties alighted/plant lines/varieties alighted/plant

16 2 10 3

26 3 30 3

19 4 20 4

6 5 6 5

Commercial rice varieties Basmati 385, Super Basmati, Basmati 2000 and Shaheen Basmati were tested with resistant check TKM-6 to study the larval preference. In a choice test, four leaf cuts of test variety along with four leaf cuts of resistant check were presented to four 3rd instar C. medinalis larvae in a perti dish arena. After 24 hours the larvae that settled on each leaf cut were counted. The data revealed that there was a difference in the larval preference to various rice varieties (Table 2). The per cent larvae settled on commercial varieties were higher than resistant check.

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Table 2: Preference of leaffolder larvae to different rice varieties

Varieties Larvae settled (%) Difference over check

Basmati 2000 75.9 46.1

TKM 6 29.8

Shaheen Basmati 68.8 35.6

TKM 6 33.2

Basmati 385 68.5 37.0

TKM 6 31.5

Super Basmati 62.8 25.6

TKM 6 37.2

In India, least number of egg masses was laid by yellow stem borer, Scirpophaga incertulas on Basmati 385, followed by Basmati 386, PH 106 and Pusa Basmati-1, whereas maximum number of egg masses was laid on Basmati 370 (Brar et al., 2001).

Resistance to shoot fly, Atherigona soccata under field conditions is primarily due to antixenosis for oviposition (Singh and Jotwani, 1980a). Under natural conditions, antixenosis for oviposition is the major component of resistance operating in cultivars TAM 2566, 1255 and SGRIL – MR-1 (Wiseman and Mc Millian, 1968).

In sugarcane, morphological characters such as plant height, leaf length, leaf width and thickness, number of leaves in a mother shoot and total tillers per plant at the time of egg laying did not have any correlation with the number of egg masses laid by the top borer, Scirpophaga excerptalis. Plant odour seems to play the major role in ovipositional preference (Mukunthan, 1986 and 1990).

The ovipositional non-preference seems to be one of the components of resistance to Heliothis armigera in pigeonpea. ICPL 5036 exihibited ovipositional non-preference mechanism against the insect (Bhadauria et al., 1996). ICPL 269 and ICPL 288 were not preferred by H. armigera for oviposition (Lateef and Pimbert, 1990). Ovipositional preferences by M. vitrata moth for hyacinth bean over cowpea and pigeonpea have been recorded (Ramasubramanian and Sundrababu, 1989). Feeding non-preference by larvae on cowpea has also been recorded (Okech and Saxena, 1990).

Antixenosis in a cultivar may be due to morphological characters or the presence or absence of allelochemicals. Morphological characteristics of plants interfere the normal behaviour of insect. Hair density, angle of insertion of leaf hair, and toughness of leaf veins are responsible for antixenosis for jassid egg laying on cotton. (Joshi and Rao, 1959). Antixenosis for oviposition of stem

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weevil is an important component of resistance. Antixenosis based on morphological characters may be long lasting and there are less chances of resistance breakdown (Dhawan 2005). Oviposition antixenosis owing to low density and less length of hair has been considered as promising component of resistance for spotted bollworm in cotton (Sharma and Agarwal, 1983). Odour of sugarcane plant seemed to play major role in ovipositional preference of sugarcane top borer, Scirpophaga excerptalis (Mukunthan, 1990).

Antixenosis for oviposition commonly exhibited resistance in chickpea. Variable susceptibility response of some chickpea genotypes to H. armigera was due to differences in oviposition preference as well as larval preference and retention on plants. The movement of H. armigera larvae from the resistant to susceptible one was due to unfavourable or repellent characters of the resistant plants (Lateef, 1985). Susceptible genotypes of chickpea supported more number of eggs and larvae of the insect as compared to resistant genotypes (Srivastava and Srivastava, 1989).

2.10.2. ANTIBIOSIS

Antibiosis is the most pre-dominant, desirable and long lasting mechanism of resistance. Antibiosis generally manifests in two ways: (i) direct mortality of insect larvae and (ii) indirectly, adverse effects on various life stages. Antibiosis refers to all adverse effects on the insect life history which results when a resistant host plant is used as food.

Antibiosis against insect pests may vary with the age and stage of plants. For example in the initial growth stage of plants, different maize genotypes both resistant and susceptible were almost equally susceptible to maize stem borer, Chilo partellus. Larval survival of the insect was almost same on 5 and 10 days old plants of Antigua Gr.1, Ganga 5 and Basi local. Larval mortality significantly increased on 15 days old plants of resistant material (Antigua, Gr.1 and Ganga 5). The most critical time for development of antibiosis seemed to be 10-15 days.

Javed (2005) studied mechanism of resistance of maize borer, C. partellus in the resistant genotypes (BR-1, BR-2, BR-3) in comparison with susceptible varieties (Margala, Gauhar, and Local). Feeding behaviour of C. partellus was observed on leaves, stem, tassels and ears. There were statistically significant differences for these traits between resistant and susceptible germplasm. Intensity of damage on leaves and number of total length of tunnels per plant showed very clear differences among the genotype of resistant and susceptible germplasms. Compared to leaf and stem, the damage to tassels and ears was less severe but the damage was significantly higher in susceptible genotypes. In resistant genotypes, none of the ear was infested. Free and no choice tests were used to observe the preference of borer for oviposition, egg hatching and survival/ presence of larvae on both types of germplasms. There were statistically significant differences for all the three characters among the resistant and susceptible genotypes. The growth and development of the borer was observed on different genotypes and on artificial media having dry leaf powder of different genotypes under laboratory conditions. The larval survival

53

and pupation on resistant genotypes were significantly lower than on susceptible genotypes. In some jars of resistant genotypes, none of the larvae survived. The weights and lengths of larvae and pupae on resistant genotypes were lower than on susceptible ones. Adult emergence was lower on resistant germplasm. In susceptible germplasms, male and female were almost equal but in resistant germplasms there were significantly more males than females. Fecundity and egg hatching of C. partellus were also affected by the resistance in resistant genotypes. The feeding stage of the development (larva) on resistant genotypes took significantly more time than on the susceptible ones.

Grain yield, plant height and stem diameter were significantly reduced in susceptible germplasms than in resistant germplasms due to stem borer damage. Leaf length and width were statistically non-significant in all genotypes indicating that these traits may have no role in resistance. In short, stem borer resistant germplasms (genotypes/ cultivars) have been identified and their resistance has been confirmed/ demonstrated by several laboratory and field tests under natural and artificial conditions. These can be further utilized through breeding selection procedures either directly or indirectly by transferring the resistance to commercial cultivars leading to a safe and environmental friendly maize production in Pakistan and elsewhere in world. The major mechanism of

resistance to C. partellus in sorghum is antibiosis (Singh and Rana, 1984) while

for chinch bug, Blissus leucopterus, moderate levels of antibiosis was observed (Shah et al, 2004).

Higher larval survival, growth index and pupal weight of leaf folder Cnaphalocrocis medinalis were found on susceptible rice variety IR36 than on resistant TKM6 (Abenes and Khan, 1990). Likewise, on resistant varieties the larval survival, population build up and pupal weight of C. medinalis was low than on susceptible varieties (Dakshayani et al., 1993).

To determine the leaf area consumed and larval development period on different rice varieties, third instar larvae, starved for 4 hours were enclosed singly in a Petri dish (9 cm diam.) with two leaf cuts (6cm long) of a test rice variety in a no-choice test. After 48 hours of release, leaf area scraped by each larva was measured. Results revealed that there was no significant difference in the area scraped of different varieties (Table 3). In an experiment larval development was studied on different rice varieties. Potted plants (40 day old) covered with mylar cage were infested with 10 first instar larvae of each test variety. Growth was measured by the number of larvae became pupae and time taken to reach the pupal stage. The insect growth index was calculated as the ratio of per centage of larvae developing into pupae to the mean growth period in days. The higher the growth index, the more suitable was the variety for C. medinalis growth (Rehman and Salim 2003).

54

Table 3: Leaffolder feeding and development responses to different rice varieties

Leaf area Larval period pupation

Variety Scraped/larva cm2) (days) (%) Growth index

Basmati 385 3.12 19.8 72 3.6

Super Basmati 2.23 21.2 71 3.3.

Basmati 2000 3.52 20.4 73 3.5

Shaheen Basmati 3.03 22.1 67 3.0

TKM 6 2.15 23.2 54 2.3

Prolonged nymphal duration and adverse effects on the population of N. lugens were observed on resistant varieties (Kalode and Krishna, 1979). Plant hopper confined on test varieties for 6 hours, lost 9-14% body weight on resistant varieties and gained 27% weight on susceptible check. The amount of honey dew excreted was 6.6-11.9 times more on susceptible check than on resistant varieties (Reddy and Kalode, 1981). Lower rates of N. lugens development, lower proportion of females and longer development period of adults have been reported. Growth index, egg hatchability, and nymphal emergence were significantly lower on resistant varieties (Adiroubane and Letchoumanane, 2002).

Adverse effect on S. furcifera including prolonged nymphal period, and reduced nymphal survival, growth index, fecundity and adult longevity resulting in reduction in the population build up indicated operating of antibiosis mechanism in S. furcifera resistant rice varieties (Salim, 1988).

Poor growth of yellow stem borer, Scirpophaga incertulas larvae due to strong antibiosis has been identified as one of the factors contributing to the expression of resistance in some rice varieties (Prakasa Rao, 1972). Less larval length and low larval weight on resistant rice varieties was recorded. Larval weight of S. incertulas was lower on the resistant variety than on susceptible one (Mishra et al., 1990).

Some overlap may occur between antixenosis and antibiosis and a problem may arise in their separation. Antixenosis refers to undesirability, i.e. avoidance by insect, whereas antibiosis refers to unsuitability, i.e. adverse effects on the insect after feeding on the host plant (Renwick, 1983). Sometimes it becomes difficult to separate the two mechanisms. For example Eruca sativa is not a preferred host of mustard aphid, Lipaphis erysimi. Its growth and development was observed to be slower on E. sativa as compared to that on Brassica species in confinement (Dilawari and Dhaliwal, 1988). The mechanism appeared to be antibiosis, but it has been found that the poor development was due to reduced ingestion in E. sativa (Dilawari and Atwal, 1987) indicating

55

antixenosis (Bhadauria et al., 1996). Feeding non-preference by larvae on cowpea has been noted (Echendu and Akingbohungbe, 1990). Wild species of pigeonpea had antibiotic effect on H. armigera larvae, affecting adversely its survival, growth and development (Shanower, et al., 1997).

High level antibiosis resistance in cotton G27 against spotted bollworms has been attributed to high gossypol (Duhoon et al., 1981). It has been responsible for increasing the post-embryonic development period (Sharma et al., 1982). Average weight of 10 day old bollworm larvae reared on a diet containing 0.1% gossypol was about ½ that of those in the check. Development was delayed by 2.7 days and mortality was 17% (Panda, 1979).

Sugarcane genotypes showing least incidence of top borer, Scirpophaga excerptalis were found to affect various biological parameters of the pest adversely (Malik, 1982; Yadav, 1985). Resistant genotypes of sugarcane reduced larval survival, larval weight, pupal weight, fecundity and increased larval and pupal duration of S. excerptalis. Antibiosis was attributed to higher content of dry matter, P, K, and lignin in resistant genotypes (Yadav, 1985).

Prolonged larval development and reduced size and weight of sugarcane internode borer, Chilo sacchariphagus was observed on resistant variety COJ 46 as compared to susceptible variety CO419 (David, 1979).

Antibiosis has been considered responsible for affecting longevity, fecundity and nymphal development of mustard aphid, Lipaphis erysimi in different genotypes of Brassica nigra, B. carinata and Eruca sativa, (Chander, 1993).

Chickpea resistant genotypes caused larval mortality, reduction in larval and pupal mass, prolongation of larval, pupal and total development period and reduced fecundity and egg viability of Helicoverpa armigera (Srivasta and Srivasta, 1990). Larvae of H. armigera reared on ICCC 506 and ICCV 7 weighed less than those reared on ICCC 37 (Cowgill and Lateef, 1996).

Fewer number of eggs of H. armigera were recorded on wild accessions of pigeonpea but no larvae were recorded indicating antibiosis to be an important component of resistance to the insect. In pigeonpea genotypes, mechanism of antibosis against H. armigera was observed. Larval period was long and larval weight was lowest when reared on pods of pigeonpea variety ICPL 86012 and the larvae fed on pods of 86005 recorded the lowest pupal weight and longest pupal period as compared to those fed on pods of ICPL 87 (Anonymous, 2001).

Wild species of pigeonpea had antibiotic effect on H. armigera larvae, affecting adversely its survival, growth and development. Seven day old larvae reared on resistant variety were lighter in weight, took longer to pupate and produced significantly smaller pupae than those reared on susceptible variety (Shanower et al., 1997).

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2.10.3 TOLERANCE

Tolerance is the ability of a plant to repel injury or to grow, to produce an adequate yield despite supporting insect population at a level capable of damaging susceptible plants (Panda and Khush, 1995). Tolerance includes all plant responses resulting in the ability to withstand insect infestation and yield satisfactorily in spite of injury levels that would debilitate non-resistant plants. Tolerance does not place any selective pressure on insect populations, therefore, plays no role in the development of insect biotypes. The main disadvantage is that insect population is allowed to build up which may affect other crops or other varieties of the same crop in the area.

Tolerance has been observed in rice against N. lugens (Kalode et al., 1978). Tolerance in rice varieties to S. furcifera was assessed in terms of loss of plant biomass. The mean biomass loss indicated that Ptb 33 suffered the least damage (47.2%), followed by suryamukhi (56.2%), bhuban (58.8%) and TNI (73.3%). Tolerance has been observed in rice varieties (OR 363-B-14, OR 821-16-2, Sharbani etc.) which gave more than 4 t/ha paddy yield despite leaf damage (78%) of leaf folder (Patnaik et al., 2000)

Enhanced tillering in certain varieties of sugarcane was observed as a result of shoot borer attack (Rao and Rao, 1961). Some sugarcane varieties like CO 416 showed the least deterioration in juice quality as compared to other varieties with similar injuries due to shoot borer infestation (Gupta, 1959). Certain sugarcane genotypes (CO 6512, CO 7304, CO 7627, CO 7706, CO 7712 etc.) were relatively tolerant to internode borer, Chilo sacchariphagus when rated on the basis of yield and commercial cane sugar output (Jayanti and David, 1984; Mehta and Jayanti, 1984). Inspite of higher incidence of the borer, variety 83 v 15 produced higher yield due to fast growing nature (Hansi and Rao, 1996).

Tolerance mechanism in some of the genotypes of chickpea has been reported. The ability to compensate pod damage due to pod borer, H. armigera attack in the chickpea genotypes can be graded from low to medium and this was considered one of the selection criteria for pod borer resistance (Lateef, 1985). Various chickpea genotypes (PG 5, BG 245, Dohad yellow) gave good yields despite sufficient pod damage (Bhalani et al., 1987).

There is wide variability in pigeonpea genotypes to compensate for H. armigera damage and this is an important criteria for selecting promising genotypes. Line ICPL 92 was high yielder despite the population build up of H. armigera (Katiyar et al., 1981). ICPL 85059 proved tolerant and is considered as good yielder inspite of attack by the insect (Gupta et al., 1990). The larval population of Maruca vitrata and pod damage of pigeonpea was similar on resistant and susceptible cultivars but the yield was significantly high in resistant cultivars. Determinate selection MGP 537-M1-2-1B and a non determinate selection MPG 664 0 M1-2-M13 showed resistance to M. vitrata through compensation mechanism. This indicates that some genotypes recover following damage by the insect (Saxena et al., 1998).

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Attack of sorghum shoot fly, Atherigona soccata on the main shoot at early stage induces the production of few synchronous tillers, which grow rapidly and most of which survive to produce harvestable ear heads, so that the yield is not much reduced. The tillers of susceptible varieties were repeatedly attacked and significant differences between resistant and susceptible varieties were found (Sharma et al., 1977). Tolerance is an important component of resistance operating in sorghum cultivars against stem borer (Jotwani et al., 1971).

Cotton varieties D-33, Stoneville, 731 N and Virnar proved tolerant to spotted bollworm, Earias spp. attack at early stage by compensating the loss in yield by producing more branches after the damage to terminal shoots (Sharma and Agarwal, 1984). Similar kind of tolerance was observed against cotton jassid in Gossypium arboreum (Bhat et al., 1985).

Inayatullah et al. (1990) developed a model for evaluating resistance of barley to aphid, Schizaphis graminum (biotype E) on the basis of antixenosis, antibiosis and tolerance components. Four varieties of barley were used during the experiment. A plant resistance index was derived for simultaneously measuring the effect of the three components of resistance. This index was easier to interpret than separate examination of data on the individual components and could be used for other insect-plant systems where antixenosis, antibiosis and tolerance values have been established.

2.11. INTENSITY OF RESISTANCE

Intensity of resistance is measured on the plant as the per centage of damage to the foliage or to the fruiting parts, the reduction of the stand, the per centage of yield reduction, and the general vigour of the plant, number of eggs oviposited, food preference, growth rate, food utilization, mortality and adult longevity (Kogan, 1982).

The degree of resistance among host plant may oscillate between two extremes, immunity and high susceptibility (Panda, 1979). Cultivars differ in their degree of resistance. There may be a gradation from extreme resistance to extreme susceptibility. From the point of view of insect,varieties vary from totally unsuitable host to completely suitable host for growth and development of the insect (Horber, 1979).

On the basis of intensity, Painter (1951) has classified resistance into immunity, high resistance, low resistance, susceptibility and high susceptibility. Heinrichs et al. (1985) categorized resistance levels into immune, highly resistant, resistant, moderately resistant, moderately susceptible and susceptible. The characterization of resistance is based on the extent of damage. The criteria differ for different insect species based on the nature of damage.

To make the point clear, characterization of resistance levels of yellow stem borer, Scirpophaga incertulas; white backed planthopper, Sogatella furcifera; army worms Mythimna and Spodoptera and leaf folder, Cnaphalocrocis medinalis on rice is described as few examples. Such methods with little modifications can be used for other pests species on different crops.

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Yellow stemborer, S. incertulas damage rating is done on the basis of ‘deadhearts’ and ‘whiteheads’

No.of deadhearts counted

Per cent deadhearts = ---------------------------------------------------- X100

Total no. of tillers observed

Conversion of % age of deadhearts to D (Damage rate):

% deadhearts in test entry

Damage rate= -----------------------------------------------------.X 100

% deadhearts in susceptible check

Transformation of the converted figures to 0-9 scale:

Scale % deadhearts % whiteheads

0

1

3

5

7

9

None

1-20

21-40

41-60

61-80

81-100

None

1-10

11-25

26-40

42-60

62-100

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Damage rating (D) for S. furcifera at seedling stage is done on the basis of extent of damage.

Scale Damage

0

1

3

5

7

9

None

Very slight damage

First and second leaves with orange tips, slight stunting.

More than half the leaves with yellow orange tips; pronounced stunting.

More than half the plants dead; remaining plants very stunted and wilted.

All plants dead

Armyworm (Mythimna and Spodoptera)

Rice Hispa, Dicladispa armigera

Scale Damage (%)

0

1

3

5

7

9

No damage

1-10

11-20

21-35

36-50

51-100

Scale Damage (% defoliation)

0

1

3

5

7

9

None

1-20

21-40

41-60

61-80

81-100

60

Leaffoder, C. medinalis

For greenhouse screening consider the extent of damage on each leaf. For each entry first examine all the leaves and rate each one 0-3 based on the extent of damage.

Grade Damage

0

1

2

3

No damage

Upto 1/3 of leaf area scraped

> 1/3 to ½ of leaf area scraped

> ½ of leaf area scraped

Based on the number of leaves with each damage grade, compute the damage rating (R):

A) = (No. of leaves with damage grade of 1 x 100) 1

Total No. of leaves observed

B) = (No. of leaves with damage grade of 2 x 100) 2

Total No. of leaves observed

C) = (No. of leaves with damage grade of 3 x 100) 3

Total No. of leaves observed

R = A + B + C ÷ 6

Calculate damage rate (R) for each test entry and the susceptible check. Then determine the adjusted damage (D) for each test entry on extent of damage in susceptible check:

R of test entry D).= ------------------------------- X 100 R of susceptible check

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Convert D to 0-9 scale

Scale Damage (%)

0

1

3

5

7

9

No damage

1-10

11-30

31-50

51-75

More than 75

A more accurate assessment is necessary in the study of mechanism of resistance. Standard procedures have been developed for studying mechanism of resistance in crops against different insect species. A good example is Heinrichs et al. (1985) for studying mechanism of resistance in rice.

2.12. REFERENCES

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CHAPTER-3 BEHAVIOURAL AND PHYSIOLOGICAL COMPONENTS OF HOST PLANT SELECTION

Herbivorous insects and plants are united by intricate relationships. Insect find nourishment and shelter from plants. It is generally believed that phytophagous insects can not exist in the absence of green plants. Insects with their overwhelming variation in form and behaviour, have been one of the forces in shaping the plant world. Probably no other interaction between two groups of organisms is comparable in type and extent elsewhere in the living world, thus, rendering plant-insect interactions a unique and scientifically fruitful area of research (Schoonhoven et al., 1998). Understanding plant-insect interactions is of crucial importance in developing host plant resistance strategies for the management of insect pests. It is, therefore, imperative to have in depth knowledge of host plant selection and establishment by phytophagous insects.

Host plant selection is the behavioural sequence by which an insect distinguishes between host and nonhost plants. Host selection is generally a function of the female moth, to deposit eggs on plants. It is composed of a sequence of a simple behavioural responses. Each activity in the sequence brings the insect into a situation in which an appropriate stimulus releases the next activity. Such temporal patterns of behavioural components are associated with internal drives such as the urge to oviposit or the need to feed. The behavioural sequences involved in host selection are sometimes very complex (Beck and Reese, 1976).

The host plant selection by the insect consists essentially of a series of take it or leave it situations in which the insect either accepts or rejects the plants at hand in response to food preference for itself or its offsprings (Thorsteinson, 1960). The host selection process is a chain of events (plant stimuli-insect responses) in which each link facilitates the unfolding of the next one (Kogan, 1975).

The steps in host selection may vary from insect to insect. For example the cabbage root fly, Hylemya brassicae selects its host through a number of steps: (i) flight to the plant, (ii) landing on leaf surface, (iii) walking on leaf surface, (iv) walking along stem, (v) circumventive walk around plant base and soil, (vi) extrusion of ovipositor, (vii) arrest of locomotion and introduction of ovipositor into soil spaces, (viii) digging with hind legs, and (ix) oviposition. In other insect species the host selection may be comparatively a simple process. For example in case of European corn borer, Ostrinia nubilalis, selection is a function of the female moth, as the insect will deposit egg on maize, potato, green pepper, gladiolus depending on their availability.

In general, host selection process consists of five major steps: (i) host-habitat finding, (ii) host finding, (iii) host recognition, (iv) host acceptance, and (v) host suitability. These steps are briefly described.

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3.1. HOST HABITAT FINDING

Adult insects usually disperse in the search of suitable habitat mainly for egg laying. Dispersal and search activities seem to be similar but some differences between them do occur. They differ in their underlying drives and reactions to stimuli. For example, alate aphids may react negatively to green foliage and positively to blue sky. Under such situation they take off and fly for long time (dispersal flight). In case of search behaviour, aphids are attracted to green surfaces, and alight on any green plant. If the plant on which the aphid lands proves to be nonhost, it reverts to the dispersal behaviour and resumes flight. The dispersing adult population usually find a general host habitat through phototaxis, anemotaxis, geotaxis, and probably due to temperature and humidity preference. Usually most of the agricultural pests stay in the general area where crops are planted. This phase is, therefore, less important in host selection (Panda, 1979).

3.2. HOST FINDING

The finding of host plant from a distance usually involves orientation on visible and chemical cues. Several aphid and whitefly species tend to alight on yellow surfaces, acridids orientate themselves towards vertical lines or objects (Mulkern, 1969). Odours may also guide insects to possible food plants. Several insect species show a positive anemotaxis in the detection of their host plants (Haskell et al., 1962; de Wilde et al., 1969). Phytophagous insects can discriminate between odours of plants through olfactory cells in their antennae. For example the vapours of aldehydes and fatty acids in plants stimulate receptory cells in the migratory locust (Kafka, 1971). Volatile chemicals are frequently involved in orientation to plants from a distance (olfactory stimuli) and as a result show directed movements toward the odour source. Apparently the role of the odour is to trigger an orientational response to other stimuli,that is wind direction and visual targets. When randomly moving, insects enter an odour plume, they may start moving upwind using optomotor stimulation and thus show positive anemotaxis. When an insect comes into the proximity of an odour source, the concentration gradient becomes much steeper, and allow chemotactic orientation (Kennedy, 1971). High concentration of an attractive odour may inhibit locomotion (arrestant effect) and consequently induce the flying insect to land (Douwes, 1968). Under close-range conditions, the insect may obtain additional olfactory information for the selection of individual plants within the same species or variety. For example, some individual plants in a cabbage field were more attractive to oviposition by cabbage butterfly, Pieris brassicae, because they contain higher than average amounts of volatile allyl nitriles, which attracts the insect (Mitchell, 1977). The tobacco hornworm moth lands nearly every time it approaches a tobacco plant, but almost always veers away as it approaches a nonhost (Yamamoto and Jenkins, 1972). Orientation to potential host plants and the discrimination of host from nonhost requires a highly developed sensory system. Usually the insect responds to the odour of a plant that is located in a stand of mixed vegetation. Specific plant odours are seldom single compounds, but are usually complexes of several volatile substances. For example the Colourado potato beetle, Leptinotarsa decemlineata is attracted to the combination of compounds (leaf alcohols and aldehydes) that is produced by the potato plant. Alfa-pinene produced by conifers, elicits a strong response to number of insect species.

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3.3. HOST RECOGNITION

Insect larvae have the sensorial equipment for host recognition. The gravid female recognizes the site of oviposition which is also suitable for its juvenile forms. Certain species of grass hoppers bite a plant to check its suitability as host before ovipositing (Panda, 1979). Insect species with tarsal receptors may detect chemicals present in the plants. Some species of apple aphid can perceive phlorizin (flavonoid) after landing on the plant (Klinguaf, 1971). Tarsal receptor of the beetle, Chrysolina brunsvicensis are stimulated by hypersin (Rees, 1969). It is evident that several typical plant substances are present in free form on plant surfaces which are detected by some insect species. Once the insect has made chemical contact with a possible food source, it may be stimulated to perform a test bite or test probe. The first bite causes other chemicals which are stored intracellularly in the plant to stimulate the gustatory receptors. The pea aphid, after arrival on a plant leaf, may execute several superficial probings before deciding to penetrate into deeper tissues. During this exploratory phase, if distasteful compounds are met, various species of insects not only stop their investigation but also spit out the repugnant material and even to regurgigate some of the foregut contents (Panda, 1979). The insects orientation behaviour is followed by recognition behaviour, in which the plant is either accepted or rejected as host. Food sources also elicit oriented locomotion and recognition behaviour pattern in most insects. Insect larvae are also capable of host selection through a certain behaviour (Beck and Schoonhoven, 1979).

Physical factors involved in host plant recognition include both visual and tactile stimuli. Butter flies searching for food (nectar) react positively to yellow, blue and in some cases ultraviolet colours. Green colour induces landing when the butterfly is searching for oviposition sites. Caribbean fruit fly, Anastrepha suspensa is attracted by orange and yellow colour (Greany et al., 1977). Light reflected from green leaves contains a distinct peak in the yellow position of the spectrum to which many whiteflies and aphids are attracted (Kring, 1972). Lepidopterous larvae of various species move toward vertical objects during their search for food. Locusts and grasshoppers are attracted by vertical striped patterns (Mulkern, 1969). Tactile factors are usually involved in the recognition of host plants. The physical characteristics of surface on which the female is willing to deposit eggs are usually of paramount importance. The insect ovipositor, generally bears mechanoreceptors and in most cases tactile stimuli seem to be the only sensory information relayed by the ovipositor. Some insect species require relatively glabrous surfaces, whereas other prefer heavily pubescent oviposition sites. The diamondback moth, Plutella maculipennis prefers an oviposition substrate with small crevices and cavities (Gupta and Thorsteinson, 1960). The weevil, Ceutorrhynchus maculaalba oviposits in young seed capsules of poppies; the convexity of seed capsule have been shown to be of decisive importance in host selection. Cabbage root fly, Chrotophila brassicae requires soil particles of certain size near the base of the host plant to lay eggs. In this case tactile information received by the ovipositor determines whether or not oviposition will take place (Zohren, 1968).

Chemical stimuli play a major role in host plant selection for both oviposition and feeding. Volatile chemicals are frequently involved in stimulating biting and probing and oviposition after the insect is in physical contact with the plant. The final recognition

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process leading to acceptance or rejection is usually mediated by non-volatile chemicals acting on contact chemoreceptors.

3.4. HOST ACCEPTANCE

Acceptance or rejection of host platn is usually mediated by non-volatile chemicals acting on contact chemoreceptors. Discrimination between host and non-host plants may be made at relatively short distances (5-20 cm). Discrimination of host from non-host requires a highly developed sensory system. Usually the insects respond to the odour of a plant that is located in a stand of mixed vegetation. The odour is usually due to complexes of several volatile substances. Once the insect has made physical contact with a plant, contact chemoreceptors located on the tarsi, antennae, mouthparts, or ovipositor receive stimuli related to the chemical characteristics of the plant surface. Special movements by the insect may intensify the chemical stimulation; such as tapping or scraping motions of the forelegs of many butterflies as they prepare for oviposition and the drumming of maxillary and labial palps by locusts. The tarsal receptors of the cabbage butterfly, Pieris brassicae were responsive to sinigrin (Ma and Schoonhoven, 1973). Surface testing by touching or piercing with the ovipositor, or by biting and probing with the mouthparts is in response to chemical factors that act as incitants. If the stimuli received from initial testing identifies the plant as an acceptable host, feeding or oviposition proceeds. These chemical factors are stimulants. If the stimuli received on initial testing indicate an unacceptable plant the behaviour pattern is interrupted and the insect abandons the plant; such stimuli are deterrents. Whereas attractants, repellents and many incitants are olfactory substances, while stimulants and deterrents are usually gustatory. Many important feeding stimulants are general nutrient substances, such as sugars and amino acids, rather than host plant specific compounds (Beck and Schoonhoven, 1979).

In some cases different chemicals appear to govern different phases of feeding behaviour. Some secondary plant substances may function mainly as incitant (Beck, 1965). In locusts, two groups of sensilla near the edge of the clypeo-labrum are probably activated first in the exploratory biting procedure, whereas the remaining sensilla located more dorsad on the clypeo labrum and on the hypopharynx act as further monitors and have to be satisfactorily stimulated to allow continued feeding (Haskell and Schoonhoven, 1969).

Lepidopterous larvae such as that of cabbage butterfly, Pierus brassicae specialized deterrent receptors detect unpalatable compounds, which render the food unacceptable. Not only taste receptors but olfactory organs may also dictate the choice of feeding on certain plants. Several taste cells appear to be stimulated by groups of chemically related compounds and sometimes the specificity of these cells seem to be fairly high. Thus cells, sensitive to one of the chemical groups (sugar, inositol, glucose, sorbitol, mustard oil glycosides, amino acids, anthocyanins) have been found In insects. Many chemicals on the other hand may affect several other cells, either directly or by increasing or reducing their sensitivity to other substances. As a consequence, complex reaction patterns are elicited when combination of various compounds imitating a natural situation are tested.

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The receptor system of different insects exhibits considerable divergences. Apparently, each species evolves a unique sensory apparatus which is tuned in such a way that an optimal discrimination capacity for hosts versus non-host plants is obtained. The chemosensory system involved transmits the information of odour and food consumption to the central nervous system in which a fairly complex pattern of nerve activities is encoded. The interpretation of the incoming peripheral information by the central nervous system is of paramount importance. In Lepidopterous larvae and certain other larvae, the sensory system allows an appraisal of the levels of several nutritive compounds such as sugars, amino acids, sterols (Hsiao and Frankel, 1968). The annual cycles of aphids are synchronized with the nutritional suitability of their hosts in a number of ways.

In short perception of the host odour or taste by a phytophagous insect is a holistic phenomena. Stimuli derived from nutrients and odd compounds are inter-twined in complex sensorial inputs. Discrimination is based on central decoding of total sensory patterns rather than sensory filtering alone (Dethier, 1967).

3.5. HOST SUITABILITY

The nutritional value of the plant and the absence of toxic compounds finally determine the adequacy of food to sustain various physiological processes related to the growth and development of larvae and the longevity and fecundity of adults. There is little sensorial involvement during this phase, which together with host acceptance is of key importance to the resistance mechanism. From the nutritional stand point, host plant suitability may be affected by the absence of some nutritional components such as vitamins, amino acids, steroids and the imbalance of dietary components. The main effects of secondary plant substances may range from lethal to sub-chronic. The main symptoms may be death of larvae in the first few stadia, abnormal growth rates, abnormal conversions of digested and ingested food into body matter, failure to pupate and abnormal adults, decreased fecundity, reduced fertility and restlessness (Painter, 1951). Host suitability is determined by the of effect of these chemicals on metabolic or physiological processes of insects (Kogan, 1975).

The essential amino acids for insects are arginine, histidine, isoleucine, leucine, lysine, methionine, phenyl alanine, threonine, tryptophan and valine. In addition to these, proline is also commonly required by many insects. Although other amino acids are not essential but they are necessary for optimal growth, because their synthesis from amino acids is energy consuming and necessitates the disposal of surplus fragments.

Carbohydrates are used as fuels by a majority of insects and may be converted to fats and may contribute to the production of amino acids. They are, therefore, important components of the diets of insects. Most insects require some carbohydrates in the diet and grow better as the proportion of carbohydrates is increased. Usually, insects are able to digest and utilize polysaccharides (starch), disaccharides (sucrose, maltose, hexose) and monosaccharides (glucose and fructose). Some of the sugars especially sucrose, fructose and glucose also act as phagostimulants for insects.

Fatty acids, phospholipids and sterols are components of cell membranes. Sterols are also required for biosynthesis of ecdysones or moulting hormones. Fatty

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acids serve as store houses for energy and as components of the insect cuticle. Sterols and polyunsaturated fatty acids are essential nutrients. Oleic, linoleic and linolenic acids meet the requirements of most insects. Insects are unable to synthesize sterols and all insects need a dietary source of sterols to meet their requirements. All insects require water-soluble B complex vitamins: thiamin, riboflavin, nicotinic acid, pyridoxine, pantothenic acid, folic acid, and biotin. Minute quantities of Na, K, Ca, Mg, Cl and phosphate are essential for the functioning of cells. Traces of Fe, Zn and Mn are also required (Arora and Dhaliwal, 2005).

If a plant is to serve as a host, it must be able to provide holistic nutrients that can support the growth, development and reproduction of insects (Beck, 1972). The host plant should not only meet the nutritional requirements of insects but also be suitable for being assimilated and converted into energy and structural compounds for normal activity and development. Morphology, nutrients, water contents and allelochemicals in the plants play key role in the regulation of herbivore populations (Bernays, 1985) The success of phytophagous insects in growth, development and reproduction depends upon their ability to efficiently ingest, digest and metabolize plant N with optimum levels of leaf water (Scriber and Slansky, 1981).

Fertilization increases the levels of soluble N compounds, amino acids and amides in plants, resulting in significant increase in digestibility for the concerned insect (Slansky Rodriguez, 1987). The involvement of allelochemicals in plant-insect relationships can determine the status of a plant either as a host (presence of kairomone) and non-host (absence of kairomone) or as resistant host (presence of allomone) and susceptible (absence of allomone) host.

Initiation and sustenance of feeding by phytophagous insects depends on the combined action and interaction of nutrients and allelochemicals. Phagostimulants (kairomones) are essential to start feeding; while deterrents (allomones) may inhibit feeding even in the presence of phagostimulants. Some of the nutrients especially sucrose also act as phagostimulant. Other nutrients like amino acids, phospholipids, certain fatty acids, sterols, ascorbic acid and various salts normally present in the host plants may also serve as phagostimulants for various insects (Dadd, 1970). Essential amino acids are effective phagostimulants in several species of Lepidoptera (van Loon and van Eeuvrjik, 1989) and termite (Chen and Henderson, 1996). Amino acid taste stimulation in phytophagous insects has been observed (Bernays and Simpson, 1982; Hedin et al., 1993; Mullin et al., 1994; Panzuto & Albert, 1998). For western corn rootworm, Diabrotica virgifera L-alanine is the most stimulatory amino acid followed by L-proline and GABA (Hollister & Mullin, 1999; Kim & Mullin, 1998). The insect response to a combination of different nutrients may be synergistic, additive, sub-additive or antagonistic (Dadd, 1970; Hollister and Mullin, 1999; Kim, 1997). Deterrent chemicals play a large part in determining both the initial choice made and the amounts of food ingested. The deterrent effects are concentration dependent and may completely prevent feeding when the concentrations are high (Schoonhoven, 1982). The balance of phagostimulants and deterrents is probably the final determinant of the palatability of a material (Neilsen, 1978a and b).

Tannins can block the availability of proteins by forming less digestible complexes. During the growth season, the protein content of oak leaves declines and the

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tannin content increases; the interaction of the two brings about a marked and accelerated shortfall in the nutritive value of oak leaves. Oak leaf tannins markedly inhibit the growth of the winter moth, Operophtera brumata larvae because of the formation of relatively indigestible complexes with the available proteins, thus reducing the rate of assimilation of dietary N (Feeny, 1969). The host plant may be deficient in certain nutritional elements required by the insect. The nutritionally deficient plant may cause antibiotic and antixenotic effects on the insect.

3.6. REFERENCES

Arora, P. K., N. Kaur, R. C. Batra, and N. K. Mehrotra. 1999. Physio-chemical characteristics of some ber varieties in relation to fruit fly incidence. J. Appl. Hort. 1: 101-102.

Arora, P. K., N. Kaur, S. K. Thind, P. S. Aulakh, and N. Kaur. 2000. Metabolites of some commercial cultivars of guava in relation to incidence of fruit fly. Pest. Manag. Hort. Ecosyst. 6: 61-62.

Arora, R., and G. S. Dhaliwal. 2005. Biochemical basis of resistance in plants to insects. pp. 84-125. In: G. S. Dhaliwal and R. Singh. (eds.) Host Plant Resistance to Insects: Concepts and Applications. Panima Publishing Co, New Delhi.

Beck, S. D. 1965. Resistance of plants to insects. Ann. Rev. Entomol. 10: 207-232.

Beck, S. D. 1972. Nutrition, adaptation and environment. pp. 1-6. In: J. G. Rodriguez (ed.). Insect and Mite Nutrition- Significance and Implications in Ecology and Pest Management. North-Holland, Amsterdam.

Beck, S. D., and L. M. Schoonhoven. 1979. Insect behaviour and plant resistance. pp. 115-135. In: F. G. Maxwell and P. R. Jennings (eds.). Breeding Plants Resistant to Insects. Jhon Wiley and Sons, New York.

Bernays, E. A. 1985. Regulation of feeding behaviour. pp. 1-32. In: G. A. Kerkut and L. I. Gilbert (eds.). Comprehensive Insect Physiology, Biochemistry and Pharmacology, Vol 4. Regulation, Digestion, Nutrition and Exceretion. Pergamon Press, Oxford.

Bernays, E. A., and S. J. Simpson. 1982. Control of food intake. pp. 59-102. In: M. J. Berrridge, J. E. Treherne and V. B. Wigglesworth (eds.). Advances in Insect Physiology, Vol 16. Academic Press, London.

Blom, F. 1978. Sensory activity and food intake: A study of input-output relationships in two phytophagous insects. Neth. J. Zool. 28: 277-340.

Butter, N. S., B. K. Vir, G. Kaur, T. H. Singh, and R. K. Rahega. 1992. Biochemical basis of resistance to white fly, Bemisia tabaci (Genn.). (Aleyrodidae: Hemiptera) in cotton. Trop. Agric. 69: 119-122.

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Chander, H. 1993. Identification of sources of resistance in some crucifers against mustard aphid, Lipaphis erysimi (Kaltenbach). PhD. Dissertation. Punjab Agric. Univ. Ludhiana.

Chen, J., and G. Henderson. 1996. Determination of feeding preference of Formosan subterranean termite, Coptotermes formosanus Shiraki, for some amino acids additives .J. Chem. Ecol. 22: 2359-2369.

Chhabra, K. S., B. S. Kooner, S. K. Sharma, and A. K. Saxena. 1990. Sources of resistance in chickpea, role of biochemical components on the incidence of gram pod borer, Helicoverpa (Heliothis) armigera (Hubner). Indian J. Ent. 52: 423-430.

Dadd, R. H. 1970. Arthropod nutrition. pp. 35-95. In: M. Florkin and B. T. Scheer (eds.). Chemical Ecology. VA. Academic Press, New York.

Darekar, K. S., B. P. Gaikwad, and U. D. Chavan. 1991. Screening of egg plant cultivars for resistance to fruit and shoot borer. J. Maharashtra Agric. Univ. 16: 366-367.

De wilde, J., L. Suverkropp, K. H. Ris, and A. V. Tol. 1969. Responses to air flow and airborne plant odour in the Colourado beetle. Neth. J. Pl. Path. 75: 53-57.

Dethier, V. G. 1967. Feeding and drinking behaviour of invertebrates. pp. 79-86. In: C.F. Code (eds.). Handbook of Physiology, Sec 6, Vol 1. Am. Physiol. Soc.

Dodia, D. A., J. R. Patel, and Y. M. Shukla. 1998. Studies on plant biochemicals imparting resistance/susceptibility against pod borer, Helicoverpa armigera in pigeon pea. pp. 41-43. In: Proc. Entomology in 21ist Century- Biodiversity, Sustainability, Environmental Safety And Human Health. 1998. Rajastan College of Agric, Udaipur.

Douwes, P. 1968. Host selection and host finding in the egg-laying female of Cidaria albulata (Lepidoptera: Geometridae). Opuscula. Entomol. 33: 233-279.

Feeny, P. 1969. Inhibitory effects of oak leaf tannins on hydrolysis of proteins by trypsin. Phytochemistry. 8: 2119-2126.

Greany, P. D., H. R. Agee, A. K. Burditt, and D. L. Chambers. 1977. Field studies on colour preferences of the Caribbean fruit fly, Anastropha suspense (Diptera: Tephretidae).Entomol. Exp. Appl. 21: 63-70.

Gupta, P. D., and A. J. Thorstreinson. 1960. Food plant relationships of the diamondback moth, Plutella maculipennis (Curt). II. Sensory regulation of oviposition of the adult female. Entomol. Exp. Appl. 3: 305-314.

Haskell, P. T., and L. M. Schoonhoven. 1969. The function of certain mouth part receptors in relation to feeding in Schistocerca gregaria and Locusta migratoria. Entomol. Exp. Appl. 12: 423-440.

Haskell, P. T., M. W. J. Paskin, and J. E. Moorhouse. 1962. Laboratory observations on factors affecting the movements of hoppers of the desert locust. J. Insect. Physiol. 8: 53-78.

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Hedin, P. A., W. P. Williams, F. M. Davis, P. M. Buckley. 1990. Roles of amino acids, proteins and fiber in leaf feeding resistance of corn to fall armyworm. J. Chem. Ecol. 16: 1977-1995.

Hedin, P. A., W. P. Williams, P. M. Buckley, and F. M. Davis. 1993. Arrestant responses of Southern corn borer larvae to free amino acids- structure activity relationships. J. Chem. Ecol. 19: 301-311.

Hollister, B., and C. A. Mullin. 1998. Behavioural and electrophysiological dose-response relationship in adult western corn rootworm, Diabrotica vergifera vergifera Le Conte, for host pollen amino acids. J. Insect Physiol. 44: 463-470.

Hollister, B., and C. A. Mullin. 1999. Isolation and identification of primary metabolite feeding stimulants for adult western corn rootworm, Diabrotica vergifera vergifera Le Conte, from host pollens. J. Insect Physiol. 25: 1263-1280.

Hsiao, T. H., and G. Fraenkel. 1968. The influence of nutrient chemicals on the feeding behaviour of the Colourado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae). Ann. Ent. Soc. Am. 61: 44-54.

Kabre, G. B., and S. A. Ghorpade. 1998. Studies on association of some chemical constituents of maize genotypes with susceptibility to maize borer. J. Maharashtra Agric. Univ. 22: 301-304.

Kafka. W. A. 1971. Specificity of odour-molecule interaction in single cells. pp. 61-70. In: G. Ohloff and A. F. Thomas (eds.). Gustation and Olfaction. Academic Press, New York.

Kennedy, J. S. 1971. Olfactory responses to distant plants and other odur sources. pp. 67-91. In: H. H. Shorey and J. J. McKelvey (eds.). Chemical Control of Insect Behaviour. John Wiley and Sons, New York.

Kim, J. H. 1997. Structure-Phagostimulatory relationships for amino acids in adult western corn rootworm, Diabrotica vergifera vergifera La Conte. M. S. Thesis. Penn. State Univ. University Park, PA.

Kim, J. H., and C. A. Mullin. 1998. Structure-Phagostimulatory relationships for amino acids in adult western corn rootworm, Diabrotica vergifera vergifera La Conte. J. Chem. Ecol. 24: 1499-1511.

Klinguaf, F. 1971. Die Wirkung des Glucosids Phlorizin auf das Wirtswahlverhalten von Rhophalosiphum insertum (Walker) and aphid, Aphis pomi. De Geer (Homoptera: Aphididae). Z. Angew. Ent. 68: 41-55.

Kogan, M. 1975. Plant resistance in pest management. pp. 103-146. In: R. L. Metcalf and W. H. Luckmann (eds.). Introduction To Insect Pest Management. John Wiley and Sons Inc. New York.

Kring, J. B. 1972. Flight behaviour of aphids. Ann. Rev. Entomol. 17: 461-492.

Ma, W. C., and L. M. Schoonhoven. 1973. Tarsal contact chemosensory hairs of the large white butterfly, Pieris brassicae and their possible role in oviposition behaviour. Entomol. Exp. Appl. 16: 343-357.

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Mitchell, N. D. 1977. Differential host selection by large white butterfly, Pieris brassicae, on Brassica oleracea sub oleracea (wild cabbage). Entomol. Exp. Appl. 22.

Mulkern, G. B. 1969. Behavioural influences on food selection in grasshoppers (Orthoptera: Acrididae). Ent. Exp. Appl. 12: 509-523.

Mullin, C. A., S. Chyb, H. Eichenseer, B. Hollister, and J. L. Frazier. 1994. Neuroreceptor mechanisms in insect gestation: A Pharmacological approach. J. Insect Physiol. 40: 913-931.

Nielson, J. K. 1978a. Host plant selection of monophagous and oligophagous flee beetles feeding on crucifers. Ent. Exp. Appl. 24: 562-569.

Nielson, J. K. 1978b. Host plant discrimination within cruciferas: Feeding responses of four leaf beetles to glucosinolates, cucurbitacins and cardenolides. Ent. Exp. Appl. 24: 41-54.

Painter, R. H. 1951. Insect Resistance in Crop Plants. The Macmillan Co, New York.

Panda, N. 1979. Principles of Host Plant Resistance to Insect Pests. Hindustan Publishing Corporation, India.

Panda, N., and G. S. Khush. 1995. Host Plant Resistance to Insects. CAB International, Wallingford. UK.

Panzuto, M., and P. J. Albert. 1998. Chemoreception of amino acids by female fourth and sixth instar larvae of the spruce budworm. Ent. Exp. Appl. 86: 89-96.

Rao, C. N., and V. P. S. Panwar. 2002. Biochemical plant factors affecting resistance to Chilo partellus (Swinhoe) in maize. Ann. Pl. Prot. Sci. 10: 28-30.

Rao, N. V., A. S. Reddy, R. Ankaioh, Y. N. Rao, and S. M. Khasim. 1990. Incidence of whitefly, Bemisia tabaci, in relation to leaf characters of upland cotton, Gossypium hirsutum. Indian J. Agric. Sci. 60: 619-624.

Rees, C. J. C. 1969. Chemoreceptor specificity associated with choice of feeding site by the beetle, Chrysolina brunsvicensis on its food plant, Hypericum hirsutum. Ent. Exp. Appl. 12: 565-583.

Sachan, S. K., and G. C. Sachan. 1991. Relation of some biochemical characters of Brassica juncea to susceptibility to Lipaphis erysimi (Kaltenbach). Indian J. Ent. 53: 218-225.

Sahoo, B. K., and H. P. Patnaik. 2002. Effect of biochemicals on the incidence of pigeonpea pod borers. Indian J. Pl. Prot. 31: 105-108.

Schoonhoven, L. M. 1982. Biological aspects of anti-feedants. Ent. Exp. Appl. 31: 57-69.

Schoonhoven, L. M., T. Jermy, and A. A. V. Loon. 1998. Insect- Plant Biology : From Physiology to Evolution. Chapman and Hall, London.

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Scriber, J. M., and F. Slansky (Jr). 1981. The nutritional ecology of immune insects. A. Rev. Ent. 26: 183-211.

Slansky (Jr), F., and J. G. Rodriguez. 1987. Nutritional ecology of insects, mites, spiders and related invertebrates: An overview. pp. 1-69. In: F. Slansky (Jr) and J. G. Rodriguez (eds.). Nutritional Ecology. John Wiley & Sons, New York.

Thayumanavan, B., R. Velusamy, S. Sadasivam, and R. C. Saxena. 1990. Phenolic compounds, reducing sugars and free amino acids in rice leaves of varieties resistant to rice thrips. Int. Rice. Res. Newsl. 15: 14-15.

Thorsteinson, A. J. 1960. Host selection in phytophagous insects. Ann. Rev. Ent. 5: 193- 218.

Van Loon, J. J. A., and F. A. Van Eeuvrijk. 1989. Chemoreception of amino acids in larvae of two species of Pieris. Physiol. Ent. 14: 459-469.

Yamamoto, R., and R. Y. Jenkins. 1972. Host plant preferences of tobacco hornworm moths. pp. 567-574. In: J. G. Rodriguez (ed.). Insect and Mite Nutrition. North Holland, Amsterdam. 567-574.

Zohren, E. 1968. Laboruntersuhungen zu Massenanzucht, Lebenweise, Eiablage und Eiablageverhalten de Kohlfliege, Chortophila brassicae Bouche (Diptera: Anthonoymiidae). Z. Angew. Entomol. 62: 139-188.

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CHAPTER-4 HOST-INSECT INTERATIONS: THEORIES

A phytophagous insect gets nourishment and shelter from the host plant. The insects and plants have intricate relationships. Plants serve as the primary source of energy rich compounds for heterotrophic organisms. Insects are considered as one of the forces in shaping the plant kingdom. Appearance and evolution of flowering plants accelerated the evolution of insects. In certain cases, insects and plants have mutualistic relationships, such as the association between flowering plants and insect pollinators. It has been estimated that about two third of all flowering plants are pollinated by insects and in return plants provide food in the form of nectar and pollen to insects (Schoonhoven et al., 1998).

Insects and plants have developed relations with each other since long. In other words two living systems: plants and insects have evolved together over hundreds of millions of years and coexisting is depicting their intricate interactions in contrasting ways. The insect plant associations involve seasonal, physiological and behavioural processes mediated by environmental factors. The adaptation of insects to the host plants involves behavioural as well as metabolic changes which enable insects to cope with the physical and chemical defence systems of plants during evolution. An understanding of insect-plant interactions requires basic knowledge of evolution of insects and plants, and the factors that promote the feeding behaviour of insects and defence mechanism of plants. These interactions are dynamic, because both partners in interactions, adapt themselves in different ways to the changing conditions. The dynamic equilibrium existing between insects and their host plants, primarily based on orientational, feeding and ovipositional factors, allomonal factors and plant nutrients etc. Phytophagous insects live in an environment which is full of plant chemicals either nutritional or non-nutritional(allelochemicals) or both. Plant chemicals released in the air may affect insects before or just after their contact with the plant surface. Phytochemicals present in plant tissues act upon the insects when they start feeding on the plant. The responses may be behavioural, physiological or ecological (Singh and Dhaliwal, 2005).

Nearly half of all existing insect species feed on living plants. It has been estimated that more than 0.4 million herbivorous insect species live on about 0.3 million vascular plant species. Important herbivorous insects belong to Lepidoptera, Hemiptera, Orthoptera, Thysanoptera and Phasmida orders. Insects feeding on plants of a single species are called monophagous. Insects feeding on plants within one family or members of closely related families are known as oligophagous and insect feeding on plants from a number of taxonomically distantly related plants are called polyphagous (Dethier, 1953). Most species of herbivorous insects exhibit association with a narrow range of host plants indicating that they are monophagous or oligophagous. Oligophagy seems to be the rule among phytophagous insects (Panda and Khush, 1995).

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Plant-insect interactions involve number of behavioural and physiological processes in host habitat finding, host finding, host recognition, host acceptance and determining host suitability. The host selection process is a chain of events in which each link facilitates the unfolding of the next one (Kogan, 1975). In the process, the insect either accepts or rejects the plant (Thorsteinson, 1958). It is discussed in detail in chapter 3.

The dispersing adult populations usually find a general host habitat by mechanisms that include phototaxis, anemotaxis, geotaxis etc. These mechanisms have important ecological implications. The finding of food from a distance usually involves orientation on visible and chemical cues (Kafka, 1971).

In general, the gravid female recognizes the site of oviposition which is suitable as a feeding site for the larvae or nymphs. During exploratory phase, if distasteful compounds are met by the insect in most of the cases it not only stops the investigation but even regurgitates some of the foregut contents.

When the chain of the host selection activities of the insect is not interrupted, it continues to feed till satiation. In some cases, different chemicals appear to govern different phases of feeding behaviour of the insect. For example sinigrin serves as an incitant and promotes biting activity of Pieris brassicae larvae (Ma, 1972). In Bombyx mori larvae, a series of compounds in mulberry leaves was associated with initial biting, swallowing and continuous feeding (Hamamura, 1970). In short, the nutritional value of the plant and the absence of toxic compounds determines the adequacy of food to sustain various physiological processes related to growth and development of larvae, and the longevity and fecundity of adults. From the nutritional stand point, host plant suitability may be affected by the absence of some nutritional components such as vitamins, amino acids, steroids and the imbalance of dietary components. Host habitat finding to host acceptance is the result of effect on behavioural processes of insects, whereas host suitability is determined because of effect on metabolic or physiological processes of insects. Plant-insect interactions are complex and highly adaptable. The survival of plants through evolutionary time is due largely to their own defence mechanisms. Selective ability of insects enables them to overcome these plant defence mechanisms. The degree to which a plant species is immune to the attack of insects is indicative of the defence strategies it has evolved as well as the evolved abilities of insects to overcome the defences (Ananthakrishanan, 1988).

4.1. THEORIES OF HOST PLANT SELECTION

Host plant selection by insects in very intricate and involves large number of processes and factors, therefore, various theories by scientists have been put forth. These theories include: botanical instinct theory, token stimuli theory, dual discrimination theory, non-preference and antibiosis theory, inhibitory chemical theory, thorsteinsons theory and nutrient imbalance theory. Based on the available knowledge these theories are briefly described.

4.1.1. BOTANICAL INSTINCT THEORY

Insects are guided by specific chemical cues from green plants in selecting suitable host plants for feeding, oviposition, and shelter (Brues, 1920). He suggested that food plants are selected on the basis of specific plant odours

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by insects and designated the phenomena as botanical instinct. The instinct might be nothing more than extreme sensitivity to a complex and physical stimuli emanating from the plants.

4.1.2. TOKEN STIMULI THEORY

Fraenkel (1953) postulated that the basic food requirements of insects were almost the same as those of higher animals, embracing the substances occurring in all living cells, including those of plant tissues. In so far as the occurrence of these substances is concerned, the composition of all leaves is very much alike, and there is a little reason to suppose that the difference in the chemical composition of “primary” substances can be responsible for the insect’s choice of a food plant. Plants also contain a vast array of the so called “secondary” plant substances (Paech, 1950). These may be conveniently grouped as glycosides, saponins, alkaloids, essential oils and organic acids, etc. Most, if not all, secondary plant substances possess characteristic odour or taste and thus may elicit sensory reactions to food. Fraenkel (1959) elaborated the role of “odd plant substances” in food plant selection of insects, in relation to the plant family Cruciferae, Umbelliferae, Legumunosae, Morceae and Graminae. He found that the insects which surrounded the plants were influenced not only by their morphology but also by their chemistry. Finally, he strongly argued that the secondary chemicals were solely responsible for guiding phytophagous insects in general to their preferred food plants and providing the chemical stimuli required to induce feeding. Fraenkel updated his theory to further include the possible effects of secondary chemicals on physiological processes and behavioural patterns of insects (Fraenkel, 1969). Some secondary plant chemicals play important function in the life of the plant itself such as the rapid turnover rate of alkaloids (Robinson, 1974), the possible involvement of steroids in plant’ s hormone functions (Heftman, 1975), and the larger number of polyphenols, flavonoids, etc., in wound reactions (Bell,1974). Beck and Rees (1976) have rightly pointed out that there is no reason to believe that this dynamic reaction is, in any way, of secondary importance to either the insect or the plant. The function of secondary plant substances in the plant ecology was also recognized by various scientists. Schoonhoven (1972) reviewed the role of secondary plant substances in determining the host plant selection by phytophagus insects. He defined “secondary plant substances “as the compounds that are not universally found in higher plants, but are restricted to some plant taxa, or occur in certain plant taxa at much higher concentrations than in others. They are of zero nutritional significance to Insects. He postulated certain principles of host selection; the insect is guided to its specific food by typical odour of its host plant. Feeding is then stimulated by the presence of substances characteristics of this plant form, provided that strong deterrents are absent. Generally, host plant selection appears to be a more complex and subtle process. In many instances, a detailed analysis of the selection procedure has revealed the insect’s capacity to perceive fairly large number of aggregate chemical constituents present in its host plant is helpful for host plant selection. The boll weevil, Anthonomus grandis is attracted to its food, the cotton bud, by an aroma composed of several components (Minyard et al., 1969). Thus, it may be stated that an insect selects

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only some out of a multitude of compounds in plant species to identify suitable host plant. The “key chemicals” involved may vary from only a few to a fairly large number. The compounds of general occurrence may serve as both “key chemicals” and secondary plant substances.

4.1.3 DUAL DISCRIMINATION THEORY

The controversy whether the “odd plant substances” or the “nutritive materials” alone determine the insect food preference tends to lose its force, and a generalization of either of the two extremes is certainly uncalled for. Examples are known in which either of the two groups of substances or even a combination of them plays a crucial role. The artificial diets for some cruciferae feeders gain considerable acceptability after the addition of mustard oil glucosides (Nayar and Thorsteinson, 1963). Feeding by the spotted cucumber beetle is stimulated by cucurbitacins (Chambliss and Jones, 1966). On the other hand, nutrients such as carbohydrates, amino acids, and also some vitamins may act as phagostimulants (Davis, 1965), indicating that they are perceived during feeding. Maxwell (1972) emphasized on the importance of nutrition in his review which cited papers dealing with the sugar content of plants and the amino acids and the mineral requirements of insects interacting with their host plant.

It is possible to prepare a mixture of nutrients, which when applied to older pith, is even more stimulating to the Colourado potato beetle than extract of potato leaves, and as yet no secondary plant substances which act as phagostimulant in this respect has been isolated. Indeed, the beetles feed continuously on a meridic diet, an observation which strengthens Jermy’s (1966) hypothesis that oligophagy in many cases is achieved by avoiding deterrents rather than by being stimulated by specific compounds. Other oligophagous species that feed on meridic diets lacking typical secondary plant substances include Manduca sexta, and Pectinophora gossypiella (Vanderzant and Reisser, 1956), Pieris brassicae (David and Gardiner, 1966), and Acyrthosiphon pisum (Auclair and Cartier, 1963). This leads support to the idea that common nutritive factors might have played a major role in the host plant acceptance by these insects.

Plant odours are usually mixtures of fairly complex compounds. Though several volatile compounds have been isolated from the plants themselves, little is known about the composition of odours as they emanate from plants. Many odourous factors, often in specific combinations, are typical of a single plant species of a higher taxon.

An attempt was made to analyse the relation between an insect and the smell of its food plant (Kamm and Fronk, 1964). The literature on insect relation to volatile factors was reviewed by various scientists. Such findings suggest that the host selection is based on the insect response to two types of stimuli, the flavour “stimuli” which originate from botanically specific biochemicals, and the “nutrient stimuli” which are apparently feeding stimulants and deterrents and may or may not constitute the required nutrients. This theory is known as the “dual discrimination” theory propounded by Kennedy and Booth (1951) and is traced to

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their study on host alteration in Aphis fabae. These aphids respond behaviouraly to at least two main classes of leaf property: one associated with the age of the leaf and the other with kind of plant. It is suggested that in nature these two sensory aphid requirements may be essentially contradictory and that the shifting pattern of aphids distribution among leaves and plants may depend on the shifting distribution of leaves offering a satisfactory compromise between them. This host alteration phenomenon is considered as a particular instance of shifting distribution pattern, probably connected with alternation of the seasons of active growth and the senescence in the winter and summer host plants.

In short the theory has the merit of flexibility in respect to the role of physiological changes in both the insect and the plant. However, it suffers from disadvantage of having been coached in rather subjective terms, such as the postulated necessity for the insect to be capable of making a nutritional assessment of the food value of its host.

4.1.4. Non-preference and Antibiosis Theory

Painter (1951, 1958) classified the resistance mechanism into three aspects: “non-preference”, “antibiosis” and “tolerance”. Non-preference denotes the plant characteristics that discourage its use for oviposition, food or shelter. Antibiosis signifies those preventive, injurious and destructive effects which host exercises by chemical means on the insect’s normal life. Tolerance specifies the host plant’s ability to sustain a pest population, damaging to a most susceptible host variety, by rapidly repairing or overcoming injury and maintaining its growth and yield. However, thinking of some scientists such as Beck(1965, 1974) is in favour of the exclusion of tolerance from the basic resistance mechanisms, since it denotes a biological relationship essentially different from the other two physical and chemical devices. Though, this sort of classification has obviously been convenient for plant breeders, it refers strictly to resistant effects and not mechanisms. However, Painter’s concept about pest population suppression through the use of resistant plants has long been considered to be an ideal control method. Preference or non-preference of host plant by the insect depends on a number of plant physical/ biophysical characters, and presence or absence of certain allelochemicals and a number of nutritional factors. These factors play an important role in the selection or rejection of host plant. Antibiosis mainly depends on the presence of allelochemicals and to some extent to the nutritional factors. The role of physical plant traits in antibiosis is negeligible.

4.1.5. CHEMOSENSORY THEORY

The two percepts advanced by Dethier (1953) to explain food plant selection by insects are: (I) though physical stimuli define the environment for oviposition and feeding, the decisive act of host selection is regulated predominantly by the chemical sense; (ii) the phytophagous insects’ behaviour towards their food plants is best understood when analyse into a series of component phases such as: (a)orientation to the food, (b) biting response, (c) continued feeding and (d) cessation of feeding, followed usually by dispersal. Each of the innate behavioural components is manifested only in response to the

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proper combinations of extrinsic releasing stimuli (plant characteristics) and intrinsic response threshold levels (chemoreceptor). Plant resistance may result from its failure to provide the releasing stimuli required for one or more components of the host finding sequence, or by the possession of characteristics having adverse effects on feeding activities. Resistance mechanisms that operate against insect feeding have usually been characterized as an example of the non-preference type resistance. Dethier et al. (1960) proposed the terminology that deals with insect feeding behaviour and other chemosensory responses. The orientation responses of insects may be directed either positively, resulting in evoking positive stimuli (attractant, incitant and stimulant) or negatively resulting in the avoidance of the host species (repellent, suppressant, and deterrent). The presence of the stimuli, which elicit positive responses from an insect species, would render the plant acceptable; on the other hand, if the specific stimuli elicit negative reactions, their presence would act as a deterrent.

On the basis of specific stimuli that are distributed in host plants, phytophagous insects have been classified according to their host specificity into three groups termed as monophagous, oligophagous and polyphagous. Monophagous is a special case where the necessary insect feeding stimulants are restricted to any one particular plant species. Oligophagy denotes the case when the necessary feeding stimulants are restricted in distribution to a relatively limited number of plants, which may be related taxonomically. In polyphagy, the necessary attractants and feeding stimulants are distributed among a wide variety of plant species and the host plant range is restricted mainly by the occurrence of repellent or toxic chemical constitute in many plant species. As insects have to find their way in a milieu of continuously increasing degree of chemical complexity, and since ‘polyphagy is luxury which can be enjoyed under certain conditions only’. Therefore many insect species are adapted to the use of very few and often closely related hosts (e.g., in the same genus). In other words, monophagous insects differ from polyphagous species in that the former tolerate fewer secondary plant substances than the latter (Jermy, 1966). Indeed, the polyphagous fall webworm, Hyphantria cuna lacks a “deterrent” receptor which typically occurs in the silkworm (Ishikawa et al., 1969).

Kogan (1977) has forwarded six distinct models of host selection strategies for both adult and larval stages of insect groups which provide an understanding of chemical basis of host selection. He has focused on the critical role of allelochemicals in the initiation and maintenance of oviposition and feeding.

In short, behavioural and electrophsiological studies indicate that the host selection activities of phytophagous insects at the various stages of their life cycle are diverse because of the interaction between the host plant allelochemicals and sensory system of insects.

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4.1.6 INHIBITORY CHEMICALS THEORY OR NEGATIVE STIMULI THEORY

Investigations on host selection by oligophagous insects such as Colourado potato beetle revealed that feeding stimuli are not markedly specific, and host selection is based primarily upon negative reactions to repellent stimuli (Jermy, 1961). Studies on chewing phytophagous insects indicated that both monophagous and polypgagous insect species are very sensitive to deterrents. From these studies it is evident that sensitivity to deterrents is more important in determining the host range than adaption to specific phagostimulants. Narrow negative specialization of the chemoreceptors to deterrent chemicals makes it possible to inhibit feeding by an oligopgagous insect and even more by a monophagous one (Jermy,1966). It can be assumed that the more the chemoreceptors are specialized to feeding stimulants, the more they are sensitive to feeding inhibitors. Such two way specialization of chemoreceptors appears to be a characteristic feature of food preference by phytophagous insects.

With the advancement of science the mechanism of the perception of inhibitory chemicals by insects has been investigated behaviourally and electro-physiologically. On the basis of behavioural studies, it is possible to identify the body parts on which the relevant receptors are found. Electrophysiological studies provide information on the reception mechanism and the manners in which the insect differentiates between stimulatory and inhibitory materials.

The experiments on insect behaviour performed indicate role of maxillae in the recognition of inhibitory chemicals (Gill, 1972), though the receptors elsewhere on the mouth parts are also involved (Kennedy, 1965).

In a few cases, a beginning has been made to the analysis of chemoreceptors by means of electro-physiological methods. The caterpillar of Pieris brassicae may serve as an illustration (Schoonhoven, 1969). Like all lepidopterous larvae, the chemoreceptors involved are located on the antennae and some on mouthparts. In the larva of P. brassicae, the odour discrimination is accomplished with an amazingly simple sensory system, each antenna having only 16 olfactory cells. Spontaneous action potential arise in many cells at a rate which varies from cell to cell. When a particular plant part odour or the scent of some pure chemical reaches the antenna, some cells may increase while others may decrease their impulse activity and thus change the total pattern in the cells affected. Because different odours induce different activity patterns, it has been concluded that each odour is coded by influencing a number of receptors which, though not identical, have partly overlapping stimulus spectra (Schoonhoven and Dethier, 1966). Besides, those found on the antennae, the maxillary palpi also have organs with an olfactory function. An investigation of eight sensilla basiconicae at the tip of the palpus with a scanning electron microscope has revealed that five of them have a small peg at their distal end; the remaining ones are blunt and seem to be covered by small pits. It seems possible that the latter represent the olfactory sensillae, and the former play a role in contact chemoreception. Each maxilla appears to bear two sensory styloconica, each

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innervated by four chemoreceptory cells. Their functional importance in the recognition of food materials has been confirmed by amputation experiments.

Dethier (1973) has, however, noted that the response to the saps of unacceptable plants by the maxillary receptors of three species of Paplio involves a number of cells. There is no consistent difference in the form of the response to unacceptable and acceptable food materials and it seems that ‘rejection is not a uniform modality’. From a practical view point, Chapman (1974) has, therefore, considered the feeding inhibitors to fall into two categories: the first one are those which act by suppressing the neuron activity in general. The antifeedants of this type may be expected to have a general effect on all insects. The second category of inhibitors are perceived by specialized receptor cells and lead to the non-preference type avoidance behaviour by the insects. The effectiveness of this inhibitor varies from species to species and in some cases it is negligible or even non-existent.

4.1.7. THORSTEINSON’S THEORY

It is essential to recognize the principle that gives link in the chain and also the one that facilitates the manifestation of the following link in which each unit response requires the simultaneous influence of more than one eliciting stimulus. The latter principle applies not only to the interaction of two different types of stimuli, i.e., colour and odour, but also to the summation effects of several stimuli of the same chemotactic. The links in the chain of stimuli are therefore, to be regarded as occurring in parallel as well as in sequence. Thus it is evident that “host finding by insects is by chance and selection by staying or leaving (Thorsteinson, 1960).

In an attempt to provide a fresh basis for the development of understanding of food plant selection, the following symbolic statement, modified from Thorsteinson (1958), is given below:

F= -I - D + E +Esn (Ep),

Where

F = optimal feeding response;

I = feeding inhibitors;

D = deterrents;

E = chemotectic stimulants;

Esn = sapid nutrient; and

Ep = piquant stimulus-special feeding stimulant of limited botanical distribution.

An optimal feeding response (F) implies that the substrate is devoid of feeding inhibitors (I) or deterrent (D), and contains the chemotectic stimulants (E) essential to elicit feeding. The essential feeding stimulants of general botanical distribution are sapid nutrients (Esn). In order to extend this general statement to the behaviour of any insect species that require special feeding stimulants of

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limited botanical distribution and presumably of no strictly metabolic consequence, the fortuitous term (Ep) is included. This represents, in anthropocentric term, a “piquant” stimulus (Ep). Piquant substances are not essential to feeding activity and food plant selection in many insects, including some with a very restricted food plant range. It, therefore, seems appropriate to regard these special stimuli as agents for respective sapid nutrients. This applies with no essential modification to the adult as well as the larvae in insect species that depend on the same food in both feeding stages, though the stimuli required to elicit oviposition or even feeding in the adult may not coincide precisely with both in the case of larvae. Perhaps the ovipositor of some phytophagous insects may be well equipped to perceive chemotectic stimuli as has been reported for hymenopterous parasites.

There is little doubt that a plant population may show an increase in the production of toxic substances (secondary metabolic products). This may restrict insects to a few species that are able to feed on plant. However, with the passage of time, more herbivorous species may undergo adaptive evolutionary changes and develop specializations, e.g., filtering mechanisms or detoxifying system, to cope with special chemical defences of the plant. Such host-induced modifications must have been a common and recurring phenomenon in the natural system, bringing about a stepwise coevolution of both plants and herbivores, during which angiosperms and insects attained an immense diversification. In other words, the evolutionary and ecological pressures kept the race even between the plants and the animals.

During third evolutionary process, over period of geologic time, many herbivorous insects became highly specific to plant forms that contain similar defensive chemicals but are toxic to majority of other organisms. For example, the flea beetle, Phyllotreta cruciferae are highly specific in their choice of food to plant families, which contain potent antibiotics like mustard oils (Caparidaceae, Cruciferae, Tropaeolaceae and Limnanthaceae) and are toxic to most animals but have no adverse effect on the flea beetle.

Such manipulative and adaptive capabilities exhibited by the insects, have forced scientists to administer the warning that insect pests are by far more resourceful than one would guess from the crude principles on which the scientific community has been working so for and, therefore, no complacency is warranted about the need of a vigorous research policy geared to a more integrated biological control.

4.1.8. NUTRIENTS IMBALANCE THEORY

Plant parts and plant constituents vary with their developmental stages, physiological conditions and plant genotypes. Such variations have considerable impact on phytophagous insects. Moreover, the dietary proportions of essential nutrients in plants seem to be of greater importance than their absolute quantities. The nutritional superiority depends on nutrient balance, i.e., the proportion of nutritionally important substances in food material with respect to the proportions required by the insects (House, 1966). The nutrients imbalance

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affects intake of food by the insects. The nutrient requirements of the insects differ subtly (rather than grossly) from one species to the other and many insects are highly selective, choosing only such food materials which are more suited to their requirements.

House (1967) designed experiments with larvae of Agria affinis in such a way that larvae had free access to four nutritionally different diets arranged in a 16-unit “Latin square”. After 5 hours the distribution of larvae was recorded, which showed a significantly greater number of larvae found on the most suitable food. Diet G (unsatisfactory) contained the same amount of glucose as the satisfactory diet (F) but a lower concentration of amino acids as F but more glucose. Thus, it seems most likely that food selection resulted from sample biting by trial and error until a satisfactory nutrient composition was found, capable to satisfactory ‘the craving’ or hunger arising from metabolic demands for certain nutrients that the other media were inefficient or inadequate to supply. After ingestion of suitable food, locomotion decreased so that the number of larvae on that food increased, whereas after ingestion of unsuitable food the larvae continued to move on.

4.2. THEORIES OF HOST PLANT DEFENCE

Evolution of plant defences has been explained by three theories. The first theory was proposed by Feeny (1976) in which he stated that defensive adaptations were selected on the basis of the probability of being found by herbivores and the host specificity of the herbivore. He suggested that apparent plants should invest heavily in highly effective and general quantitative defences, whereas unapparent plants need only to invest in low-cost quantitative defences against generalists.

The second theory was advanced by Coley et al. (1985). He proposed that the availability of resources creates the selection pressure for different defensive strategies. The level of defence will increase as plant growth rates or resource availability decreases. Thus, slow growing plants on nutrient impoverished sites will invest more in costly quantitative defences, while fast growing species on nutrient rich sites will tend to use cheaper qualitative defences.

Rhoades (1979) proposed optimal defence theory. The theory is based on three assumptions: (i) defence is costly to the plant, (ii) the impact of herbivory on plant’s fitness depends on the characteristics of the affected tissue and (iii) there is higher likelihood for some plant parts or tissues to be attacked more than others. From these assumptions, it is evident that a particular trait will evolve only if the benefit to fitness exceeds its cost. It is also apparent that the benefit of a particular trait depends upon the impact of a particular herbivore on plant fitness.

It is hypothesized that the cost of defence might be a result of several factors such as a differential allocation of the resources to produce more defences, self toxicity, disruption of biochemical pathways when a new resistance trait arises, and finally ecological mechanisms (Sims, 1992). This means that the production of defensive chemicals should have negative effect on the reproductive growth of plants. Such effect differs in different plant species and with different species of insect pests. For example studies on Oak tree showed a decrease in seed production following herbivory (Crawley,

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1985). In other cases, opposite results were obtained which could be attributed to over compensation in plants due to herbivory (Trumble et al., 1993).

4.3. THEORIES OF CO-EVOLUTION

4.3.1. EHRLICH AND RAVEN (1964) THEORY.

The co-evolution of plants and insects have received considerable attention by scientists. It has been postulated that there was a wave-like, or parallel, evolution of groups of plants with groups of animals adapted to them. In these studies special emphasis has been laid on the ability of certain species of butterflies to sequester and store secondary plant substances. Ehrlich and Raven (1964) have analyzed the stepwise reciprocal selective pressures that are likely to have governed the evolutionary interaction of plants and butterflies. The existence among butterflies of certain phyletic units that have become specialized for the exploitation of certain complexes of related species of plants can be explained. The postulate that the defensive chemistry of such butterflies matches that of plants which they feed upon is gaining support from the new knowledge being generated on this aspect. The use of these plants for food has a double basis; it provides them with a feeding niche in which they have relatively little competition and it may supply them with the substances, or precursors of substances that make them unpalatable to predators. The distasteful groups of butterflies apparently have evolved physiological changes that render them immune to toxic or repellent plant substances and thus enable them to turn the chemical defences of plants to their own advantage. According to this coevolutionary theory, a succession of chemical defences against herbivores are developed by host species in a stepwise fashion. At each step, some herbivores become extinct, while others circumvent the special challenge, forcing the plants to evolve further deterrent compounds. This process can be looked upon as a kind of herbivore-filter, which through time tends to reduce the number of higher taxa of herbivores which are able to feed on the plant. Being freed of competition, the remaining herbivores might undergo adaptive radiation on the same host plant which itself has a diversified period of relative protection. When only one or a few kinds of herbivores are important to a plant, the evolution further proceeds in providing a highly specific weapon to plants such as the trichomes which may be more advantageous in terms of cost in contrast to the benefit occurring from the development of further complex chemicals. The plant may also face the problem of increased autotoxicity caused by the addition of such chemicals, but by evolving hooked trichomes the host plant might surpass its insect pests in the co-evolutionary race. For example the trichomes of Passiflora adenopoda act as a specific and highly efficient deterent against heliconiine butterfly, Heliconius melpomene larvae. The heliconiine larvae feeding on these plants were caught by the hooked trichomes and all were dead and desiccated. The defensive function of trichomes was verified with a scanning electron microscope. The micrographs showed that the entire plant surface, including the tendrils, was covered with these hooked structures which were of such length and spacing as to surely hook each larval proleg with 3-5 trichomes.

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The theory claims that the trophic relations of the phytophagous insects result from a very tight evolutionary interaction between plants and insects in which, on one hand, the selection pressure represented by insect attacks induces the appearance of resistance mechanisms in the plants against the insects, while on the other hand, some insects succeed in overcoming this resistance by adapting themselves to these substances which may become feeding stimulants. It is also supposed that such insects enter a new adaptive zone where they are free to diversify largely in the absence of other competing phytophagous insect species. The co-evolutionary theory is proved mainly by the fact that often closely related insect species feed on closely related plant species characterized by the presence of a group of specific plant chemicals.

Critical analysis of the theory reveals following questions (Jermy, 1976):

o Are phytophagous insects really important selection factors determining plant evolution?

o Are trophic plant-insect interactions always antagonistic from the evolutionary aspect i.e. are the insects’ attacks always ‘noxious’ to the plant as a species?

o Is the plant-insect interaction the principal or perhaps the single reason for the existence of secondary plant substances (allelochemicals)?

o Are parallel evolutionary series proving co-evolutionary processes common in nature?

o Is the plant-insect interaction the principal or perhaps the single reason for the existence of secondary plant substances (allelochemics)?

o Are parallel evolutionary series proving co-evolutionary processes common in nature?

o Is the interspecific competition between phytophagous insects considerable?

There is need of serious consideration to assess the authenticity of the theory of co- evolution expounded by Ehrlich and Raven (1964) and by various other scientists in one way or the other.

Most of the phytophagous insect species are rare in natural communities. Their population densities are often extremely low compared to the available food resources. The great bulk of plant material never goes through the animal part of the energy-food cycle. In natural communities, most of the phytophagous insects are found only as a fraction of the individual specimens of their host-plant species. Since they affect only a very small part of the host- plant’s population, their role as selection factor must be negligible compared to such potent environmental factors like soil, climate, plant-plant interactions within the community, affecting regularly the whole or at least the major part of the population of a given plant species in a biotype. It has been observed that those plant groups which are seldom attacked by phytophagous insects do not appear in great biomasses, while those groups which serve as a food for many insect

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species are most abundant in natural communities disappearing a noticeable population controlling affect of phytophagous insects.

In the animal kingdom, there is no indication of the development of resistance against metazoan parasites. It is assumed that fatal parasitism is a built in regulatory mechanism preventing overcrowding of the host which would result in exhausting the host’s resources. In many plant species normal development is impeded by high population densities, therefore, it is logical to suppose that phytophagous insects may serve as specific regulators of host-plant abundance. In such cases the insects adapted to a given plant species serve as natural thinning-out factors prompting normal development of the remaining plant individuals, therefore, a resistant mutant of the plant would be handicapped in further spread as compared to the sensitive ones. Such plant-insect relationships are mutualistic instead of antagonistic.

Appearance of the secondary plant substances (allelochemics) cannot be solely the result of insect-plant interactions. These substances are intermediary steps of plant metabolism. These are important defence substances against and also important information carriers in plant-plant interactions (allelopathy). Not all characters of an organism are involved in co-evolutionary selection mechanisms. There are characters which are neither advantageous nor disadvantageous for survival. Secondary plant substances represent at least in part, only different possible pathways of metabolism which are in the same way species specific like many morphological characters, therefore regarding evolution , they are also not necessarily advantageous or disadvantageous but often neutral. Natural selection is a great destroyer, but a very feasible creator, which means that many characters were not created by selection. Many characters created by mutations are not destroyed by selection not only if they are advantageous but also if they are neutral for survival, like many morphological characters, and possibly also biochemical ones.

If there were an effective mutual selection pressure between plants and insects decisively determining the evolution of both groups of organisms, then parallel evolutionary lines should exist but rarely so happens in the real world. Such relationships which most frequently occur in nature, cannot be the result of insect and plant co-evolution as supposed by Ehrlich and Raven (1964) and other scientists. In the natural communities the population densities of most phytophagous insect species is generally very low compared to the copiousness of available food. In homogenous distribution as well as differences in the phenology of insect species and differences in their feeding niches further reduce the probability of competition for food. It is evident from this discussion that competition must be negligible as a factor in the evolution of phytophagous insects.

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4.3.2. SEQUENTIAL CO-EVOLUTION

The host plant is not merely something fed on, it is also something lived on. Plants are the most sensitive bio-indicators of a whole complex of ecological factors being effective in a given biotype, therefore, the choice of a plant species most often means also the choice of a well defined combination of biotic and abiotic factors. Thus, the insect’s chemoreceptors, being tuned to a plant species’s biochemical profile consisting of primary and secondary plant substances, represent a reliable clue not only to a suitable source of food but also to a specific ecological situation to which the insect is adapted in all its requirements for development and reproduction. This must be the advantage of food speciation (Kennedy, 1953).

Closely related insect taxa are living on very different host- plants. This is most common in natural communities. Various insect groups such as aphids, leafhoppers, fruitflies, bruchids, sawflies etc. are sometimes difficult to distinguish by morphological characters but very well by their feeding habits (host plants). It is therefore, logical to suppose that in many cases mutational changes in feeding behaviour represented enough isolation for sympatric speciation, thus, the flowering plants with their immense biochemical diversity serve as a base for the evolution of the phytophagous insects without an evolutionary feedback mechanism, i.e. in most cases without an appreciable selection effect exerted by the insects on plants. This means the evolution of phytophagous insects follows the evolution of plants. The latter being one of the most important selection factors in the evolution of insects. This is the essence of the proposed theory of sequential evolution of phytophagy. This theory seems to be suitable for the evolutionary interpretation of all existing insect-hostplant relationships (Jermy, 1976).

Another pertinent question relating to this theory is, what is the adaptive advantage of narrow host specificity for phytophagous insects? This question is logical and needs some serious consideration. It is logical, since narrow specificity in feeding behaviour very often results in high mortality caused by the exhaustion of food resource by total defoliation of the individual host plant before larval development is completed, thus, monophagy and oligophagy reduces the probability of survival, therefore, selection for polyphagy must be strong.

Schoonhoven et al. (1998) are of the view that in several cases an evolutionary time lag has been found between the emergence of plant species or group of species and their colonization by herbivorous insect species. So far, no study has proved that switchover of an insect species has changed the traits of that plant. Therefore, at the present state of knowledge, the sequential theory most plausibly explains how the exact insect-plant relationships came about.

4.3.3. DIFUSE CO-EVOLUTION

The theory proposes that, instead of the pairwise reciprocal evolutionary interactions co-evolution must be considered in a community context and not simply as an isolated two species interaction. The community affecting any one plant species includes several types of herbivores, alternative host plants,

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potentially competing plants and species in higher trophic levels that may respond directly to the herbivores or to the plants changes induced by one member of the community may affect many other species. However, it has not been proved so far (Schoonhoven et al., 1998). The theory also proposes that interactions between the members of a community are so tight that the evolution of any member affects the evolution of all other members (Van Valen, 1973).

4.3.4. GEOGRAPHIC MOSAIC THEORY

According to the theory, the reciprocal evolutionary change in interacting species necessarily co-evolve. The salient features of the theory are as follows (Thompson, 1994):

i) Interspcific interactions commonly differ in outcome among populations. These differences result from the combined effects of difference in the physical environment, the local genetic and demographic structure of population and the community content in which the interaction occurs.

ii) As a result of these differences in outcome, an interaction may co-evolve some populations, affect the evolution of only one of the participants in other populations, and have no effect on evolution in yet other local populations.

iii) Populations differ in the extent to which they show extreme specialization to one or more species. Some populations may specialize and sometimes co-evolve locally with only one other species. Other populations may specialize on and perhaps coevolve with different species, and yet others may coevolve simultaneously with multiple species.

iv) Inter populational differences in outcome and specialization create a geographic mosaic in interactions.

v) Gene flow among populations and extinction of some demes reshape the geographic mosaic of coevolution as the adaptations and patterns of specialization developed locally spread to other populations are lost.

vi) There is continually shifting geographic pattern of co-evolution between any two or more species.

In short, the geographic mosaic theory suggests that the co-evolution process is much more dynamic than is apparent from the study of individual populations (Thompson,1997).

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Jacobson, M. 1966. Chemical insect attractants and repellents. Ann. Rev. Entomol. 11: 403-422.

Jermy, T. 1961. On the nature of oligophagy in Leptinotarsa decimlineata Say (Coleoptera: Chrysomelidae). Acta zool. Acad. Sci. Hung. 7:119- 132.

Jermy, T. 1966. Feeding inhibitors and food preference in chewing phytophagous insects. Entomol. Exp. Appl. 9: 1-12.

Jermy, T. 1976. Insect – host plant relationships – coevolution or sequential evolution ? Symp. Biol. Hung. 16: 109-113.

Jermy, T. 1984. Evolution of insect / host plant relationships. Am. Nat. 124: 609-630.

Kafka, W. A. 1971. Specificity of odour – molecule interaction in single cell. pp. 61-70. In: Gustation And Olfaction. G. Ohloff and A.F. Thomas (eds.), Academic Press, New Yark.

Kamm, J. A., and W. D. Fronk. 1964. Olfactory response of the alfalfa-seed chalcid, Bruchophagus roddi Guss, to chemicals found in alfalfa. Wyo. Agr. Exp. Sta. Bull. 413: 36.

Kennedy, J.S. 1953. Host plant selection in Aphididae. Trans. IXth Inst. Congr. Ent. 2: 106-113.

Kennedy, J. S. 1965. Mechanisms of host plant selection. Amm. Appl. Boil. 56: 317-322.

Kennedy, J. S., and C. O. Booth. 1951. Host alteration in Aphis fabae Scop. 1. Feeding preferences and fecundity in relation to age and kind of leaves. Ann. Appl. Biol. 38: 25-64.

Kogan, M. 1975. Plant resistance in pest management. pp.103-146. In: Introduction to Insect Pest Management, R.L. Metcalf and W. H. Lucknann (eds,) Ohm Wiley and Sons, New Yark.

Kogan, M. 1977. The role of chemical factors in insect/plant relationships. pp. 211- 227. In: Proceedings V, D.White(ed.), International Congress on Entomology. Entomological Society of America, Washington, D.C.

Ma, Wei-Chun. 1972. Dynamics of feeding responses in Pieris brassicae Linn. as a function of Chemosensory input: a behavioural ultra – structural and electrophysiological study. Meded. Landb Hoogesch. Wageningen 72, 162 pp.

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Maxwell, F. G. 1972. Host plant resistance to insects- nutritional and pest management relationships. pp. 599- 609. In; J.G.Rodriguez (ed.), Insect And Mite Nutrition, Amsterdam, London: North-Holland Publishing Co.

Minyard, J. P., D. D. Hardee, R. C. Gueldner, A. C. Thompson, G. Wiygul, and P. A. Hedin. 1969. constituents of cotton bud. Compounds attractive to the boll weevil. J. Agr. Food. Chem.. 17: 1093-1097.

Mulkern, G.B. 1969. Behavioural influences on food selection in grasshoppers (Orthoptera: Acrididae). Ent. Exp. Apple. 12: 509-523.

Nayar, J. K, and A. J. Thorsteinson. 1963. Further investigations into the chemical basis of insect-host relationships in an oligophagous insect, Plutella maculipennis (Curtis) (Lepidoptera: Plutellidae). Can. J. Zool. 41: 923-929.

Paige, K. N. 1992. Overcompensation in response to mammalian herbivory: from mutualistic to antagonistic interactions. Ecology 73: 2076-2085.

Painter, R. H. 1951. Insect Resistance in Crop Plants. Mac Millan, New York.

Painter, R. H. 1958. Resistance of plants to insects. Ann. Rev. Ent. 3: 267-290.

Panda, N. and G. S. Khush. 1995. Host plant Resistance To Insect. CAB Inter-national, Wallingford.

Panda, N. 1979. Principles of Host Plant Resistance to Insect Pests. Hindustan Publishing Co. India.

Paech, K. 1950. Biochemie und Physiologie der sekundaren Pflanzenstoffe. Berlin: Springer.

Rhoades, D.F. 1979. Evolution of plant chemical defence against herbivores. pp. :3-54. In: Herbivores: Their Interaction With Secondary Plant Metabolites, G. A. Rosenthal and D. H. Janzen (eds.), Academic Press, New York.

Robinson, T. 1974. Metabolism and function of alkaloids in plants. Science. 184: 430-435.

Schoohoven, L. M. 1969. Gustation and food plant selection in some lepidopterous larvae. Entomol. Exp. Appl. 12: 555-564.

Schoohoven, L. M. 1972. Secondry plant substances and insects. pp. 197-224. In: V.C.Runeckless, (ed.), Structural And Functional Aspects Of Phytochemistry. New York: Academic Press.

Schoohoven, L. M., and V. G. Dethier. 1966. Sensory aspects of host-plant discrimination by lepidopterous larvae. Arch. Neerl. Zool. 16: 497-530.

Schoonhoven, L. M., T. Jermy, and J. A. A.Van Loon. 1998. Insect – Plant Biology: From Physiology to Evolution. Chapman and Hall, Landon.

Sims, E. 1992. Cost of plant resistance to herbivory. pp: 392-425. In: Plant Resistance To Herbivores and Pathogens. Ecology, Evolution and Genetics. The University of Chicago Press, Chicago.

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Singh, R. and G. S. Dhaliwal. 2005. Insect – plant interaction and host plant resistance. pp. 3-41. In: Host Plant Resistance to Insects, G.S. Dhalival and R. Singh (eds.). Panima Publishing Corporation, New Delhi, Bangalor.

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Thorsteinson, A. J. 1960. Host selection in phytophagous insects. Ann. Rev. Ent. S: 193-218.

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Vanderzant, E. S., and R. Reisser. 1956. Studies on the nutrition of the pink bollworm using purified casein media. J. Econ. Entomol. 49: 454-458.

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CHAPTER-5 MORPHOLOGICAL BASIS OF RESISTANCE IN PLANTS TO INSECTS

Plants deter insect pests through morphological and chemical plant defences. The morphological characteristics of a plant which confer resistance to pests are trichomes on plant surface, surface waxes, hardness of plant tissues, thickness of cell walls and cuticle, anatomical modifications, silica content, colour, shape and size. Morphological characters of plants may act as barriers to insect’s normal feeding and oviposition. These may also indirectly cause nutritional imbalances either through restrictive feeding because of texture or shape which would reduce the amount of nutritive material being ingested, or through limiting the digestibility and utilization of food by insects.

Morphological, physical or biophysical resistance factors interfere physically with locomotors mechanisms especially with the mechanism of host selection, feeding, ingestion, digestion, mating or oviposition. Allomones affecting insect behavioural and metabolic processes may also occur in plant morphological structures (trichomes or bracts). Morphological characters responsible for resistance in plants against insect pests are briefly discussed.

5.1. COLOUR AND SHAPE

Host selection behaviour of phytophagous insects is associated with the colour and shape of plants. Colour and shape of plants remotely affect the plant host selection behaviour of phytophagous insects and thus are associated with resistance. Colour related insect resistance in plants does not exist but genetic manipulation of plant colour usually has an effect on some fundamental physical plant processes (Norris and Kogan, 1980). Foliage colour and tree shape and size play role in discrimination between hosts and non-hosts by Rhagoletis fly (Boller and Prokopy, 1976). Alate aphids are attracted to leaves reflecting about 500nm, regardless of the species of plant, because they seem to be attracted to plants at a physiologically suitable stage of growth (Kennedy et al, 1961). Yellow green plants were preferred to green plants by the pea aphid, Acyrthosiphon pisum (Cartier, 1993). Red cotton plants were less attractive to the boll weevil, Anthonomus grandis, than green plants (Stephens, 1957).

Colour of cabbage leaves affected host selection by Brevicoryne brassicae. Red leaved cabbage varieties were less susceptible to Pieris brassicae (Verma et al., 1981) and were least selected by alate aphids but proved suitable host plant after infestation (Radcliffe and Chapman,1966). Purple pigmented sorghum lines exhibited a high level of resistance to the shoofly, Atherigona soccata, whereas they were more susceptible to the red mite, Oligonichus indicus. (Singh et al., 1981). Oat cultivars with red tiller bases were less susceptible to frit fly, Oscinella frit (Peregrine and Catling, 1967). Certain

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morphological characteristics may be linked to other resistance factors. For example okra leaved cotton suffered less damage by majority of insect pests (Dean, 1982).

Red foliaged Brassica cultivars were comparatively less attacked by insect pests such as Pieris rapae, Pieris brassicae, Myzus brassicae, Bemisia tabaci, Brevicoryne brassicae, as compared to green foliaged cultivars (Dunn and Kampton, 1976). Brassica genotypes with purple foliage and apetalous flowers were resistant to Lipaphis erysimi (Rohilla et al, 1999). Cotton cultivars having red leaves were less attacked by Helicoverpa armigera (Wu et al, 1997).

Plant shape is known to bring some behavioural changes in insects. In cotton, Okra leaved cultivars suffered less damage by pink bolloworm, Pectinopphora gossypiella (Wilson et al,. 1981), Heliothis species (Wu et al, 1997), Anthonomus.grandis (Jones et al, 1978), Bemisia tabaci (Ozgur and Sekeroglu, 1986) and Earias species as compared to normal leaved cotton cultivars. Stemmed varieties of soybean have proved resistant to the bean fly, Ophiomyia centrosematris (Chiang and Norris 1984). Turnip varieties that have strong, round, long roots were more tolerant to the turnip maggots, Hylemya florales than the varieties with thin roots (Varies, 1958).

Sorghum cultivars with shorter glumes and shorter floral structures were resistant to earhead midge, Stenodiplosis sorghicola (Naik et al, 1995). Longer and narrow leaves coupled with hardness of leaf sheath proved resistant to A. soccata in sorghum (Jotwani, 1978). Liewise, narrow leaves and yellowish green colour of leaves were associated with A.soccata resistance in sorghum (Mote et al, 1956).

5.2. THICKENING OF CELL WALLS

Thickening of cell walls results from deposition of cellulose and lignin. As consequences, the tissue becomes tougher or more resistant to the tearing action of mandibles or to the penetration of the proboscis or ovipositor of insects. Feeding rates and larval growth of mustard beetle, Phaedon cochlariae was comparatively less on tough turnip and Brussels sprout leaves (Tanton, 1962). Thickness of the pod wall of cowpeas interfered the penetration by the Chalcodermis aenes. Thicker hypodermal layers of rice were considered a resistance factor to stripe stem borer, Chilo suppressalis (Patanakamjorn and Pathak, 1967). Resistance in sorghum to the sorghum shoot fly, Atherigona soccata was attributed to the presence of cells with distant lignification and thicker walls enclosing the vascular bundle sheaths within the central whorl of young leaves (Blum, 1968).

Seed damage due to alfalfa seed chalcid, Bruchophagus raddi was less in medicago species which had highly lignified pod-walls (Springer et al, 1990). Thickness of all categories of veins had positive correlation with egg laying by Amrasca biguttula in okra and cotton (Sharma and Singh, 2001).

Close association between resistance in sugarcane and its mid-rib lignification was observed; higher the lignification of mid-rib, greater would be resistance to the top borer, Scirpophaga nivella (Verma and Mathur, 1950). The relative toughness of the mid-rib in resistant sorghum cultivars interfered free Chilo partellus larval movement (Kishore, 1991). In brinjal, leaf thickness,and mid-rib thickness was positively correlated with jassid,

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Amrasca biguttula infestation (Gaikwad et al, 1991). In chickpea, resistance to Callosobruchus maculatus and C. Chinensis was associated with roughness and toughness of the seed coat (Singh, 1985).

5.3 STRUCTURE AND TEXTURE OF TISSUES

Toughness and thickness of plant stems altered the insect- plant interactions. Resistance to certain stem borers was related to the nature of the stem tissues. Solid stems were responsible for the resistance of several wheat varieties to the wheat stem sawfly, Cephus cinctus. A certain degree of stem solidness resulted in damaged and dessicated eggs and impaired larval movements (Wallace et al, 1973). Hardness of the rind and fiber content of stalks were key factors in sugarcane against Diatrea saccharalis (Martin et al, 1975). Hard woody stems of cucurbits with closely packed, tough vascular bundles were the main resistance factors against the squash vine borer, Melittia cucurbita. Thick cortex in the stem of wild tomato relative, Lycopersicum hirsutum prevented the potato aphid, Macrosiphum euphorbiae from reaching the vascular tissue (Quiras et al, 1977). Insect pest resistance in cotton was positively correlated with the genotypes possessing compact tissues (Timmaiah et al, 1994).

In medicago resistance to potato leafhopper, Empoasca fabae was due to avoidance of tough stems for oviposition (Brewer et al, 1986). Entry of the larvae of spotted bollworm, Earias vittella in hard skinned, tough varieties was difficult as compared to soft skinned, smooth surfaced varieties of okra (Teli and Dalaya, 1981), Sugarcane varieties with very strong hard mid-ribs in the leaves were found resistant to sugarcane top borer, Scripophaga nivella as compared to those with weak mid-ribs.

Seed hardness and seed coat thickness in chickpea were not associated with resistance to Callosobruchus maculatus. Brinjal cultivar (Aushey) was less susceptible to fruit and shoot borer, Leucinodes orbonalis due to toughness of the skin and hardness of the pulp of the fruits (Lall and Ahmad, 1965). Highly significant correlation between tomato fruit toughness and Spodoptera exigua mortality was observed (Juvick and Stevens, 1982). Leaf toughness was also greater in cotton cultivars resistant to Amrasca biguttula. Leaf-morphic characters (leaf hair density and leaf hair length) in cotton were found contributing resistance against whitefly, Bemisia tabaci adults, jassid, Amarasca devastans adults and nymphs of Thrips tabaci. Leaf hair densities on leaf midrib, vein, lamina and leaf hair length were positively correlated with the population of whitefly, jassids and thrips (Aslam et al, 2004), while hair density on midrib and lamina and gossypol glands on midrib showed significantly negative correlation with jassid population (Mansoor ul Hassan et al, 1999, Murtaza et al, 1999), also the correlation between CLCV incidence and thrips population was significantly positive (Aheer et al, 1999). Hair density on midrib, vein, and lamina was significantly positive for thrips infestation (Ali et al, 1995). Likewise, in soybean leaf area and moisture content showed positive correlation with whitefly infestation and leaf hair density on abaxial surface of leaf had significantly negative correlation (Ihsan ul Haq et al, 2003).

Toughness of leaf veins (Afzal and Ghani, 1948), thickness of lamina (Tidke and Sane, 1962), and plisade cells (Batra and Gupta, 1970) of cotton affected its resistance to the cotton jassid, Amrasca devastans. Thick-hulled varieties of southern peas had fewer ovipositional punctures, eggs and larvae of the cowpea cuculio, Chalcodermus

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aeneus, than thin-hulled varieties (Chalfant et al, 1972). Hardness of various plant tissues has been reported to be correlated with the resistance to insect pests. Toughness and thickness of various plant parts adversely affected penetration and feeding by insects. In cowpea, tough pod wall, required force to penetrate at pod maturity, is an important factor contributing to the less feeding damage.

Varietal resistance to the squash vine borer, Melittia cucurbitae, in cucurbits (Howe, 1949) to the shoot and fruit borer, Leucinodes orbanalis, in egg plant (Panda and Khush, 1995) and to the stem weevil, Apion virens, in red clover (Kokorin, 1973) has been attributed to the structure and compactness of vascular bundles in the plants. The solid stemmed nature of certain wheat varieties was the major cause of their resistance to the wheat stem fly, Cephus cinctus (McNeal et al, 1971; Wallace et al, 1973), the eggs of which were mechanically damaged and desiccated, and the movement of the hatching larvae was limited. In sorghum, seed coat and glume toughness influenced the feeding of sorghum earhead bug, Calcoris angustatus (Ramesh, 1992).

5.4. INCRUSTATION OF MINERALS IN CUTICLE

The high silica content of some plants and crystalline material in the leaves may confer resistance by acting as feeding barrier (Pathak, 1969). Deposits of silica and calcium carbonate were found in a number of plant species (Martin and Juniper, 1970). Calcified and silicified hairs exist on many plants (Uphof, 1962). Silica contents absorbed by the plants get deposited in the tissues of several plant species and in certain species contribute to resistance against insect attack (Lanning et al, 1980). In Sorghum, cultivars with high silica content in 4-6 leaf stage had low incidence of shoot fly, Atherigona soccata (Bothe and Pokarhar, 1985). In rice, a highly significant negative correlation was recorded between silica content of the stem and the susceptibility to the stripe stem borer, Chilo suppressalis (Hanifa et al, 1974) and Chilo zacconius (Ukwangwa and Odebiyi, 1985). Deficiency of silica induced susceptibility in rice plants to yellow stem borer, Scirpophaga incertulas (Mandras, 1991).

Young cuticle is usually preferred over mature cuticle by certain species of insect pests for feeding and oviposition. Other insect species may prefer mature cuticle. Soybean looper, Pseudoplusia includens preferred the two fully expanded leaves nearest to the apex over the third or lower leaves (Reynolds and Smith, 1985). Leaf area and moisture contents showed positive correlation with soybean looper infestation and leaf hair density on abaxial surface of leaf had significantly negative correlation (Ihsan ul Haq et al, 2003).

Bayberry whitefly, Parabemisia myricae could not feed on mature lemon leaves due to inhibitory properties of mature cuticle (Walker, 1988). Mature leaves of citrus were repellent to probing by aphids (Zettler et al, 1969). Younger leaves of lemon were preferred for probing, oviposition and survival of P. myricae over mature leaves (Walker and Aitken, 1985).Larvae of H.zea preferred to feed on older leaves of soybean plants over younger ones (Nault et al, 1992).

The narrow leaflet isolines tolerated defoliation better than the wide leaflet soybean isolines. In sorghum, the varieties with greater height, greater distance between two leaves, and smaller leaf angle were less susceptible to the aphid (Mote and

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Shahane,1994). The thrips population was related to the leaf insertion angle in onion; the greater the insertion angle, the lower the thrips population densities on the plant (De deverira et al., 1995). Longer and tougher leaf blades of rice plants contributed to resistance against leaf folder Cnaphalocrocis medinalis . Width, length and toughness of rice leaf blade may play a vital role in resistance to leaffolder (Islam and Karim, 1997).

Many plant species have deposits of silica on the epidermal walls, which may play a role in resistance mechanism. It has been reported that mandibles of C. suppressalis worn out in high silica lines of rice (Pathak et al., 1971). Larval survival of C. zacconius is affected on rice varieties with high level of silica (Ukwungwa et al., 1990). Low incidence of A. varia soccata was observed on high silica varieties of Sorghum (Bothe et al., 1985). Low incidence of A. devastans was recorded on cotton varieties with high silica content (Singh 1970). Incidence of Dacus cucurbitae was comparatively low on musk melon varieties with high level of silica than on those varieties with low level of silica (Chelliah and Sambandorn 1974). Low incidence of Amrasca biguttula was observed on high silica varieties of okra (Singh, 1989).

5.5. SURFACE WAXES

All substances of a waxy nature isolated from a plant are considered under the term wax. Chemically a wax refers to an ester formed of a long-chain fatty acid and a high molecular weight aliphatic alcohol. Plant waxes vary from a fraction of a per cent to several per cent of the dry weight of a plant. The cuticles of most vascular plants are covered with a thin layer of largely hydrophobic constituents. Cuticular waxes generally are complex mixtures including mainly n-alkanes (C7 to C62) as well as alcohols and acids. The waxy coating of leaves, function primarily in the mechanisms of water balance of the plant, but also contains substances that interfere with insect attacks (Norris and Kogan, 1980).

Surface waxes over the epicuticle, protect the plant surface against desiccation, insect feeding and diseases. Epicuticular waxes affect the feeding behaviour of insects by acting as phagostimulants or feeding deterrents. Due to the presence of waxes on plant surface the sense organs on the insect tarsi and mouth parts receive negative chemical and tactile stimuli from the plant surface resulting in resistance of the plant to insect attack (Ram et al, 2005).

Surface waxes do not impart resistance against all insect species. For example, the cabbage aphid, Brevicoryne brassicae and the whitefly, Aleurodes brassicae, developed large colonies on normal waxy plants of Brassica oleracea var acephala, but did not colonize non waxy plants. It has been observed that waxes affected the rate of initial infestation, but once established, aphid displayed no significant difference in reproductive rates (Thompson, 1963).

Leaf epicuticular wax is an important antixenotic factor that affects the rate and pattern of feeding of flea beetle, Phyllotreta cruciferae in Brassicaceae. Diamond backmoth, Plutella xylostella larvae had non-preference for leaf wax in glossy-leaved resistant Brassica oleracea (Eigenbrode and Shelton, 1990). Glossy lines of Brassica species had low population of cabbageworm, Artogeia rapae, cabbage aphid, B. brassicae and P. xylostella (Stoner, 1990). In bloom cultivars, the culm is heavily waxed

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and the neonate larvae face considerable difficulty in climbing because their prolegs stuck in the wax and never reach the feeding site (Bernays et al, 1983). Increased activity of the brown planthopper, Nilaparvata lugens on rice variety IR 64 over that observed on varieties IR 22 and IR 62 was shown to be due to the chemical composition of the surface wax (Woodhead and Padgham, 1988).

Surface waxes may enhance the level of resistance against certain pest species or may be useful for the insect pests and harmful for plants. Broccoli with waxy leaves was more resistant to Phylotrata olbionica than broccoli with a glossy leaves (Anstey and Moore, 1954). Sugar cane stalk surface wax contributed to resistance against sugarcane borer, Eldana saccharina (Rutherford and Staden, 1996). B. brassicae and A. brassicae did not colonize on nonwaxy plants (Thompson, 1963).

5.6. ANATOMICAL ADOPTION OF ORGANS

Slight variations in the morphological structure of plants may result in altered fitness to herbivores. They may alter the effectiveness of other factors causing mortality.of insects A positive correlation was observed between number of outer leaves and aphid, B. brassicae intensity in 14 varieties of cabbage due to unwrapping of infected leaves during head development (Verma et al, 1999). The tightness of leaf sheath enveloping the stem and forming acute angle in combination with presence of lingular hairs, and erect hair prevented the establishment of C. partellus larvae in sorghum (Kishore, 1991).

Rice varieties with tight leaf sheath wrapping were less susceptible to striped stem borer, C. suppressalis (Patanakamjorn and Pathank, 1970). Spring wheat genotypes with tight leaf sheath, dense hairs at the basal part of leaves and with thick waxy leaf layer were resistant to Hydrellia griseola (Zhu, 1981). Awns in wheat impart resistance to grain aphid, Sitobion avenae (Acreman and Dixon, 1986). In corn, silk balling is associated with resistance to corn earworm, H. armigera by presenting physical barrier to ear penetration (Osuna et al, 1983). In sugarcane, plants with loose-fitting leaf sheaths were more damaged by the internode borer, Chilo saccahariphagus indicus than those with tight leaf sheaths (Agarwal, 1969) and the reverse was observed in case of mealy bugs (Mehla et al, 1981).

Number of gossypol glands in ovary surface of cotton imparted resistance against ballworm, H. armigera (Rajarajeswari and Subbarao, 1997). The presence of gossypol glands on the upper edge of the sepals of Gossypium hirsutum flower bud conferred resistance to H. zea and H. virescens (Calhoun, 1997).

Gossypium arboreum genotypes with high gossypol gland density on ovary surface showed low incidence of H. armigera, Earias viettella and Pectinophora gossypiella (Mohan et al, 1994).

Various anatomical characters of plants have certain affects on host–insect interactions. Sugarcane varieties with intact leaf sheath were comparatively more resistant to sugarcane borer, Diatraea saccharalis (Mathes and Carpentirs, 1963) and internode borer, Chilo sacchariphagus indicus (David, 1979). Sugarcane varieties with loose fitting leaf sheath were resistant to mealybug, Saccharicocus sacchari (sithanantham, 1983) and scale insects (Agarwal, 1969).

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Absence of nectaries in cotton reduced oviposition of pink bollworm, P. gossypiella, American bollworm, H. zea and H. virescens (Davis et al.,1973), H. armigera and Earias spp (Agarwal et al.,1976), confered resistance to tarnished bug,(Bailey et al.,1984), but induced susceptibility to jassid, A. devastans and whitefly, B. tabaci (Baloch et al.,1982). Frego bract cotton has been reported to be resistant against H. zea (Lincoln and Waddle, 1969), Anthonomus grandis (Jenkins and Parrott, 1971) but susceptible to Lygus hespenus (Leigh et al., 1972). The effect of gossypol glands on vein and thickness of leaf lamina were highly significant and positive for jassid, Amrasca devastans infestation (Ali et al, 1995). Hair density and leaf lamina were found to contribute towards resistance against aphid, mite and bollworms (Javed et al, 1998).

Wheat with solid stem desiccated eggs and impaired larval movement of wheat stem sawfly, Cephus cinctus (Wallace, et al., 1973). Sugarcane varieties with hard rind and fibre content of stalks were resistant to D. saccharalis. Likewise denticles on the leaf midribs, lignification of cell walls etc. conferred resistance to the pest (Martin et al.,1975). Feeding by Chilo sacchariphagus and Chilo infuscatellus was less on sugarcane varieties with hard stem and rind hardness (Rao et al., 1956). Lignified vascular bundles of alfalfa reduced oviposition of Empoasca fabae (Brewer, et al., 1986). Cotton with stiffness of stem tips was resistant to aphids (Kadapa et al., 1998)

Rice varieties with thicker sclerenchymatous layer than parenchymas were resistant to stem borer (Chaudhary et al.,1984. Rice varieties having broader and thicker sclerenchymatous hypodermics, compact parenchyma ground tissue, small air spaces in the ground tissue, more vascular bundles with narrower spaces between vascular ridged stem surface containing vascular bundles and narrower pith are considered to be characters for resistance. Thinner sclerenchymatous hypodermics, loose parenchyma cells of ground tissue, larger spaces between vascular, wider pith and larger air cavities, might be responsible for the susceptibility to yellow stem borer, S. incertulas. (Shahjahan, 2004).

The resistance in maize cultivars to the shootfly, A. soccata maggots was affected significantly by the changes in plant height, thickness of leaf midrib, length of leaf sheath wrapping, tenacity of the leaf sheath wrapping and its nature, diameter of stem, thickness of leaf sheath and the intensity of the leaf colour. The morphological character under reference had a highly significant impact as resistance which mainly came through the thickness of leaf sheath as well as through the angle of leaf divergence from the stem and the diameter of stem at ground level. These three characters accounted collectively for 100% of the changes in resistance and that 99.92% of them came alone through the first one of them. Every unit decrease in this character will increase resistance of the maize cultivars against the A. soccata maggots (Kasana and Wahla, 1996).

Khaliq et al. (2003) studied the effect of various morphophysical plant factors (leaf sheath hairiness, leaf area, stalk girth, cane height and moisture contents in leaves) on pest infestations in sugarcane (cvs. SPSG-26, BF-162, CP-72/2086, CP-43/33 and L-118). None of the variety was found resistant to infestation at tillering. CP-43/33 was found comparatively more susceptible (8.55% infestation) and BF-162 more resistant (4.69% infestation). All morphophysical plant factors differed significantly among varieties. Plant height, cane girth, leaf area and leaf sheath hairiness showed negative

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and significant correlation, whereas thickness of leaf lamina and moisture contents showed positive and significant correlation with pest infestation at tillering stage.

5.7. TRICHOMES

Trichomes are outgrowths from the epidermis of leaves, shoots and roots (Uphof, 1962). The collective trichome cover of a plant surface is called pubescence. The epidermis of plants bears hair like outgrowths called trichomes or hairs. Trichomes occur in several forms, shapes and sizes (Jeffree, 1996). These serve many critical physiological and ecological functions in plants. In general, the purely mechanical effects of trichomes depend on their main characteristics such as density, erectness, length and shape. The trichomes are either glandular or non-glandular. Glandular trichomes secrete chemicals which are toxic to the insects. The exudates inhibit movement of insects on plant surfaces and act as repellents and deterrents, disrupt feeding, affect development, reproduction and survival of insects. The effect of glandular trichomes may depend on the nature of exudates. It may be composed of allochemicals (Johnson, 1975) or sticky in nature. Exudates with allelochemicals may kill insects on contact or act as repellents. Sticky exudates glue the insect legs and impede locomotion. The non-glandular trichomes affect locomotion, attachment, shelter, feeding and survival of insects. Trichomes are one of the most important morphological adaptations of plants against insect pests. The effects of trichomes on host insect interactions are briefly described.

5.7.1. LOCOMOTION, ATTACHMENT AND RELATED BEHAVIOUR

First instar larva of cereal leaf beetle, Olema melanopa were critically affected by the pubescence of wheat plants (Schillinger and Gallun, 1968). Hairiness on the plant surface affected the insect behaviour by acting as a barrier. Bimesia tabaci are trapped by the glandular hairs of tomato leaves (Kisha, 1984). Hair density on midrib and leaf lamina of cotton were found to contribute towards resistance against these pests. The situation demands more attention to varietal resistance against pests and its effective application in integrated control programmeme.

Certain varieties of bean plants possess hooked trichomes which impaled leafhoppers and aphids etc. (Johnson, 1953). Four lobed glandular hairs on leaves and stems of S. polyadenium discharged a sticky substance on contact with larvae of Colourado potato beetle, Leptinotarsa decemlineata. This sticky substance accumulated on the tarsi, immobilized some larvae and caused others to fall off the plants (Gibson, 1976).

Twenty two cotton genotypes (BH-121, BI-1-125, BH-147, CIM-473, CIM-482, CIM-499, CIM-511GE, CIM-707, CRIS-467, CRIS-468, DNH-157, FNH-945, FNH-1000, MNH-633, MNH-635, MNH-636, NIBGE-1, RH-510, SLH-244, SLH-257, VH-141 and VH-142) were evaluated under unsprayed conditions for their comparative resistance or relative susceptibility to whitefly, Bemisia tabaci adults; jassid, Amarasca devastance adults and nymphs and Thrips tabaci adults and nymphs at Multan, Pakistan. The relationship between leaf-morphic characters (leaf hair density and leaf hair length) and population of sucking insect pests was also investigated. The maximum mean seasonal

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population 1.3 whitefly adults was on CIM-473, 1.7 jassid adults and nymphs on BH-147 and 3.1 thrips adults and nymphs per leaf was observed on genotype FNH-945. The minimum mean seasonal per leaf population of whitely was 0.5 on genotype BH-121 and CRIS-467, jassid 0.6 on MNH-635 and thirps 0.8 on CIM-499. Leaf hair density and length were important morphic characters contributing some resistance against sucking insect pests. Leaf hair densities on leaf midrib, vein, lamina and leaf hair length were positively, negatively and positively, correlated with the population of whitely, jassid and thrips, respectively (Aslam et al, 2004)

Cotton variety CRIS-7A, due to resistance against jassid required one or two less insecticide applications for control. However, maximum population of whitefly was observed on this variety indicating the positive correlation of hairiness and whitefly attack. CRIS-7A recorded minimum bollworm damage per cent as compared to commercial checks NIAB-78 and CRIS-9. CRIS-7A also proved as boll rot disease resistant and seedling rot disease tolerant variety when compared with other advance strains and standards. This variety has proved itself better among the high yielding varieties also (Soomro et al, 2000).

5.7.2. INSECT FEEDING AND DIGESTION

It was observed that early first instar larvae of cereal leaf beetle, Oulema melanopa were critically affected by the pubescence of wheat plants. High mortality of its larvae was explained by the fact that larvae had to eat the hairs to reach the epidermis. In doing so they ingested usually large amounts of cellulose and lignin, the basic constituents of hairs. Death of the young larvae was attributed to unbalanced diet overly rich in fibrous material. Larval weight was negatively correlated with increase in pubescence (Schillinger and Gallun, 1968). In addition to the dietary imbalance the larvae that fed on pubescent wheat leaves were suffered with undigested hairs, some of which pierced the gut wall of the insect (Wellso, 1973). Pubescence reduced the quality of the food ingested and caused greater larval mortality (Kogan, 1972). It has been observed that hairiness on the plant surface affected feeding, development and survival of insect pests by acting as barrier to normal feeding. Hariness of plants interfers with feeding of certain insect species (Webster, 1975). Beach straw berry clone CL5 was resistant to feeding by adult black rice weevil, Otiorhynchus sulcatus partly due to dense covering of hairs on their abaxial surface (Doss et al., 1987).

Density of hairs on cotton affected the feeding behaviour of cotton aphids, Aphis gossypii by reducing the feeding time and increasing the first non feeding time and penetration frequency (Jiang and Guo, 1996). Weight gain and leaf feeding capacity of Spodeptera littoralis and H. armigera was smaller on hairy cultivars than on the glabrous cultivars (Navon et al., 1991). Wheat varieties with dense and long pubescence (Hope and CI 8519) were highly resistant to larval feeding (Hoxie et al., 1975).

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5.7.3. INSECT DEVELOPMENT AND POPULATION

Cotton variety (CIM 70) which had low density of hairs and shorter hair length had a larger population of aphids (Naveed et al., 1995). High hair density and more hair length on lower leaf surface of okra genotypes affected the development of jassid (Hooda et al., 1997).

5.7.4. INSECT MORTALITY

Pubescence caused greater larval mortality of cereal leaf beetle, Oulema melanopa (Kogan, 1972). Hooked hairs on Phaseolus valgaris traped and killed aphid, Aphis carccivora. Glandular hairs on tomato traped aphids (Johnson, 1953). Specialized hairs on certain Phseolus bean cultivars captured or impaled both nymphs and adults of potato leafhopper, E. fabae (Pillemer and Tingey, 1978). Hooked trichomes made numerous puncture wounds in the larval integuments, immobilizing the larvae, leading to their death by starvation (Rathake and Poole, 1975).

First instar nymph of aphid, Aulacorthum solani was trapped physically by trichomes on kidney bean leaves. The traped aphids died due to starvation. Mortality of early instar aphids was higher on cultivars with trichomes that had heavily curved tips (Mizukoshi and Kakizaki, 1995).

5.7.5. OVIPOSITION

Hariness of plants interferes with oviposition of certain insect species (Webster 1975). Pubescence in wheat greatly reduced cereal leaf beetle Olema melanopus oviposition (Lampert et al., 1983). Wheat varieties with dense and long pubescence (Hope and CI 8519) were highly resistant to oviposition (Hoxie et al., 1975). In wheat, Hessian fly, Mayetiola destructor laid fewer eggs on the pubescent leaved (Vel) wheat than on the glabrous leaved (Arthur) (Roberts et al., 1979). Pubescence on sugarcane plants (NG 77-147) adversely affected the oviposition of sugarcane borer, D. saccharalis (Sosa, 1988). The number of trichomes on leaves of maize cultivar (IC2-T) deterred oviposition of corn earworm, C. partellus (Kumar, 1992). Hair density and hair length had negative correlation with the number of eggs laid by leafhopper, A. biguttula on malvaceous plants (Sharma and Singh, 2001).

In some cases, trichomes may enhance oviposition by some insect species. Glabrous cotton strains were less favourable for egg laying than pubescent strains for oviposition by H. zea and H. virescens (Stadelbacher and Scales, 1973). Number of eggs was strongly correlated with hair density. Positive correlation have been observed between oviposition and pubescence. For example H. virescens preferred to lay eggs on the abaxial leaf surfaces where greater densities of trichomes occur in different plant species (Navasero and Ramaswamy, 1991). Trichome length on the upper surface of the leaves of cotton showed positive correlation with oviposition of H. armigera (Butter and Singh, 1996). Oviposition on pubescent cultivars of cotton was high by B. tabaci (Navon et al., 1991) and E. vittella (Sellammal, 1982). In brinjal, whitefly, B. tabaci preferred to oviposit on hairy varieties (Balaji and veeravel, 1994).

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Likewise, higher oviposition by B. argentifolii occurred on leaves of tomato with dense trichomes (Heinz and Zalom, 1995).

Hairy varieties received more eggs of H. armigera than the glabrous varieties (Sivaprakasam, 1996 a and b). Whitefly, B. argentifolii laid more eggs on hairy soybean than on glabrous soybean (McAuslane, 1996).

Leaf trichomes interferred with the oviposition of A. devastans on okra (Teli and Dalaya, 1981). Effective hair length on the ventral surface of mid veins of cotton showed a significant negative correlation with the number of eggs laid by A.devastans (Khan and Agarwal, 1984). Leaf hairiness in cotton positively correlated with the number of eggs laid by E. vittella (Sharma and Agarwal , 1983). Lygus hesperus laid 31% less eggs on smooth leaves than on pilose leaves (Benedict et al., 1983). In sorghum, the presence of trichomes on leaf surface was related to less frequency of oviposition by shoot fly, A. soccata (Maiti et al., 1980). Pubescence in wheat greatly reduced oviposition by the cereal leaf beetle, Oulema melanopus (Lampert et al.,1983). Alfalfa varieties with long glandular hairs were resistant to seed chalcid, Bruchophagus roddi (Pierce, 1983). The apple codling moth preferred the glabrous leaf for oviposition over the pubescent leaf surface (Plourde et al., 1985).

5.7.6. INSECT INFESTATION OR PLANT DAMAGE

Infestation by codling moth was comparatively less on apple cultivars with pubescent on lower leaf surface (Plourde et al., 1985). The larvae of Mexican bean beetle, Epilachna verivestis caused 3-13 times more damage on the glabrous isolines than on the normal and dense isolines (Gannon and Back, 1996). In cowpea, significant negative correlation was obtained between total trichome density on pods, pod infestation and damage severity by legume pod borer, Maruca testulalis (Oghiake et al., 1992). In lima bean trichome density on underside of leaves was highly and negatively correlated with the damage (Lyman and Cardona, 1982). Ashgourd with highest trichome density was least infested with Aphis gossypii (Khan et al., 2000). Hair length and hair density on brinjal leaf were negatively correlated with jassid, A. biguttula infestation (Gaikwad et al., 1991). Number of gossypol glands on leaf and ovary surface of cotton had significantly negative relation with H. armigera incidence (Rajarajeswari and Subbarao, 1997).

5.7.7. PUBESCENCE ASSOCIATED WITH ALLELOCHEMICAL FACTORS

The action of glandular hairs demonstrate the close interaction of physical and biochemical plant defences. In addition to entrapping, the glandular trichome exudes certain chemicals which are toxic to the insects. Glandular hairs serve as cues in host plant finding for those pest species that have evolved the mechanisms to utilize such resources. These also act as insect repellents due to large variety of non-volatile and volatile chemicals.

Trichomes on Nicotiana species exude materials which produce toxic effects on aphids such as leg paralysis, loss of equilibrium and death.Trichome exudates from the leaves of Nicotiana and Petunia species were toxic to the

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larvae of the tobacco hornworm, manduca sexta. Wild Solanum spp produced high mortality of adults and larvae of Colourado potato beetle, L. decemlineata (Pelletier et al, 1999). The glandular exudates from wild hirsutum, contained large amounts of methyl ketones which were toxic to E. vervestis, M. sexta, A. gossypii, M. persicae, H. zea etc. (Farrar and Kennedy, 1987). The phenolic compounds in grandular trichomes of the tomato plant were found to inhibit the larval growth in H. zea (Duffy and Isman, 1981). Oxalic acid levels in trichome exudates on chickpea leaf and pod surface reduced the infestation of H. armigera (Yoshida et al., 1997). Leaf trichomes of yellow swurash significantly reduced the oviposition of pickleworm moth, Diaphania nitidalis (Peterson et al., 1994). The principal mechanism of resistance of wild tomato, Lycopersium esulentum to leaf minor, Liriomyza trifollii was the secretion of allomone (acylglucoses) from glandular trichomes (Hawthorne et al., 1992).

Density, erectness, length and shape of trichomes sometimes play an important role in host-insect interactions. The glandular trichomes exude different types of allelochemicals. Such allelochemicals may kill insect on contact or act as repellent. Some substances may interfere with locomotion of Insects or prove toxic to the insects (Webster, 1975).

Glandular trichomes exude certains chemicals that induce resistance in plants to certain species of insect pests. For example, resistance of wild tomato to Helicoverpa zea (Dimoch and Kennedy, 1983) and Leptinotarsa decemlineata is attributed to high levels of 2-tridecanone present on the tips of trichomes. Trichomes in different plant species contain number of chemicals which protect the plants against insect pests such as presence of 2-tridecanone and 2-undecanone in Lycopersicon hirsutum. These chemicals are toxic to Spodoptera enigua (Lin et al., 1987). The action of glandular hairs demonstrates the close interaction of physical and biochemical plant defences. Glandular hairs are also serving as cues in host plant finding for few insect species (Sadasivam and Thayumanavam, 2003).

5.8. TRICHOMES AND RESISTANCE

Resistance in Brassica juncea to mustard aphid, Lipaphis erysimi was correlated with hairiness of leaf surface (Lal et al., 1999). In pigeonpea, resistance of pods to H. armigera larvae was due to high density of trichomes (Romies et al., 1999). Tolerance in plants to fleahopper, Pseudatomoscelis seriatus increased with increase in trichome density. Trichome density on sorghum leaves imparted resistance to shoot fly, A. soccata. Susceptibility of okra genotypes to Earias spp was positively correlated with the hair density on fruits (Kumbhar et al., 1991). Many plants possessing hairs and spines on their epidermis are resistant to insect pests. For example in cotton long hairs on the leaves confered resistance to leafhopper. Hairiness in cotton increased resistance to a number of insects such as jassid, Amrasca spp. (Evans, 1965), cotton aphid, Aphis gossypii (Kamel and Elkassaby, 1965); cotton leafworm, Spodoptera littoralis and spider mites, Tetranychus spp. (Abdul Nasr, 1960); bollweevil, Anthonomus grandis (Stephens and Lee, 1961) and pink bollworm, P. gossypiella (Smith et al., 1975).

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Hairiness of wheat leaves provided protection against the cereal leaf beetle, Oulema melanopus (Hoxie et al., 1975). Orientation of hairs on plant surface is considered important for resistance to the potato leafhopper in soybeans (Broersma et al., 1972). The presence of gossypol glands on the upper edge of the sepals of G. hirsutum flower buds confered significantly higher level of resistance to H. zea and H. virescens.

Trichomes can trap, immobilize or impale many insect pests. For example, hooked trichomes present on certain varieties of French bean impaled the aphids, Aphis craccivora (Lampe, 1982). Trichomes on seedling stem offered resistance to the spotted alfalfa aphids, Therioaphis maculate through non-preference (Manglitz and Kehr, 1984).

Resistant varieties of okra had a greater number of trichomes on the mid rib and lamina versus fewer and shorter trichomes on susceptible varieties. Laminar trichomes without adequate length or long trichomes without adequate density were not effective in providing resistance in okra (Uthamasamy, 1985). Pubescence interfered with feeding of B. tabeci, A. varia soccata and Spodoptera littoralis (Webster, 1975). The youngest leaves of soybean resistant cultivar, which were closely covered with trichomes were the most resistant to larval feeding of Trichoplusia ni. When these trichomes were shaved off, such leaves became as susceptible to attack as sparsely trichomed covered leaves (Khan et al., 1986). Trichomes density on green gram pod surface and thickness of pod wall had a positive correlation with nymphal period and negative correlation with a body weight of nymphs and adults of Riptortus linearis (Baruah and Dutta, 1994).

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CHAPTER-6 BIOCHEMICAL BASIS OF RESISTANCE IN PLANTS TO INSECTS

Plants and insects have intricate interactions with each other based on plant characteristics and biological parameters of insect pests. Host plant resistance mainly depends on these interactions. Brues (1920) stated that host selection is through botanical instinct. Fraenkel (1953) postulated that host selection was governed by secondary plant biochemicals. Nutritional factors such as sugars, amino acids, sterols, etc. have been shown to stimulate feeding of some insect species (Auclair, 1959).

Plants remain surrounded in an external environment of volatile compounds which emanate from the outer layers of different parts of plants. These compounds serve as olfactory stimuli for insects. The internal environment of plants consists of a large variety of chemicals, some of which are of nutritional value to insects and may also act as feeding stimulants. There may also be other compounds which deter or inhibit feeding or may cause physiological disturbances affecting development, survival and reproduction of insect pests. The chemicals present in the external and internal environments of plants are perceived by the insect’s sensory organs and are decoded by the central nervous system into an expression of “suitable host”, “unsuitable host” or “non-host”. Chemicals based resistance is a major factor in plants for defence against insect pests and other herbivores. Susceptibility or resistance of plants is the result of a series of interactions between plants and insects which influence the ultimate degree of establishment of insect populations of plants. Bio-chemical factors of plants are far more important in imparting resistance to insects as these chemicals affect behaviour and physiology of insects in a number of ways. These factors may be nutritionally based or may include allelochemicals that affect insect behaviour, growth, health or physiology. Some of these allelochemicals have been found to be associated with repellence, feeding deterrence, toxicity or other effects on insects (Whittaker and Feeny, 1971).

Establishment of insects on plants involves orientation, feeding, metabolic utilization of ingested food, growth, survival, adult longevity and fecundity. The capacity of a plant to cause the interruption of any of these biological parameters of insect pests may account for its resistance against them. Such interruptions may be caused by the biochemical or biophysical characteristics of a plant. Insect resistance in plants is often due to their chemical constituents rather than physical or nutritional factors.

Semiochemicals are involved in behaviorual or physiological interactions between organisms. Intraspecific semiochemicals are termed as pheromones. While, Interspecific ones are called allelochemicals, which include kairomones, allomones and synomones. Kairomones are the chemicals that benefit the receiver, allomones benefit the emitter and a synomone is a substance produced or acquired by an organism such that when it act on another species, it envokes a behavioural or physiological reaction

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from the receiver that is adaptively favourable to both emitter and receiver. Allelochemicals usually have a deterring role, but some are phagostimulatory.

Various interactions between allelochemicals and nutrients may affect the suitability of insect food. Thus, not only presence of nutrients but also their “bioavailability” may be significant (Hagen et al., 1984). Allelochemicals also influence the availability of N to herbivores by forming nonutilizable chemical complexes with proteins and through effects on the herbivore nutritional physiology (Beck and Rees, 1976).

Many allelochemicals reduce insect growth. The allelochemicals can be considered ecological and chemical requirements of plants since these serve to tie the insects to their hosts and protect plants from other insects, pathogens and general herbivores that have not broken the chemical defences of these plants. An allelochemic is dominant in the insect/plant interactions. The allelochemics are principally shared between two groups of interspecifically active chemicals known as allomones and kairomones. In general allelochemicals occur in plants in diverse ways to protect them from their enemies (Whittakar, 1970).

The evolution of successful plant life by necessity carried with it a chemistry of defence. The early history of botany is largely a history of man’s adaptations of phytochemical defences for uses in human medicine. Thus we have known of chemical defences in plants against pests since antiquity, but our use of such knowledge to breed plants more resistant to insects developed quite recently. The early emphasis was on whether resistance was inherited, genetically manageable, and reasonably stable in practical uses. Afterwards knowledge of the chemistry of plants mushroomed. Several key mechanisms of chemically based resistance to insects were elucidated by the mid 1960s and provided stimuli for further research. A molecular code controlling the inherited nature of living systems has given new potentialities for using genetic techniques to “construct” plants possessing enhanced chemical defences. Interspecific hybridization of nucleic acids is a method of this type currently being developed (Norris and Kogan, 1979).

The qualitative and quantitative differences in allomones in various parts of a plant may vary as significantly as among plant species. Quantity of allomones may also vary with the age of same plant part. In natural ecosystems, phytophagous insects coexist in a complex relationship with plant communities. The evolution of defensive mechanisms in plants in response to selection pressure by insect pests has been an important factor in the coexistence of green plants and phytophagous insects. A wide variety of traits expressed by plants are associated the avoidance, resistance or tolerance of herbivory. These effects are the outcome of the absence of some nutritional factors, the presence of toxic metabolites or antienzymes. The presence of allomones or the absence of kairomones and the presence of synomones are important in host-insect interactions.

It is well established fact that the investigations on the biochemical basis of planta is very helpful to develop resistant varieties against insect pests. The defence chemicals present in plants have been classified in to different categories: non-protein aminoacids, lections, cynogenic glycosides, glucosinolates, alkaloids, phenolics,

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terpenoids/isoprenoids, epicuticular lipids etc. Lot of information is available on these defence chemicals. These biochemical basis of resistance in plants against insect pests are briefly described.

6.1. NON-PROTEIN AMINO ACIDS

The amino acids that are not found in proteins are known as non-protein amino acids or unusual amino acids. Many non-protein amino acids interfere with growth of vertebrates. However, bioactivities against insects are known for a few non-protein amino acids such as canavanine, cyanoalanine, canaline, diaminobutyric acid, propionic acid, ß-Pyrozole-1-yl-L-alanine, mimosine, albizziine and 3,4-dopa.

Non-protein amino acids are found in a number of plant species especially legumes. The protective effect of non-protein amino acids is elicited through their structural analogy to the commonly occurring amino acids. The harmful effects of non-protein amino acids on insects are partly due to the fact that the analogue molecule gets incorporated into protein synthesis of the insects. The plants having these chemicals, avoid auto-toxicity by avoiding their incorporation into proteins (Rosenthal, 1991). Some non-protein amino acids also act as enzyme inhibitors.

Canavanine protects seed and young foliage of plants from insect pests and possess significant insecticidal properties. Small quantity of canavanine (88 ppm) in the diet of the fifth instar tobacco hornworm, Manduca sexta significantly reduced the overall weight of the resulting moth (Rosenthal and Dahlman, 1975). It reduced both the fecundity and fertility of M. sexta (Palumbo and Dahlman, 1978). The growth rate of M. sexta larvae on tobacco plants treated with canavanine was comparatively slower than on control plants. A slower growth rate exposed the insects to environmental stress and natural enemies for longer time.

β-cyano-L-alanine (BCNA) in Vicia sativa was toxic to locust at physiological levels in the diet. Administration of BCNA to adult locusts resulted in a significant decrease in hemolymph volume within a day and caused death of the insect within five days. The death of locust was caused due to impaired ability to reabsorb water into the hemolymph from the rectal lumen. Chronic BCNA toxicity irreversibly disrupted the control of water balance in the insect (Schlesinger et al., 1976). Larvae of yellow mealworm, Tenebrio molitor treated with 2, 4-diaminobutyric acid and β-cyanoalanine perished (Schlesinger and Applebaum, 1977). Canaline is a potent neurotoxin to the adult moth of M. sexta. It elicited severe larval developmental aberrations, prevented successful larval – pupal ecdysis and caused deformities in the pupae of insect (Kammer et al., 1978).

At 0.005-0.55, OHS (O-oxalythomoserine) and Y-ODAB showed antifeedant activity whereas, HS and β-ODAP showed phagostimulant activity against certain species of insect pests. (Bell et al .,1996).

Pyrazolin cucumis had high deleterious effect on Acyrthosiphon pisum (Chen et al.,1999). Mimosine, β-cyanoalanine, and azetidine-2-carboxylic acid in the diet produced high mortality of the larvae of seed eating beetle, Callosobruchus maculatus at 0.1% level. Cananine proved lethal to the larvae of. C maculatus at a concentration of 0.5 % and was markedly toxic at 1% (Janzen et al.,1977).

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6.2. LECTINS

Lectins or phytohaemagglutinins are proteins of non-immunoglobulin nature capable of specific recognition and reversible binding to the carbohydrate moieties of complex carbohydrates without altering the covalent structure of any of the recognized glycosyl legends. The term lectin was first introduced by Boyd and Shapleigh (1954). Lectins are found in a wide range of plant species mainly in the family leguminosae. Leguminaceaous lectins are abundant in seeds. In red kidney bean and soybean, low levels of lectin were detected in the roots, stems and leaves of young seedlings (Lis and Sharon, 1981). Lectins are also found in the Graminaceous and Solanaceous plants. In cereal grains the lectins are confined largely to the embryo and absent in the endosperm. The cotyledons of seeds of legumes are mostly rich in lectins. To some extent lectins are also found in the vegetative parts of plants. However, lectins in vegetative parts are different from the seed lectins.

Lectins are synthesized at high molecular weight precursors. Lectins serve as plant defence compounds. When eaten raw, the common bean is toxic to many animals. For example PHA binds to intestinal mucosa of rats, resulting in lesions, disruptions and abnormal development of microvilli. It also inhibits the absorption of nutrients across the intestinal wall. (Pusztai et al .,1979).

The toxicity of PHA was studied on Bruchid beetle and cow pea beetle. Soybean lectin was toxic to the larvae of M. sexta (Shukle et al., 1983). The size of the Myzus persicae was reduced to 30% when the pest was fed with Con A in an artificial diet. Feeding on transgenic potato plants expressing the Con A protein showed similar results. In tomato, retarded larval development, reduction in larval weight and reduced intake of transgenic plant tissues by M. sexta was observed. Likewise Con A expressing potato plants reduced the fecundity of M. persicae upto 45% (Sauvion et al., 1996). PHA also damage the cell structure of the small intestine or effect the pancreas and production of digestive enzymes when eaten by humans or animals (Pusztai et al., 1993). Reduction in the biomass of Lacanobia oleracea larvae after 21 days was 32% and 23%, respectively in an artificial diet containing 2% w/w GNA (Galanthus nivalis Agglutinin) and excised leaves of transgenic potato containing 0.17% GNA. It caused 59% reduction in mean daily consumption of artificial diet by the insect. Reduction in larval survival was 40% (Fitches et al., 1997). There was direct relationship between the level of GNA in the potato transgenic plant and its resistance to the larvae of L. oleracea (Down et al., 2001). It caused 80% corrected mortality in first to third instar nymphs of N. lugens at a concentration of 1.0 g/L in the diet (Powell et al., 1993). GNA caused 42% mortality and 50% growth inhibition of M. persicae nymphs at 1500 μg/ml (Sadasivam and Thayumanauan, 2003). TEL (Talisia esculenta lectin) produced about 90% mortality of Callosobruchus maculatus when incorporated in artificial diet at a level of 2% (w/w) (Macedo et al., 2002). Lectin molecules when ingested by C. maculatus bound to the mid gut epithelial cells and these cells were disrupted (Gatehouse et al., 1984). The cells GNA, AHA, Con A and P-lec have been genetically engineered and transgenic plants were toxic to insect pests.

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6.3. CYANOGENIC GLYCOSIDES (CGs)

CGS are nitrogen containing secondary metabolites. They are intermediately polar, water soluble compounds that often accumulate in the vacuole of the plant cell. These are formed in the cytoplasm but stored in the central vacuole. Several reviews on cynogenic compounds have been published. It has been estimated that CGS are found in 2650 plant species belonging to more than 550 genera and 130 families (Aikman et al., 1996). The important families in which CGS commonly occur are Araceae, Asteraceae, Euphorbiaceae, Fabaceae, Flacourtiaceae, Proteaceae, Rosaceae, Spindaceae, Turneraceae, Compositae, Gramineae and Leguminaceae.

CGS in some plant species participate in defence mechanism. CGS can act as feeding deterrents or phagostimulants (Poulton, 1989). Bitter almond plants are resistant to larvae of buprestid, Capnodis tenebrionis, due to high concentration of CGS (Malagon and Garrido, 1990). CG dhurrin in sorghum act as an oviposition activator for Atherigona soccata and Chilo partellus (Alborn et al., 1992). In cassava tubers CG increased the level of resistance against bug, Cyrtomenus bergi (Belloti and Arias, 1994).

The biosynthetic pathway for CGS involves mainly three enzymes of which two are multifunctional cytochrome P450s and a third a-VDPG-glycosyltransferase. Through gene technology, it is feasible to insert the pathway for CG synthesis in acynogenic plants (Cases et al., 1993). It is also possible to reduce the level of CGS in plants by antisense or cosuppresion technique by blocking the synthesis of CG biosynthesis (Estruch et al., 1997).

6.4. GLUCOSINOLATES

Glucosinolates are sulphur or nitrogen containing distinctive secondary compounds.. These are amino acid derived secondary plant products containing a sulphate and thioglucose moiety and are a uniform class of hydrophilic, nonvolatile, water soluble anionic compounds.

Glucosinolates appear to contribute in the effective chemical defences against a majority of non-adapted phytophagous insects. Some adapted (crucifer feeding) insects are able to utilize glucosinolates in host seeking or host recognition behaviour. Specific concentration of different glucosinolates also stimulate larval feeding by adapted species. In general, damage by herbivorous insects tends to decline as the concentration of glucosinolates increases.

More than 100 glucosinolates have been isolated from plants and all have similar general structure. These compounds are non-volatile and can, therefore, be recognized by insects only on contact. These chemicals act as plant defence chemicals against generalized insects including aphids, grass hoppers etc., interfere with the oviposition and egg hatchability and interact with larval chemoreceptors and have profound effects on larval and adult beetles. They also function as fumigants, parasite attractant and nematicidal.

Glucosinolates stimulate oviposition in insects associated with species of Cruciferae. Adult females of Pieris brassicae possess tarsal contact chemoreceptors that are sensitive to glucosinoates and are probably involved in oviposition (Ma and

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Schoonhoven, 1973). Adult females of Delia brassicae possess tarsal chemoreceptors that respond to sinigrin with a thresholds of 10-5 to 10-4 M (Stadler, 1978). P. brassicae lay eggs on non-hosts painted with allyl glucosinoate. Allyl glucosinolate generally stimulated oviposition activity in P. rapae and P. napi (Renwick and radke, 1983). Among glucosinolates, the indoles have been demonstrated to be the most powerful oviposition kairomone for P. brassicae and P. rapae (Huang et al., 1993). Allyl glucosinolate was the most toxic to the midge, Dasineura brassicae at 10ppm. This concentration is within the range in Brassica juncea cultivars (Ahman, 1986).

Glucosinolates are the active contact stimuli in the feeding specificity of crucifer insects. These compounds stimulated feeding activity in larvae of Plutella maculipennis (Thorsteinson, 1953). Studies have revealed that glucosinolates could provide a basis for caterpillar preference among crucifer plants containing several glucosinolates in different proportions.

Glucosinolates and their degradation products are considered to function as phagostimulants and to have a major role in host plant selection and colonization by insects. High concentrations of glucosinolates can protect plants against aphid, Lipaphis erysimi, and the desert locust, Schistocerca gregaria (Nielson et al., 1979). Root extacts of Brussels sprouts were very toxic to Drosophila melanogaster. Macerated root tissue was also toxic to the D. melanogaster and Musca domestica (Lichtenstein et al., 1964).

Glucosinilates and their degradation products function as part of the plant’s defence against insect attack to act as phagostimulants, host selection and colonization. They also play a role in finding of insects by their parasitoids. Their insecticidal activity is a result of changes in the metabolism of the insect specially the inhibition of the glycolysis, Krebs cycle by decreasing the total O2 uptake and CO2 expired. Glucosinolates serve as gustatory stimulants for insect pests of cruciferous plants and their cleavage products (Sadasivam and Thayumanavan, 2003).

Genetic engineering has helped developing plants that combine high glucosinolate level in leaves that produce plant-pest, plant-pathogens and plant-herbivore interactions advantageous to plant with low seed glucosinolate level to impart quality advantages (Bak et al., 1998).

6.5. ALKALOIDS

The alkaloids represent one of the most diverse and complicated group of secondary plant metabolites. More than 12000 alkaloids have been described so for. The term alkaloid, meaning “alkali” or compound possessing some basicity. It is difficult to define alkaloids because they encompass a wide variety of structures. The definition has been updated several times to include their ever increasing numbers. An alkaloid is defined as a hetrocyclic compound containing nitrogen. Compounds with exocyclic nitrogen bases were called as psuedoalkaloids. Protoalkaloids include compounds which are derived from amino acids without any heterocyclic ring system. An appropriate definition includes all nitrogen-containing natural products that are not otherwise classified as nitrogen amino acids, amines, cynogenic glycocides, glucosinolates, peptides, cofactors, phytohormones, etc.

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Alkaloids occur in 20% of all plants belonging to 150 families (Hartman, 1991). Important alkaloids are given below:

Alkaloids Sources

Nicotine Nicotina spp

Cocaine Solanaceae

Hyoscyamine Datura, Atropa

Stachygrine Medicago sativa

Quinine Cinchona

Morphine Papaveraceae

The alkaloids are a heterogeneous class of natural products that occur in all classes of living organisms especially in plants. Alkaloids generally include basic substances that contain one or more nitrogen atoms, usually in combination as part of cyclic system. Generally, each alkaloid-bearing species displays it into its own unique, genetically defined alkaloid pattern. Numerous alkaloids have been reported to be toxic or deterrent to insects. Many alkaloids interfere with the key components of acetylcholine transmission in the nervous system. A large number of alkaloids such as pyrrolizidines, quinolizidines, indole alkaloids, benzylisoquinolines, steroid alkaloids and methylxanthines are feeding deterrents to many insects. Some of the alkaloids are also sequestered by insects who use them for their own protection from natural enemies. (Arora and Dhaliwal, 2005)

Alkaloids are synthesized in younger tissues, older tissues, specialized cells of highly differentiated tissues etc. Seeds are in some cases the site of alkaloid synthesis. The site of synthesis of an alkaloid is ususlly not the site at which it is found in highest concentration. Generally alkaloids are removed from their site of synthesis and transported to other sites in neighbouring tissues or even other parts of the plant or they are excreted into extra cellular space. Alkaloids accumulate in the vacuoles. This accumulation process is mainly considered to be an important factor in the physiological control of alkaloid formation. Synthesis of alkaloids in the roots and their transport to the aerial parts is well known process, but the reverse process also seems to take place (Panda, 1979).

Alkaloidal allomones remained effective relatively over long periods in plant evolution. Alkaloids have been used as insecticides against many insects species. Their effects on insects include deterrence of larval and adult feeding and inhibition of the growth rate of larvae (Norris and Kogan, 1979).

Many alkaloids such as Pyrrolizidines, quinolizidines, indole alkaloids, benzylisoquinolines, steroid alkaloids and methyl xanthines act as feeding deterrents to large number of insect species. These are considered as part of the plant’s constitutive chemical defence. Generally alkaloids are synthesized and stored at strategically important sites or vulnerable plant parts and therefore are in a position to defend from insect pests and other biotic stresses. Several alkaloids are toxic to insect and vertebrates.

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In vitro assays indicated that the degree of DNA intercalation, was positively correlated with inhibition of DNA polymerase, reverse transcriptase and translation at the molecular level and with toxicity against insects and vertebrates at an organismic level. Inhibition of protein biosynthesis was positively correlated with animal toxicity. Receptor binding and toxicity was correlated in insects. Many alkaloids are compound with a broad activity spectrum. They are multipurpose compounds which may be active in more than one environmental interactions. Alkaloids repel or deter the feeding of many organisms due to their bitter or pungent taste. Many alkaloids are toxic to insects and other organisms.

Several alkaloids biosynthetic genes have been cloned. The genes that regulate alkaloid metabolism must be elucidated. Molecular cloning of putative alkaloid-specific master genes may require some ingenucity and time, but it should be rewarding considering the biotechnological applications. Many aspects of alkaloid biosynthesis, such as elaborate subcellular compartmentation of enzymes and the intercellular translocation of pathway intermediates, need more attention. Emerging knowledge of the biochemistry, molecular biology, and cell biology of alkaloids biosynthesis will also lead to exciting opportunities to engineer alkaloid metabolism in transgenic plants (Sadasivam and Thayumanavam, 2003).

6.6. PHENOLICS

Aromatic rings bearing a hydroxyl group, including functional derivatives such as esters, methylethers, and glycosides are called phenolics. A number of derivatives and polymers of phenols occur in plants and perform different functions. These compounds are found throughout the plant kingdom. Vascular plants contain a full range of polyphenols. All ferns, gymnosperms, and angiosperms, have lignin in the cell wall. Important phenolics are flavonoids, anthocyanins, biflavonoids, tannins, isoflavonoids, xanthones etc. Phlorridzin, phloretin, naringenin, and catechin were all feeding deterrents, whereas quercetin and rutin were all feeding stimulants to adult Japanese beetle, Popilla japonica (Fulcher et al.,1998).

Corn earworm, Helicoverpa zea fed on the diet containing maysin. Concentration more than 0.2% maysin in the diet reduced larval growth to over 50% than that of control, and higher levels of maysin (upto 1%) reduced weights of the larvae to more than 80%. Rutin possessing the requisite ortho-dihydroxy structure was found to be as active as maysin. Chlorogenic acid having an ortho-dihydroxy phenyl structure was found to be active against the corn earworm H. zea resulting in 80% reduction of growth at 20.5M concentration. Galactosyl luteolin was almost as active as maysin in reducing the growth of the insect. The natural resistance of corn silks to the corn earworm has been attributed to the presence of a single flavone called maysin (Snook et al., 1994). Maysin was active against the fall armyworm, Spodophera frugiperda.

Several plant phenol glycosides have been identified as attractants or stimulants for herbivores. Mixtures of quercetin, quercetin – 7 -0-glucoside and quercetin 3-0-glucoside were stimulants for the boll weevil, Anthonomus grandis (Hedin, et al., 1968). A mixture of chlorogenic acid and luteolin 7-0-(6-0-malonyl)-β-D-gluco-pyranoside isolated from carrot foliage served as an oviposition stimulant for the black swallotail

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butterfly (Feeny et al., 1988). Four leaf beetle species selected their favoured host plants, Salix spp. based on phenol glycoside content (Tahavanainen et al., 1985).

Phenol glycosides have adverse effects on insect pests. Rutin and several other flavonoids adversely affected larvae of the tobacco budworm, the cotton bollworm, the pink bollworm and greenbug. Simple coumarin is ovicidal to Colourado potato beetles and toxic to mustard beetles. Xanthotoxin in artificial diet at 0.1% caused 100% mortality of the Southern armyworm. Coumarins exhibited a tremendous range of effects on insects. It is also ovicidal to D. melanogaster. Furanocoumarins are deterrents as well as toxic to a variety of insects such as Helicoverpa zea, H. virescens, Spodoptera litura, Tetrarhychus urticae.

Terthienyl and related furanocoumarin compounds are among the most promising chemicals for insect control under field conditions. Quercetin and glycosides (2-rhamnoside, 3-glucoside and 3-rutinoside) repelled feeding by the tobacco budworm, H. virescens; tobacco bollworm, H. zea and cotton pink bollworm, P. gossypiella when fed on low concentration. These flavonoids killed the larvae at 0.2% concentration or greater when applied in the diet (Shavor and Lukefahr, 1969). The cotton boll weevil, A. grandis was stimulated to feed 3-glucoside and 3-rhamnoside. Cabbage butterfly, P. brassicae larvae are highly sensitive to flavonoids when fed with artificial diets (Schoonhoven, 1981). Isoflavonoid – based rotenoids are highly toxic to many insect species. Rotenoids have low mammalian toxicity but are highly toxic to fish. Rotenone controls many insect pests such as leaf chewing beetles, caterpillars, flex beetles and aphids. Coumestrol have antifeedant and antibiotic activities against cabbage semilooper, Trichoplusia ni feeding. Glyceolin had strong antifeedant activities against the Mexican bean beetle. High concentration of rutin and chlorogenic acid were toxic to corn earworm (Isman and Duffey, 1982).

6.7. TERPENOIDS/ISOPRENOIDS

Plant terpenoids comprise a structurally diverse group of compounds including monoterpenes, resquiterpenes, triterpinoids, saponins and various other steroids. A wide range of allomonic effects have been demonstrated among these chemicals. The effects include altered insect behaviour, sensory physiology, metabolism, insect development rates, metamorphosis, fecundity and longevity. Monoterpenoids may interfere with basic metabolic, biochemical, physiological and behavioural functions of insect. Some terpenoids have acute toxicity; others function as repellents, attractants or antifeedants; some others affect growth, development and reproduction of insects. Repellents in plants constitute the first barrier against insect attack. Oil of camphor and citronella were used commercially as insect repellents. Eucalyptus oils were used as insect repellents. Many terpenoids act as insect attractants. Attractants can be used to lure insects to insecticides or mask repellent properties of insecticides. They may also be used to attract beneficial insects to desired habitats. Certain terpenoids might have dual utility as attractants to beneficial insects and repellent to certain insect pests.

Sublethal or chronic effects of monoterpenoids are considered more important in the plant’s defence than any acute toxicity. A number of feeding deterrents are found in plants. Feeding deterrents interfere with the insects’ ability to ingest and utilize food, which ultimately results in reduced growth and delayed development. One of the best

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examples of monoterpene derivatives that are toxic to insects are pyrethroid. Pyrethroids are neurotoxins, causing hyperexcitation, uncoordinated movement and pyralysis of insects. They are very active broad spectrum insecticides and are generally not hazardous under normal circumstances due to their rapid metabolism and excretion in mammals. The commercially important pyrethrum plant species are Tanacetum cinerariifolium (chrysanthemum) and T. coccineum. The dried plant material contains 1.3% pyrethrins (active ingredient). The highest concentration of pyrethrin is found in fully opened flowers. Natural pyrethrins are unstable in air and light, therefore, their principal use has been limited against household pests.

Plant derived terpeniod juvenile hormone (JH) mimics are used as effective insecticides. Plant derived mono- and sesquiterpens have insect sterilizing effect as JH analogues. Citronellol acted as an oviposition deterrent to the leafhopper, Amrasca devastans. Cineole proved as a feeding deterrent as well as an ovipositional repellent against adult yellow fever mosquitoes, Aedes aegyptii. Limonene inhibited egg hatch of western corn rootworm, Diabrotica virgifera (Gershenzon and Croteau, 1991). Monoterpenoids such as d-limonene, linalool, Alpha – terpineol, and β-myrcene significantly influenced the nymphal developmental period of German cockroach. Growth and development of the European corn borer were affected by monoterpeniod compounds, and of these compounds 1-menthol, pulegone, MTEE-25 and MTEE-P acted as insect growth inhibitors (Lee et al., 1991). Limonene inhibited embryonic development in the cat flea, Ctenocephalides felis and pulegone decreased larval growth of the Southern armyworm, Spodoptera eridania (Hassanali and Lwande, 1989).

Sesquiterpene lactones are poisonous to several lepidopterans, flour beetles, and grasshoppers. Sesquiterpene polyol ester angulatin A was strongly antifeedant and insecticidal against a variety of insects. The presence of sesquiterpene lactones detered oviposition and feeding by armyworm species and other generalist insect feeders. The sesquiterpene caryophyllene was an aphid repellent and the epoxide of this compound inhibited H. virescens larval growth by 60% when incorporated as a 5 M concentration into the diet (Gegory et al., 1986). Alantolactone acted as a potent antifeedant against the confused flour beetle, Tribolium confusum (Yano, 1987). Some sesquiterpene lactones can function as insect oviposition deterrents, antifeedants and act as growth inhibitory agents. Mutagin has moderate anti-feeding activity against C. partellus.

JHO, JHI, JHII and JHIII represent the JH activity of majority of insects. JH is required in the adults of many insect species in the ovarian development, yolk synthesis, maturation of eggs in females, pheromone function and accessory reproductive gland development in males. Regulation of diapause is also under JH control. It plays role in the regulation of moulting by affecting the synthesis of alpha-ecdysone in the prothoracic gland. Several compounds with JH activity are found in plants. Juvocimene II was isolated from the oil of sweet basil, Ocimum basilicum and proved effective against milkweed bug, Oncopeltus fasicatus. JH analogues have been found in various plants and may interfere to some extent with the control of several species of insects. Many plants have compounds with JH activity (phytojuvenoids) such as juvabione, farnesol, juvocimene I and II, sesamin, sesamolin, thujic acid, sterulic acid, tagetone, ostruthin, echinolone, bake-chiol and juvadecene. Plant – derived terpenoid JH mimics are effective insecticides (Nishida et al., 1984).

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Sesterpenoids such as helicocides isolated from glands of leaves and bolls of cotton plants are toxic to Helicoverpa spp. and appear to be involved in the resistance of certain varieties of cotton to insect pests. Hemigossypolone, gossypol and helicocides H1 to H4 are toxic and deterrent to feeding of several generalist lepidopteran insects. In the stored grains, neem leaves are mixed to save grains from the attack of insect pests. Bitter constituents of neem include nimbin, nimbinin and nimbidin. Azadirachtin is considered the most important active ingredient in neem seed kernels. It possesses feeding deterrent, antiovipositional, growth disrupting of insects. Neem-based botanicals appeared to be relatively benign to beneficial insects and are well suited in IPM. A methanolic extract of bitter gourd leaves inhibited feeding of the armyworm, Spodoptera litura larvae (Yasui et al., 1998).

Phytoecdysteroids and their antagonists are found in plants which can play an important role for the control of insect pests. Several plant derived steroids are analogues to the insect moulting hormone ecdysone. By preventing moulting, these compounds act as potent insecticides.

6.8. EPICUTICULAR LIPIDS

The aerial parts of all plants are covered with a protective cuticle composed of a lipid polymer and a mixture of lipid compounds. These compounds are collectively called epicuticular lipids or surface waxes (Eigenbrode et al., 1991). The physical and chemical properties of these compounds play a key role in plant resistance to a variety of stresses including biological stresses like attack of insect pests. They also influence the uptake and efficiency of pesticides.

Population of the cabbage aphid, Brevioryne brassicae and diamond back moth Plutella xylostella on glossy Brassica oleracea was about one tenth the size of population on plants with normal wax. On the contrary the aphid population on glossy genotypes of barley varied from 2-10 fold greater than on normal plants (Tsumuki et al., 1989).

The cuticular compounds interfere with oviposition, feeding and population development of insects on plants. Artificial removal or disruption of epicuticlar lipids with mechanical polishing or detergents resulted in greater oviposition on B. olearacea leaves by P. xylostella (Uematsu and Sakanoshita 1989) and Delia radicum (Prokopy et al. 1983). Similar results were obtained in case of olive fly, Dacus oleae on olive plants (Neuenschwander et al., 1985). Studies indicated that P. Xylostella oviposited on cabbage leaves more when the surface lipid layer was removed (Walton, 1990).

Epicuticular lipids played an important role in the early stages of host plant selection by Aphis fabae (Powell et al., 1999). The chemical composition of epicuticular lipids have been correlated with resistance of plants to insect pests. High levels of docosanol (C22 alcohol) in tobacco were associated with resistance to tobacco budworm, H. virescens (Johnson and Severson, 1984). Epicuticular compounds function as feeding deterrents or stimulants. Surface lipid extracts from rice varieties resistant to brown planthopper, Nilaparvata lugens deterred feeding and increased its restlessness when the extract were applied to the surface of susceptible plants.

The toxicity of epicuticular components was also exhibited to the insect pests after the ingestion of food materials. The growth of Spodoptera frugiperda was grealer

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when larvae were fed diet containing corn foliage from which epicuticular lipids had been extracted than when they were fed diet containing unextracted foliage. Similar growth retardation was also observed in Helicomerpa zea on corn silk (Yang et al., 1992).

6.9. TANNINS

Tannins are phenolic polymers. The term tannin has long been applied to plant polyphenols used to convert animal hides in to leather. Most of the phenolic groups in tannins are free and are able to bind proteins. Tannins are known to inhibit enzymes, coagulate proteins and bind to other materials.

The most common effects of tannins on insects are reduced food consumption, decreased weight gain and decreased efficiency of food utilization. Tannin contents help to protect cotton bolls from pink bollworm, P. gossyxtpiella attack. Jassid resistant cultivars contained higher content of tannin in leaves (Taneja et al., 1988). Population of white fly was low on cotton varieties with high tannin content . It indicates that tannins impart resistance in cotton against this insect(Butter et al., 1992). Infestation of H. armigera was significantly low on cotton with high content of tannins (Rajeshwari et al., 1999).

6.10. PROLINE

Incidence of cotton jassid and pink bollworm was comparatively low on cotton varieties with high content of proline in leaves (Sharma and Agarwal, 1983b).

6.11. SUGARS

Sugars are used as fuels by majority of insects. These are important components of the diets of insects in most of the cases. It has been observed that in general insects require some sugars in the diet, and grow better as the proportion of sugars in the diet is increased. Some of the sugars such as sucrose, fructose and glucose also act as phagostimulants for insects. Jassid population was negatively correlated with total free sugars (Sharma and Agarwal, 1983a). Likewise, negative correlation was observed between the non-reducing sugars and the leafhopper egg production in cotton (Singh and Agarwal, 1988). Sugar content in the leaves was positively correlated with the susceptibility in cotton to whitefly (Jayaswal and Pundarikakshudu, 1989). Cotton varieties resistant to bollworms were with high reducing and total sugar content in plants (Ahuja et al., 2001). Similarly jassid resistant varieties had comparatively high reducing and total sugar content in leaves and stem.

6.12. BIOCHEMICAL BASIS OF RESISTANCE IN MAJOR CROPS

6.12.1. COTTON

The qualitative and quantitative composition of chemicals in cotton plant can have the adverse effect on the physiological processes of the herbivores and can be exploited for the management of insect pests of cotton. These effects on an insect pest feeding on resistant cotton cultivar can be lethal, sub-lethal or very mild, but affect its growth, development and survival (Dhawan, 2005).

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Gossypol in cotton affected the growth and development of bollworms and caused high mortality of these insects on cotton varieties with high gossypol content therefore such varieties had low incidence of bollworms (Rao et al., 1993). High gossypol in cotton conferred resistance and caused reduction in the weight and growth of spotted bollworm (Duhoon, et al., 1981). High gossypol increased the post-embryonic development period of spotted bollworm, Earias vittella larvae (Sharma et al., 1982). It not only influenced the abundance of cotton bollworms but also had significant effect on the population density of cotton jassid (Reddy, 1986). Increase in the quantity of gossypol in cotton leaves significantly reduced the number of eggs of cotton jassid (Singh and Agarwal, 1988) and jassid population (Sharma and Agarwal, 1983a). Low population of jassid and low infestation of bollworm was recorded on cotton with high gossypol content (Ahuja et al., 2001). Low bollworm infestation was recorded on cotton genotypes with high gossypol (Rajeshwari et al., 1999). High gossypol content of cotton leaves was responsible for imparting resistance against grey weevil in cotton (Agarwal et al., 1972).

Total mineral and calcium content showed significant effects on jassids, A. devastans and thrips, Thrips tabaci. It was found that reducing sugars played a significant role in reducing the population of jassids. Thrips population was found significantly affected by nitrogen, protein, total lipids and manganese with positive response and zinc with negative response. Likewise, higher quantity of nitrogen, protein and calcium in top leaves, total lipids, reducing sugars, calcium and zinc in lower and bottom leaves while lower amount of total mineral, copper and zinc in top leaves contributed resistance against thrips. Significant negative correlation with total lipids, reducing sugar, magnesium and calcium in middle and bottom leaves with jassid population was found. Total minerals played significant positive role in reducing jassid population (Ali et al, 1995).

6.12.2. SUGARCANE

Total mineral, manganese and copper contents did not show significant correlation with the top borer infestation. Nitrogen, potassium, calcium, magnesium and ferrous contents played a positive and significant role. Phosphorous, carbohydrates, fats and zinc played a negatively significant role on infestation of top borer, Scirpophaga nivella at tillering stage (Ashfaq et al, 2003).

6.12.3. MAIZE

Maize varieties susceptible to maize borer, Chilo partellus generally contained high contents of amino acids, sugars, nitrogen and phosphorus and low contents of silica and polyphenols. Shoot fly, Atherigona soccata resistant maize varieties possessed low contents of chlorophyll, carotenoid, nitrogen and crude protein as compared to susceptible varieties (Bhanot et al., 2005). Polyphenols and K content were significantly higher in stem borer resistant genotypes of maize (Kaber and Ghorpade, 1997). Silica and Fe contents contributed to resistance in maize against stem borer (Sharma and Chatterjee, 1971).

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6.12.4. OIL SEED CROPS

Higher concentration of total sugars, iron and sulphure content in Brassica spp. were negatively correlated with mustard aphid, Lipaphis erysimi reproduction and population build up. Higher content of iron, sulphur and falvonoid imparted resistance in the plant against L. erysimi. Generally total protein and free amino acids, were higher in susceptible Brassica varieties to the insect. Negative correlation between the total glucosinolate content in plant to aphid fecundity and population multiplication has been reported (Singh and Basappa, 2005).

6.12.5. RICE

A wide array of chemical substances including inorganic chemicals, primary and intermediary metabolites and secondary substances are known to impart resistance to a wide variety of insect pests. Plant chemicals influence the feeding behaviour of insect which reduce the probability for survival or may interfere with the physiological processes such as growth, metamorphosis and reproduction.

Larvae of striped stemborers, Chilo suppressalis feeding on high silica containing rice varieties exibibited typical antibiotic effects and had worn out mandibles (Pathak et al., 1971). Benzoic acid and silicic acid were recorded as larval growth inhibitors in rice plant (Munakata and Okamoto, 1967).

At IRRI, Philippines, allelochemicals, mainly plant volatiles were obtained by steam distillation. The distillate was extracted with diethyl ether and after vaccum evaporation, a yellow oily residue was recovered. The oil extract of Rexoro (susceptible) and TKM 6 (resistant) varieties was tested against stripe stem borer, C. suppressalis. When moths were allowed a choice of an untreated paper strip, a strip treated with the extract of Rexoro variety, the female moths of stem borer oviposited heavily on Rexoro extract treated strips and very few on untreated strips. Likewise, Rexoro-extract was both a strong attractant for moths and stimulated strong egg-laying response. A complete reversal in the ovipositional behaviour of stem borer moths was obtained by spraying the TKM 6 extract on Rexoro plants on which females laid only one tenth the number of eggs laid on normal Rexoro plants. On the other hand TKM 6 plants sprayed with Rexoro extract received nearly four times more the number of eggs laid on untreated TKM 6 plants. Topical application of TKM 6 extract on eggs adversely affected embryonic development and reduced the egg hatch (5%), against control (98%). Treatment of 4th to 5th instar stem borer larvae with TKM 6 extract disrupted their growth and development and all the larvae died within three days. Larvae treated with Rexoro extract pupated normally and moths emerged successfully from them. These results point to qualitative and quantitative differences between the allelochemics of stem borer-susceptible and resistant varieties. The insect’s greater preference for egg laying on Rexoro can be attributed to the presence of attract factors and oviposition inducers. It explains the change in ovipositional behaviour of stem borer moths on TKM 6 plants treated with Rexoro extracts and vice versa. It may be possible to use these

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chemicals for interrupting stem borer moths oviposition on a rice crop, or alternatively make a trap crop to attract stem borer moths to lay all eggs on it. Probably, the “switch on” response of the newly emerged, mated moths to start egg laying on a Rexoro, extract-treated surface is due to an “imprinting” of message from the behaviour and metabolism regulators to which the insect is exposed during younger stages in life. The insect uses this information to its advantage for recognition and colonization of the susceptible host plant (Sexena, 1985).

Silica plays an important role in imparting resistance in ric plant to insect pests. Silicated cells in the form of buliform tablets were closer and double in number per unit area in yellow stem borer, Scirpophaga incertulas resistant variety (Pundia) than in susceptible variety (TNI). Higher silica content was recorded in resistant varieties to yellow stem borer, Scirpophaga incertulas than susceptible varieties (Mishra et al., 1990). Soluble silicic acid and onalic acid in the rice plant acted as brown planthopper sucking inhibitor (Yoshihara et al., 1979).

Resistance of mudgo rice to brown planthopper, Nilaparvata lugens was probably due to low concentration of amino acids, particularly asparagine which stimulates its feeding (Sogawa and Pathak, 1970). Asparagine has been identified in rice as stimualtor and ristosterol as a sucking inhibitor to brown planthopper (Shigematsu et al., 1982). Allelochimicals and nutritive balance of rice varieties were important in eliciting optimal or suboptimal responses, thereby affecting brown planthopper ability to establish on rice plants. The stem distillate extracts of resistant varieties were repellent and when applied topically, caused high mortality of brown planthopper, Nilaparvata lugens nymphs. In contrast, extracts of susceptible varieties attracted the Insect and were relatively non-toxic to the insect.

Total amino acids and phenolic compounds were high in brown planthopper resistant rice varieties, while soluble sugars were high in susceptible varieties. Brown planthopper resistant rice varieties had low levels of amino acids than susceptible varieties. Ptasssium content was more in resistant varieties. Protein content of the leaf sheath was more in brown planthopper resistant varieties.

Green leafhoppers, Nephotettix spp. are primarily a phloem feeder on susceptible varieties, but they primarily feed on xylem tissue on resistant varieties of rice. Distinct differences in waveforms for probing, salivation, phloem feeding and xylem feeding on green leafhopper resistant and susceptible rice plants were observed. Feeding behaviour of the insect on susceptible TNI plants sprayed with the steam distillate extract of resistant ASD 7 plants was monitored with the electronic monitoring device and the lignin specific dye. Application of such extract disrupted the normal feeding behaviour of the insect and phloem feeding of the insect was significantly less on TNI plants sprayed with ASD 7 extract, than on control TNI plants sprayed with acetone/water mixture. The reduced phloem feeding on the extract treated plants was associated with a significant increase in probing frequency and an increase in duration of salivation

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and xylem feeding (Khan and Saxena, 1985). The odoriferous and volatile nature of steam distilate extract exerts a strong influence on the total chemical environment of the rice plant and is of ecological significance in determining the susceptibility or resistance of rice plants to insect pests.

Rice varieties resistant to green leafhopper, Nephottettix virescens and N. nigropictus had lower total amino acids than susceptible varieties (Singh and Dhaliwal, 2005). Soluble carbohydrates were lower in resistant varieties (Reddy and Misra, 1995).

More amino acids and phenols but less sugars were found on the leaf tips of gall midge, Pachydiplosis oryzae resistant rice varieties (Peraiah and Roy 1979). Gall midge resistant rice varieties had higher content of free amino acids and less sugars in shoot apices than susceptible ones (Vidyachandra et al., 1981). Total phenols and ortho-dihydroxy phenols contribute resistance in rice to gall midge (Singh and Salam, 1997).

Rice varieties resistant to whitebacked planthopper, Sogatella furcifera had low content of total amino acids and total sugar content (Rath and Misra, 1998). Salim (1988) carried out detailed studies on whitebacked planthopper, Sogatella furcifera, a major insect pest of rice in Pakistan and various other rice growing countries. Rice cultivars with different levels of resistance against S. furcifera were grown in culture solution with different levels of salinity (10 EC or 12 EC). Likewise, insect resistant (IR 2035) and susceptible (TNI) rice cultivars were grown in culture solution with different levels of K (3, 40 and 200 ppm) and Fe (0.02, 2 and 40 ppm). Plants of TNI and IR 2035 were also grown under different temperature regimes (26/18, 29/21 and 35/27 oC day/night temperature). Determined the effects of these factors on the production of allelochemics in test varieties.

Leaf sheaths of about 40-day-old rice plants grown under different conditions were harvested and ground with an electric grinder. About 200 g sample was steam distilled for 4 hours, during which approximately 900 ml distillate was collected. The distillate was extracted with diethyl ether by shaking a mixture of the two together in separatory funnel for 5 minutes. Diethyl ether absorbed essential oils and other volatiles and the mixture settled above the water layer in the funnel. The water layer was discarded. The ether extract was pooled in a glass beaker, to which 100 g of anhydrous sodium sulphate was added. The beaker was then covered with aluminium foil and held overnight to allow the sodium suophate to absorb traces of water from the extract. The extract was evaporated further to 10 ml and decanted into a weighed glass vial, which was then covered with perforated aluminium foil and placed inside a desiccator. Ether was evaporated under a vacuum, leaving behind a yellow oily residue. The residue was diluted in acetone to 4 g/l concentration.

Determined the quantity of allelochemicals in resistant and susceptible rice plants grown under different conditions. The quantity of allelochemics in rice plant was comparatively more in resistant cultivars than in susceptible ones. Even under stressful growing conditions the quantity of allelochemics in resistant

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cultivar IR 2035 was higher than susceptible TNI plants. However, the quantity of allelochemics was comparatively low when rice plants were grown at low level of K (Fig.1), very low and very high level of Fe (Fig.2) or at low temperature (Fig.3) or high temperature regimes than the plants grown under optimal conditions. Likewise, plants grown under salinity stress had low quantity of allelochemics than those of unstressed rice plants (Fig.4). The magnitude of decrease in the quantity of allelochemics in rice plants, increased with increase in the level of salinity stress.

Fig.1 Effect of potassium(K) appliction in culture solution on the quantity of allelochemicals in susceptible and resistant rice cultivars to S.furcifera

0

2

4

6

8

10

12

14

TN1 IR 2035

3 K 40 K 200 K

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Fig. 2: Effect of iron (Fe) appliction in culture solution on the quantity of allelochemicals in susceptible and resistant rice cultivars to S. furcifera

0

2

4

6

8

10

12

14

16

TN1 IR 2035

Qu

an

tity o

f alle

loc

hem

icals

(mg

)

0.02 Fe

2 Fe

40 Fe

Fig. 3: Effect of temprature stress on the quantity of allelochemicals in susceptible and resistant rice cultivars to S.furcifera

0

2

4

6

8

10

12

14

16

TN1 IR 2035

Qu

an

tity o

f alle

loc

hem

icals

(mg

)

26/18 C

29/21 C

35/27 C

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To determine the effect of allelochemics on orientation of S. furcifera eight single tillers of 35-day old TNI plants with roots immersed in vials of water were each painted with 0.2 ml of 4 g/litre acetone solution of steam distillate extract of salinity stressed or control plants. This rate of application of the extract demonstrated cultivar differences in resistance or susceptibility to S. furcifera. Control plants were painted with acetone. After the acetone had evaporated, the extract treated and control plants were placed at opposite ends of the orientation chamber outside the nylon net wall. The orientational response of the brachypterous S. furcifera females was recorded by using horizontal cylindrical glass chembers (each 70 mm long 130 mm diameter) having nylon net walls at both ends. The newly emerged females were introduced into the middle of the chamber. The females could respond to the plants but could not come in contact with them because they were outside the nylon net walls. Females alighting on the end walls of the chamber facing the plants were counted at 5 minute intervals for one hour to observe their orientational response. Chi-square tests were performed to determine whether observed ratios of females attracted to stressed and unstressed plants deviated significantly from the no preference ratio of 1:1.

Application of allelochemics (steam distillate extract) from insect resistant IR 2035 plants to susceptible TNI plants rendered them less attractive to the insect. Chi-square values were significant for deviations from means of total times the females alighted on nylon net walls facing control TNI plants (Tables 1 and 2).

Fig.4: Effect of salinity stress on quantity of allelochemicals in rice cultivars with different levels of resistance against S. furicifera

0

2

4

6

8

10

12

14

TN1 IR2035 Nona Bokra Pokkali

Qu

an

tity o

f alle

loc

hem

icals

(mg

)

EC 10 Control EC 12 Control

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Single tillers of 35-day-old resistant IR 2035 or susceptible TNI were painted with 0.2 ml of 4 g/liter acetone solution of extract of salinity stressed or unstressed plants of test cultivars, while another set of control plants was treated with acetone. After the acetone evaporated, S. furcifera females in batches of three were enclosed in parafilm sachets mounted on the tillers. After 24 hours each batch of females and their excreta weighed. The amount of food ingested and assimilated was calculated. Intake and assimilation of food by the insect decreased significantly on TNI plants painted with IR 2035 extract. In contrast, it increased significantly on IR 2035 plants treated with TNI extract (Table 3 and 4).

Single tiller of 35 day old TNI plants were painted individually with 0.23 ml acetone containing 4 g/liter steam distillate extract of plants of TNI and IR2035 grown at different levels of K, or Fe and at three temperatures regimes. Control tillers were painted with acetone. Treated and control tillers were infested with first instars in batches of 10 each and mortality was recorded after 72 hours. Application of extract from susceptible variety on susceptible variety caused higher mortality of nymphs (5.0-10.0) than control (2.5-7.5%). Nymphal mortality increased drastically when steam distillate extract from resistant variety applied on susceptible rice variety (more than 31%) (Table 5).

Allelochemics are involved in plant defences. TNI extract showed that even susceptible plants possess allomones or defence chemicals, however, their toxicity was less. Even the most susceptible plants are remarkably well defended against insect attack when compared to artificial diet. Although the allomones have not been identified in rice plant, against S. furcifera, a large group of low molecular weight compounds, such as essential oils, terpenoids, alcohols, aldehydes, fatty acids, esters and waxes have been obtained.

Table 1: Effect on S. furcifera orientation to TNI plants painted with 4000 ppm

steam distillate extract of Fe-stressed and unstressed rice plants.

Treatment Orientation

TNI (0.02 ppm Fe) 0.7 ns

TNI (2 ppm Fe) 0.6 ns

TNI (40 ppm Fe) 0.9 ns

IR2035 (0.02 ppm Fe) 16.6*

IR2035 (2 ppm Fe) 21.2**

IR2035 (40 ppm Fe) 28.8**

Ns, *, ** = not significant, significant at 5% and 1% levels, respectively by chi-square.

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Table 2: Effect on S. furcifera orientation to TNI rice plants painted with 4000 ppm steam distillate extract of salinity stressed and control rice plants.

Treatment Orientationa

EC 10. EC 12

TNI (stressed) 1.0 ns 5.0 ns

TNI (control) 1.2 ns 1.8 ns

IR2035 (stressed) 14.4* 31.7**

IR2035 (control) 13.2* 16.8**

Nona Bokra (stressed) 0.5ns 0.6 ns

Nona Bokra (control) 0.3 ns 0.4 ns

Pokkali (stressed) 0.5 ns 0.1 ns

Pokkali (control) 0.7 ns 0.4 ns

a. Chi-square tests were performed to determine whether observed ratios of females attracted to treated and untreated plants deviated significantly from the no preference ratio of 1:1.

Ns, *, ** = not significant, significant at 5% and 1% levels, respectively by chi-square.

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Table 3: Effect of extract application on IR 2035 plants obtained from salinity

stressed (EC 12 ds/m) and control rice plants on intake and assimilation of food

by S. furcifera.

Cultivar EC 10 Control Mean Diff.

Food intake (mg)/female/24 h

TNI 6.4 a 5.7 a 6.1 a 0.7 ns

IR2035 4.2 b 3.6 b 3.9 b 0.6 ns

Nona Bokra

5.1 b 4.7 ab 4.9 ab 0.4 ns

Pokkali 5.3 b 4.1 ab 4.7 b 1.2 ns

Food assimilated (mg)/female/24 h

TNI 0.21 a 0.18 a 0.19 a 0.03 ns

IR2035 0.14 b 0.11 b 0.13 b 0.03 ns

Nona Bokra

0.19 ab 0.17 b 0.18 ab 0.02 ns

Pokkali 0.17 ab 0.18 ab 0.17 ab -0.01 ns

Honey dew excreted (mg)/female/24 h

TNI 6.2 a 5.5 a 5.9 a 0.7 ns

IR2035 4.0 b 3.5 b 3.8 b 0.5 ns

Nona Bokra

5.0 ab 4.5 ab 4.8 ab 0.5 ns

Pokkali 5.1 ab 3.9 ab 4.5 ab 1.2*

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Table 4:Effect of extract application on TN1 plants obtained from salinity stressed (EC 12 ds/m) and control rice plants on intake and assimilation of food by S. furcifera.

Cultivar EC 12 Control Mean Diff.

Food intake (mg)/female/24 h

TNI 17.9 a 16.4 a 17.2 a 1.5 ns

IR2035117-3 11.9 b 10.3 b 11.1 c 1.6 ns

NonaBokra 15.7 ab 14.3 ab 15.0 ab 1.4 ns

Pokkali 13.9 ab 11.7 ab 12.8 bc 2.2 ns

Food assimilated (mg)/female/24 h

TNI 0.62 a 0.58 a 0.60 a 0.04 ns

IR2035-117-3 0.49 b 0.41 b 0.45 c 0.08 *

Nona Bokra 0.54 ab 0.50 b 0.52 b 0.04 ns

Pokkali 0.53 b 0.49 ab 0.51 bc 0.04 ns

Honey dew excreted (mg)/female/24 h

TNI 17.3 a 15.8 a 16.6 a 1.5 ns

IR2035-117-3 11.4 b 9.8 b 10.6 c 1.6 ns

Nona Bokra 15.2 ab 13.8 ab 14.5 ab 1.4 ns

Pokkali 13.3 ab 11.3 ab 12.3 bc 2.0 ns

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Table 5: Mortality of first instar S. furcifera nymphs on TNI rice plants painted with 4000 ppm extract of TNI and IR2035 plants grown at different levels of K, Fe and day/night temperatures

Treatment % Mortality

TNI (3 ppm K in culture solution) 5.0 bc

TNI (40 ppm) 8.8 b

TNI (200 ppm) 7.5 b

IR2035 ( 3 ppm) 26.3 a

IR2035 (40 ppm) 30.0 a

IR2035 (200 ppm) 31.3 a

Control 3.8 c

TNI (0.02 ppm Fe in culture solution) 6.3 b

TNI (2 ppm) 7.5 b

TNI (40 ppm) 7.5 b

IR2035 (0.02 ppm) 27.5 a

IR2035 (2 ppm) 31.3 a

IR2035 (40 ppm) 30.0 a

Control 2.5 c

Temperature oC (day/night)

TNI (26/18) 10.0 c

TNI (29/21) 15.0 bc

TNI (35/27) 12.5 bc

IR2035 (26/18) 22.5 ab

IR2035 (29/21) 30.0 a

IR2035 (35/27) 25.0 ab

Control 7.5 c

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CHAPTER-7 FACTORS AFFECTING HOST PLANT RESISTANCE

The interactions between plants and herbivorus insects are very complex. Characteristics of the plant that influence attractiveness, suitability and tolerance to insect pests are affected by various environmental factors (Tingey and Singh, 1980). Crops are generally grown under diverse climatic and edaphic conditions and may face unfavourable conditions and in some cases are affected by physiochemical stresses. These stresses may have significant effects on plant physiological and biochemical processes and as a result may profoundly affect host-insect interactions.

In physical sciences or engineering, stress is defined as the force applied per unit area and the result is strain. However, in biology, stress is considered as any factor that disturbs the normal functioning of an organism. Stresses usually cause changes in growth and biomass of plants by making changes in the normal functioning of plants. Plants can minimize or prevent stress by avoiding thermodynamic equilibrium with the stress by physical, chemical or metabolic barriers. In contrast, stress tolerance is a type of resistance in which plant is resistant in spite of permitting the stress to enter its tissues, provided that it decreases or eliminates the strain. Stresses induce potentially injurious strains in the plants, either reversibly by inhibiting metabolism or growth or irreversibly by injuring or killing the cells. Biologists have adopted the term stress for any environmental factor potentially unfavourable for living organisms and stress resistance to the ability of the plant to survive the unfavourable factor or even to grow in its presence under stress. The living organisms may show a physical strain or a chemical strain. If the strain is sufficiently severe the organism may suffer a permanent injury or death. The plant may be able to repair the strain by an active expenditure of metabolic energy (Levitt, 1980).

Stresses reduce the size of energy and nutrient budget of plants or require its expenditure to accommodate the stress and may result in a lower commitment for defence. Effects of stresses on the production of allelochemics in plants are complex. In addition to physiochemical plant stresses human interventions in the production of crops may also have effects on host-insect interactions. Because the well being of phytophagous insects depends in part upon the chemical make up of their host plants, therefore, factors that affect plants should also affect insect pests (Rodriguez, 1960). Plants and insects have intricate relations; therefore, any change in plant may have profound effect on host-insect interactions (Salim, 1988)

Brief description of the effects of different factors affecting the level of resistance/susceptibility in plants due to different categories of plant stresses such as temperature, salinity, nutritional deficiencies, excessive amounts of nutrients, nutrient toxicity etc. on host-insect interactions are described.

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7.1. TEMPERATURE STRESS

Crops are grown under diverse temperature conditions. For example rice is grown at latitudes 40o S to 53o N and elevations ranging from below sea level to 7200 meter height; the temperature regimes vary widely in these diverse areas. Temperature regimes greatly influence the growth duration as well as growth pattern of rice plants. Within a range of 21-31 oC, growth rate of rice increases almost linearly with increasing temperatures but declines above 35 oC (Yoshida, 1981). Each species or even variety of plants grows well under a certain range of temperature, increase or decrease from this range causes the plants in stress.

High as well as low temperatures, increase the abundance of phytophagous insects (Wellington et al., 1950). Temperature is a major environmental factor influencing fundamental plant and insect pest physiological processes, including the magnitude and expression of resistance in plants to insects. Both low and high temperatures can lower the level of insect resistance in plants. Temperature can modify host-insect interactions through influence on plants, effect on the insect or through the combination of both direct and indirect effects on physiological processes or direct effects on pest behaviour and biology.

Scientists have carried out studies on a number of crops and large number of insect species. Here the purpose is not to review the literature but to site some examples to explain the effects of temperature stresses on plants and insect pests. It has been reported that Wheat plants were comparatively more susceptible to Hessian fly, Mayetiola destructor when grown at high temperatures (Sosa and Foster, 1976). Survival of M. destructor larvae increased significantly at 28 oC to 31 oC, particularly on resistant plants, suggesting a temperature induced interaction (Tyler and Hatchett, 1983). Likewise, resistance of wheat to green bug, Schizaphis graminum decreased at high temperatures (Painter, 1958). Sorghum resistance to aphids increased as temperatures increased (Wood and Starks, 1972). It has been investigated that temperature modified resistance in alfalfa against spotted alfalfa aphid, Therioaphis maculata. Populaion of aphid on selected clones of alfalfa was less at 24 oC. At 16 o C nymphal mortality of aphid was less that at 27 oC. Its population on resistant alfalfa clones was influenced by an increase in reproductive rate with increasing temperature and through an increase in expression of resistance with rising temperature. At low temperature breakdown of resistance was observed in alfalfa clones against T. maculata and Acyrthosiphon pisum. Antixenosis in resistant clones against T. maculata was more evident at constant 27 oC.temperature Level of such resistance was reduced at constant remperature of 10 oC. It is evident from these studies that low temperature reduced the level of resistance in alfalfa clones against T. maculata. Fluctuating mean temperatures of 20, 22, 24 oC generally led to greater fecundity and nymphal survival of T. maculata on susceptible alfalfa clones than the same constant temperatures. On the other hand, fluctuating temperatures affected its performance on resistant clones to smaller extent (Kindler and staples, 1970). Resistance in plants to insect pests has been associated with optimum temperature for plant growth (Schweissing and Wilde, 1978).

Insect development may take place within a fairly narrow range of temperatures. A developmental threshold or the developmental null point exists below which the insect can survive but cannot grow and develop (Bursell, 1964). Temperatures above the

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developmental threshold increase the rate of development in almost a linear fashion upto the optimum, beyond which, insect growth and development is retarded.

Measurements of pest resistance and assessment of resistance mechanisms are generally made at constant temperatures. However, under natural conditions, plant and insects physiological processes are subject to day-night fluctuations in temperature. Results obtained at constant temperatures may be different from those obtained under natural conditions. Therefore, Salim and Saxena (1991) carried out detailed studies on the effects of temperature stresses on the levels of resistance of IR2035 and TN1 rice cultivars against S. furcifera. Seeds of IR2035 and TN1 were sown in soil in a tray (260 by 320 by 115 mm) under 29/21 oC day/night (12/12 hours) temperature, minimum 70% relative humidity, and natural daylight of 12 hours. Two weeks after sowing, seedlings were transplanted into plastic pots (150 mm high, 120 mm diam) or porcelain pots (200 mm high, 160 mm diam) at two and four seedlings per pot, respectively. Two days later, potted plants were shifted to three Koitotron growth cabinets (Model 3SA-L # 35-4708, Koito Industries, Tokyo) maintaining 26/18, 29/21 and 35/27 oC day/night (12/12 h) temperature regimes, respectively, 70% relative humidity, and 12-hour daylight. To further demonstrate subtle temperature-induced effects on plant resistance and insect responses, potted plants were also kept in another set of growth cabinets maintaining 24/16,29/21, and 36/28 oC, respectively. Plants grown at optimum (29/21 oC) temperature served as the controls in all experiments. Data were recorded on growth and biomass of rice plants. Fresh and dry shoot and root weights were recorded for plants maintained in growth cabinets.

Both high and low temperature regimes decreased the heights of TN1 and IR 2035 plants, but IR2035 plants at 35/27 oC were as tall as the control (Fig.1). The effect of temperature on the number of tillers was inconsistent (Fig. 2 and 3). Temperatures stress significantly reduced shoot weights and caused changes in the chemical composition of rice plants (Table 1).

The orientational and settling responses of whitebacked planthopper sogatella. furcifera to temperature stressed and control plants did not differ. The females alighted almost equally on nylon net walls facing temperature-stressed and control plants. Compared with nymphal development on resistant plants grown at optimum temperature significantly more nymphs became adults on resistant IR2035 plants exposed to low or high temperatures. Nymphal development on susceptible TN1 plants grown at low or high temperature was high at optimum temperature. Insect development was prolonged on plants grown at low temperature(table 2).

Adult longevity decreased with increase in temperature on susceptible TN1 plants, but increased on resistant IR2035 plants at both high and low temperatures(Table 3). Compared with IR2035 plants grown at optimum temperature, insect population increased significantly on IR2035 plants grown at high temperature(Table 4). Food intake, increased significantly on temperature stressed resistant IR2035 and susceptible TN1 plants. Assimilation of food increased on TN1 plants grown at 35/27 oC, but was basically unaffected on IR2035 plants grown at either low or high temperature. It is evident from this study that both low and high temperature stresses reduced the level of resistance and induced susceptibility in rice plants to S. furcifera.

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Planthoppers are known to harbour yeast like intercellular endosymbiotes in their fat bodies which are transmitted to the offsprings through the egg. The endosymbiotes provide sterols for protection of hosts (Fredenhagen et al., 1987). In a study biosynthesis of cuticle was terminated and the body cholesterol was reduced to 1/10 of normal nymphs of the small brown planthopper, Laodelphax striatellus kept at 35 oC for three days on sterol free diet (Noda and Saito, 1979).

Fig. 1. Effect of temperature on height of rice plants

0

100

200

300

400

500

600

700

800

TN1 IR 2035

Pla

nt H

eig

ht (m

m)

(mm

)

24/18 oC

oC 29/21 oC

36/28 oC

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Fig.2. Efffect of Temperature 24/16 to 36/28 oC on tillers per plant of TNI and IR2035

rice cultivars

0

1

2

3

4

5

6

TN1 IR 2035

Tille

rs (p

er p

lan

t)

24/16 oC

29/21 oC

36/28 oC

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Fig. 3. Effect of Temperature 26/18 to 35/27 oC on tillers per plant of TN

1 and IR 2035 rice cultivars

0

1

2

3

4

5

6

7

TN1 IR 2035

Tille

rs

26/18 oC

29/21 oC

35/27 oC

Fig .4. Effect of Temperature on shoot biomass of TN1 and IR 2035 rice cultivars

0

10

20

30

40

50

60

70

80

TN1 IR 2035

Sh

oo

t Bio

mas

s (g

)

24/16 oC

29/21 oC

36/28 oC

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Fig. 5. Effect of rice plants grown under temperature stress on assimation

of food by S.furcifera

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

TN1 IR 2035

26/18 oC

29/21 oC

35/27 oC

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Table 1: Chemical composition of rice plants grown at different day/night

temperature (oC) regimes.

Constituents

TN1 IR2035

26/18 29/21 35/27 26/18 29/21 35/27

N (g kg-1) 41.3 37.1 36.6 32.1 30.4 30.4

P (g kg-1) 3.4 2.4 2.2 2.9 2.7 2.1

K (g kg-1) 33.2 49.7 46.8 36.3 50.0 42.0

Mg (g kg-1) 2.6 2.5 2.5 2.4 2.3 2.0

Ca (g kg-1) 1.6 1.8 1.6 2.0 1.7 1.5

Si (g kg-1) 30.9 42.3 50.8 31.5 35.7 48.3

Soluble Proteins (g kg-1)

64.5 71.7 75.4 59.2 61.2 70.4

Total Free Sugar(g kg-1)

98.0 48.8 42.4 71.4 51.0 57.0

Zn (mg kg-1 ) 16.0 8.9 11.9 6.2 5.0 7.4

Fe (mg kg-1 ) 59.7 29.8 45.4 43.4 35.1 43.0

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Table 2: Growth and development of S. furcifera nymphs on susceptible TN1 and

esistant IR2035 rice plants grown at different day/night temperature

regimes.

Temperature oC

Day/

Night

WDMT

Nymphs to adults (%)

Development period (days)

Growth index

TN1 IR2035 Dif. TN1 IR2035 Dif. TN1 IR2035 Dif.

26/18 22 92a 57a 35** 21.3a 24.4a -3.2** 4.3b 2.3a 2.0*

29/21 25 95a 22b 73** 14.9b 18.6b -3.7** 6.4a 1.2b 5.2**

35/27 31 80a 55a 25** 14.1b 17.2b -3.1** 5.7a 3.2a 2.5**

24/16 20 92a 65a 27** 26.5a 28.8a -2.3** 3.5b 2.3a 1.2**

29/21 25 92b 27b 65** 14.8c 17.5c -2.7** 6.2a 1.5b 4.7**

36/28 32 15b 10c 5NS 18.5b 20.3b -1.8** 0.8c 0.5c 0.3NS

*,** Significant at the 0.05 and 0.01 probability levels, respectively, by the least significant

difference (LSD)

Within columns and within each gouping of three temperature regimes, values followed by the same letter are not significantly different at the 0.05 probability level by Duncan’s Multiple range Test (DMRT).

WDMT: Weighted Daily Mean Temperature

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Table 3: Longevity and fecundity of S. furcifera adults on susceptible TN1 and resistant

R2035 rice plants grown at different day/night temperature regimes.

Temperature oC

Day

Night

WDMT

Male longevity (days)

Female longevity(days)

Fecundity(eggs laid by 10 females)

TN1 IR2035 Dif. TN1 IR 2035

Dif. TN1 IR 2035

Dif.

26/18 22 14.7a 10.7a 4.0**

20.5a 12.8a 7.7** 1574 220a 1354*

*

29/21 25 14.3a 6.1c 8.2**

18.2b 8.2b 10.0** 1387b

47c 1340*

*

35/27 31 11.2b 9.3b 1.9**

14.5c 11.8a 2.7** 1473a

107b 1366*

*

24/16 20 15.3a 9.4a 5.9**

21.5a 12.2a 9.3** 2017a

374a 1643*

*

29/21 25 13.3b 6.5b 6.8**

17.3b 8.1b 9.2** 1854a

112b 1742*

*

36/28 32 6.4c 4.7c 1.7**

9.5c 5.6c 3.9** 29b 25c 4NS

** Significant at the 0.01 probability levels, by LSD test

† Within columns and within each grouping of three temperature regimes, values

followed by the same letter are not significantly different at 0.05 probability level by

DMRT.

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Table 4: Population of S. furcifera at 40 days after infestation on susceptible TN1 and resistant IR2035 rice plants grown at different day/night temperature regimes.

Temperature (oC) Population from 5 pairs (No)

Day/night WDMT TNI IR2035 Difference

26/18 22 428b 68a 360**

29/21 25 586a 35b 551**

35/27 31 594a 75a 519**

26/18 22 442b 72a 370**

29/21 25 520a 40b 480**

35/27 31 25c 21b 4NS

** Significant at the 0.01 probability level by LSD

Within columns and within each grouping of the three temperature regimes, values

followed by the same letter are not significantly different at 0.05 probability level by

DMRT

7.2. SALINITY STRESS

In South and Southeast Asia, nearly 60 million ha of land are affected by salinity (Akbar and Senadhira, 1985). It is a major obstacle to high yields on about 27 million ha of lands in deltas, estuaries and coastal fringes in Asia, which are physiologically and climatically suited to rice production. In inland saline soils, the constraints of rice production are salinity and deficiencies of N,P,K, and Zn, while in coastal saline soils, salinity, sodicity, P deficiency and Zn, B, and Fe toxicities limit high yields in rice. Rice yield is drastically reduced at 10 EC (Akbar and Senadhira, 1985).

At early seedling stage, root length, seedling height, emergence of new roots and dry matter decreased significantly at 5-6 EC (Akbar and Yabuno, 1974). During the vegetative growth, plant height, root length, number of tillers/plant, straw weight and dry weight of roots were affected by salinity (IRRI 1968). Salinity significantly reduced plant height, root length, and biomass in all test cultivars: TN1, IR2035, Nona Bokra and

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Pokkali. The reduction in growth parameters was greater in salt sensitive cultivar IR2035 than in salt tolerant cultivars Nona Bokra and Pokkali (Salim et al., 1990).

In general, salinity increased the concentration of Na in the rice plant (John et al., 1977). Increase in Na content in the shoot of rice plants due to salinity is important and affects various physiological and biochemical processes in rice plant. Increase in N in rice plant was recorded with increase in the level of salinity (Shimose, 1956). Rashid (1983) found that the N content in the shoot, straw and grain increased with increasing salinity. The increase of N was more pronounced in the shoot than in the grain and straw. Salinity reduced K concentration in the shoot, straw and grain.

Salt tolerance in crop plants was generally correlated with maintenance of a high K/Na ratio in the cytoplasm and a high Na/K ratio in the vacuole, exclusion of Na at the plasma membrane, mobilization of Na to older leaves and transport of K to younger leaves (Pitman, 1984).

Plants serve not only as food but also provide shelter and protect phytophagous insects. Ecology, biology and behaviour of insect pests depend largely on the physical and chemical makeup of host plants. Chemical factors of plants are very important in host- insect interactions. Insects use chemical factors (allelochemics) as signals for several purposes: (i) to attract adult insects to suitable host plants, (ii) to aid the adults in locating opposite partners and (iii) help larvae in choosing an appropriate host plant via pheromones left by the female in eggs (Edwards and Wratten, 1980). Even minor changes in the physical and chemical attributes of plants can affect their suitability as hosts (Salim and Saxena, 1991).

Salinity is one of the most important stresses which induces various changes in rice plant and as a result can alter significantly host-insect interactions. However, the information on this aspect is yet at infancy stage. Salim et al., (1990) are pioneers in studies on the effects of salinity on host-insect interactions. They investigated the salinity effects on various parameters of whitebacked planthopper, Sogatella furcifera, a major pest of rice in Pakistan and elsewhere. Since it is a first study of its nature in the world, therefore, it is being discussed in detail.

Salim et.al. (1990) carried out detailed studies on the effect of salinity stress on the growth and chemical composition and in the production of allelochemics in rice plants and on the level of resistance/susceptibility of rice plant against S. furcifera. IR2035, Nona Bokra, Pokkali and TN1 rice cultivars were used in these studies. IR2035, an improved line resistant to S. furcifera is highly sensitive to salinity. Nona Bokara and Pokkali; traditional, tall, salt tolerant cultivars are susceptible to the pest. TN1 is highly susceptible to the pest and sensitive to salinity.

Rice seeds of selected rice cultivars were surface sterilized with HgCl2 solution @ 1g/kg rice seed for 2 min., washed thouroughly with water and soaked in water for 24 hours, and then incubated at 30 oC for 48 hours in Petri dishes lined with moist filter paper. Pre-germinated seeds were sown singly in holes (15 mm diam.) in culture foam plastic sheets with nylon-net bottoms that rested on 12-L plastic trays filled with the standard culture solution. Each culture foam plastic sheet (320 by 270 by 13 mm) had 40 holes bored equidistantly in six rows and was glued with epoxy on the lower side of a foam plastic frame (50 mm wide, 20 mm thick) along its inner border. The emerging

181

seedlings were grown for 2 weeks in the culture solution, the level of which was kept in contact with seedling roots. The pH of the culture solution was adjusted daily to 5 using either 1 M NaOH or 1 M HCl. The culture solution was changed at weekly intervals. The seedlings were subjected to salinization by adding NaCl and CaCl2 (1:1 by weight) to the culture solution to get the desired salinity levels of 10 EC or 12 EC. using an electrical conductivity meter. Control plants were grown in the standard culture solution. Salinization continued from 35 to 75 days, depending on the duration of each experiment. The orientational response of brachypterous S. furcifera females was tested using horizontal cylindrical glass chambers (each 70 mm long, 130 mm diam.) having nylon net walls at both ends. Plants that were salinity stressed for 30 days were placed singly with roots immersed in the nutrient solution in eight vials at one end of the chamber, while unstressed control plants were placed at the opposite end of the chamber. Ten newly emerged females (starved for 2 hours but water satiated) were introduced into the middle of the chamber using a blowing glass tube. The females could respond to the plants but could not come in contact with them because they were outside the nylon-net walls. Females alighting on the end walls of the chamber facing the plants were counted at 5 minutes intervals for 1 hour to observe their orientational response. Chi-square tests were performed to determine whether observed ratios of females attracted to stressed and unstressed plants deviated significantly from the no preference ratio of 1:1.

In another experiment, eight tillers of 35-days old TN1 plants with roots immersed in vials of water were each painted with 0.2 mL of 4 g/L acetone solution of steam-distillate extract (allelochemics) of salinity stressed or unstressed plants. This rate of application of the extract demonstrated cultivar differences in resistance or susceptibility to S. furcifera. Control plants were painted with acetone. After the acetone had evaporated, the extract treated and control plants were placed at opposite ends of the orientation chamber outside the nylon-net walls and insect orientation was determined, as described above. Newly emerged branchypterous females (starved for 3 hours but water satiated) were weighed in batches of three and enclosed in air-tight parafilm sachets (50 by 50 mm), through each of which passed the leaf sheath of a 35 days stressed or unstressed plant. After 24 hours each batch of females and their excreta were weighed. To quantify the loss in insect body weight due to catabolism, a control was similarly established, in which the females were given access to a moist cotton swab to prevent desiccation. The amount of food ingested and assimilated was calculated.

In another test, single tillers of 35-days old resistant IR2035 or susceptible TN1 plants were each painted with 0.2 mL of 4 g/L acetone solution of extract of stressed or unstressed plants of a test cultivar, while another set of control plants was treated with acetone. After the acetone evaporated, S. furcifera females in batches of three were enclosed in parafilm sachets mounted on the tillers and their intake and assimilation of food was determined.

Plants salinity-stressed for 35 days were placed singly in plastic pots (140 mm deep, 120 mm diam) containing salinized solution. Roots of each plant were kept immersed in the solution while tillers emerged through a median hole (15 mm diam) in a circular styrofoam lid (20 mm thick, 122 mm diam.) on the pot. A mylar-film sleeve (360 mm high, 122.5 mm diam.) was slipped over the styrofoam lid up to the base of the pot. A cylindrical mylar-film cage (1000 mm high, 120 mm diam) with nylon-mesh top and side windows enclosed each plant and snugly rested on the styrofoam lid within the collar of

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the mylar-film sleeve. Similarly the unstressed control plants in pots containing standard culture solution were enclosed in mylar cages. Salinity stressed and control plants were infested with first instar nymphs in batches of 10 each. The pH of the culture solution was adjusted daily to 5.5. Growth was measured by the number of nymphs that became adults and the time taken to reach the adult stage. The insect growth index on stressed and unstressed plants was calculated as the ratio of per centage nymphs becoming adults to the mean growth period in days.

Plants stressed for 35-days and kept in salinized culture solution and unstressed plants kept in standard culture solution in plastic pots were infested with 10 pairs of newly emerged females (brachypteropus) and males, covered with mylar-film cages. Insect mortality was registered daily until all individuals died. The total number of nymphs emerged on each plant was recorded and represented the number of viable eggs laid by the females. At the end of the nymphal emergence, unhatched eggs also were counted by dissecting leaf sheaths under a 20 x binocular microscope. The total number of nymphs emerged and the number of unhatched eggs represented the fecundity of S. furcifera females.

Salinity stressed and control plants grown in salinized and standard culture solution, respectively, at four plants per pot (180 mm deep, 160 mm diam) were infested with five pairs of newly emerged females (brachypterous) and males using mylar cages (1000 mm high, 190 diam). Insects, both nymphs and adults, were counted 40 days after infestation.

Data for tests on food intake and assimilation, growth, adult longevity and fecundity, and population increase were subjected to analysis of variance and the means were compared using Duncan’s (1951) multiple range test at the P < 0.05 levels.

Salinity significantly reduced plant height, root length, and biomass in all rice test cultivars (Table 5). The reduction in growth parameters was greater in salt-sensitive cultivar IR2035 than in salt-tolerant cultivars Nona Bokra and Pokkali. Salinization also influenced the chemical composition of rice plant. Nitrogen, Ca and Fe increased, while K content decreased at EC 12 in the leaf sheath (Table 6). Both salinity levels decreased the quantity of allelochemics recovered as steam distillate extracts from all cultivars (Table 7), but the reduction was greater at 12 EC than 10 EC. The reduction in the quantity of allelochemics produced was greater in salt-sensitive IR2035 than in salt-tolerant cultivars Nona Bokra and Pokkali. Likewise, K content decreased more in IR2035 than in salt-tolerant Nona Bokra and Pokkali. Reduction in quantity of allelochemicals produced was of great magnitude in IR2035 than other rice varieties due to salinity stress.

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Fig.6. Effect of salinity on growth of rice plants raised in culture solution

Salinity stress did not alter plant attractiveness to S. furcifera females. However, application of steam distillate extract from salinity stressed or unstressed insect resistant IR2035 plants to susceptible TN1 plants rendered them less attractive to the insect. Chi-square values were significant for deviations from means of total times the females alighted on nylon-net walls facing control TN1 plants and plants treated with extract of stressed (x2 = 14.4* and 31.7** ) or unstressed IR2035 plants (x2 = 13.2* and 16.8**).

Salinity stress of EC 10 did not influence the insect’s intake and assimilation of food (Table 8) but at EC 12, food intake and assimilation on TN1 and Nona Bokra plants increased significantly. Food intake increased on Pokkali at EC 12, but assimilation of food was not affected. Salinity did not increase food intake on IR2035 but assimilation of food increased significantly. Regardless of salinity level, intake and assimilation of food by S. furcifera was significantly greater on TN1 than on other cultivars.

Intake and assimilation of food decreased significantly on TN1 plants painted with IR2035 extract than on TN1 plants painted with TN1 extract (Table 9). In contrast, it increased significantly on IR2035 plants treated with TN1 extract, regardless of whether TN1 plants were stressed or unstressed.

Growth duration was longer and significantly fewer nymphs became adults on IR2035 resistant to S. furcifera and on salt tolerant Pokkali and Nona Bokra than on susceptible TN1 plants (Table 10). However, nymphal growth and development increased significantly on all test cultivars at EC 12 but not at EC 10, except on IR2035.

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Table 5: Effect of salinity (EC 12) on growth and biomass of four rice cultivars

Cultivar

Plant height (mm) Root length (mm) Dry weight (g)

Salinity

Control

Diff.

Salinity

Control

Diff.

Salinity

Control

Diff.

TN1 720c† 850c 130**

180c 256d 76** 23.0c 37.8c 14.8**

IR2035 640d 830d 190**

267b 391b 124**

12.6d 26.3d 13.7**

Nona Bokra

1050b 1160b 110**

352a 414a 62** 36.3b 50.6b 14.3**

Pokkali 1100a 1240a 140**

261b 321c 60** 38.5a 52.5a 14.0**

** Significant at the 0.01 probability level by LSD

Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

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Table 6: Effect of salinity (EC 12) on the chemical composition of four rice cultivars

Mineral

TN1 R2035 Nona Bokra Pokkali

Control

EC 12

Control

EC 12

Control EC 12

Control EC 12

N (g/kg) 12.5 16.4 9.3 14.5 9.1 11.9 10.5 14.2

P (g/kg) 3.7 3.3 3.2 4.1 2.8 3.0 3.4 3.3

K (g/kg) 21.0 20.1 20.8 13.4 26.0 18.0 29.6 22.0

Mg (g/kg) 2.5 1.5 2.0 2.1 2.8 1.6 3.2 1.7

Ca (g/kg) 1.0 2.0 0.8 3.4 0.7 2.0 1.1 2.5

Si (g/kg) 1.1 1.3 20.5 31.5 2.9 2.8 2.7 3.0

C (g/kg) 410 400 400 410 420 410 410 400

SP† (g/kg) 37.1 36.1 41.5 46.1 37.4 35.8 34.6 40.4

TFS†(g/kg)

63.8 68.4 64.4 56.6 36.0 52.4 53.2 42.3

Zn (mg/kg) 34 23 30 31 25 26 33 28

Fe (mg/kg) 117 149 56 68 80 120 127 143

† TFS = total free sugars, SP = soluble proteins

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Table 7 : Effect of salinity stress on production of allelochemics in leaf sheath of rice plants.

Weight of allelochemics (mg k-1)

Cultivar EC 10 Control EC 12 Control

TN1 35.5 40.0 30.8 35.7

IR2035 52.5 62.5 45.8 63.8

Nona Bokra 42.0 45.5 35.7 45.0

Pokkali 43.0 47.5 37.3 45.8

Table 8: Intake and assimilation of food by S. furcifera females on salinity

stressed plants of four rice cultivars

Cultivar EC 12 Control Diff. EC 10 Control Diff.

Food intake, mg/female/24h

TN1 14.1a† 12.5a 1.6NS 16.5a 13.4a 3.1**

IR2035 5.3c 3.9c 1.4NS 4.3c 3.2c 1.2NS

Nona Bokra 9.8b 9.3b 0.5NS 10.5b 8.2b 2.3*

Pokkali 9.1b 9.0b 0.1NS 11.7b 8.9b 2.8*

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Food assimilated, mg/female/24h

TN1 0.62a 0.59a 0.03NS 0.56a 0.51a 0.05*

IR2035 0.18c 0.14c 0.04NS 0.17c 0.12c 0.05*

Nona Bokra 0.51b 0.50b 0.01NS 0.39b 0.34b 0.05*

Pokkali 0.54b 0.52b 0.02NS 0.38b 0.35b 0.03NS

NS,*,** = Non-significant and significant at the 0.05 and 0.01 probability levels.

respectively, by LSD.

Values followed by the same letter in each column are not significantly different at the

0.05 probability level by DMRT

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Table 9: Intake and assimilation of food by S. furcifera females on susceptible TN1 and

resistant IR2035 plants painted with steam distillate extract of salinity-stressed (EC 12) or unstressed plants of four rice cultivars.

Cultivar TN1 plants IR2035 plants

Extract EC 12 Control Diff. EC 12 Control Diff.

Food intake, mg/female/24h

TN1 17.9a 16.4a 1.5NS 6.4a 5.7a 0.7NS

IR2035 11.9 10.3b 1.6NS 4.2b 3.6b 0.6NS

Nona Bokra 15.7ab 14.3ab 1.4NS 5.1b 4.7ab 0.4NS

Pokkali 13.9ab 11.7ab 2.2NS 5.3b 4.1ab 1.2*

Food assimilated, mg/female/24h

TN1 0.62a 0.58a 0.04NS 0.21a 0.18a 0.03NS

IR2035 0.49b 0.41c 0.08* 0.14b 0.11b 0.03NS

Nona Bokra 0.54ab 0.5ab 0.04NS 0.19ab 0.17ab 0.02NS

Pokkali 0.53b 0.49bc 0.04NS 0.17ab 0.18a -0.01NS

NS,*,** = Non-significant and significant at the 0.05 and 0.01 probability levels respectively, by LSD Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

Table 10: Growth and development of S. furcifera nymphs on salinity- stressed plants of

four rice cultivars

Cultivar EC 10 Control Diff. EC 12 Control Diff.

Nymphs becoming adult, %

TN1 95.0a 92.5a 2.5NS 97.5a 92.5a 5.0*

IR2035 42.5c 30.0c 12.5* 42.5c 32.5c 10.0*

Nona Bokra 82.5b 77.5b 5.0 NS 80.0b 70.0b 10.0*

Pokkali 80.0b 75.0b 5.0 NS 85.0b 75.0b 10.0*

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Development period, days

TN1 14.6c 14.8c -0.2 NS 14.8c 15.3c -0.5*

IR2035 18.2a 18.9a -0.7** 18.4a 19.1a -0.7**

Nona Bokra 15.6b 15.7b -0.1 NS 15.7b 16.3b -0.6*

Pokkali 15.6b 15.7b -0.1 NS 15.9b 16.3b -0.4*

Growth index

TN1 6.49a 6.24a 0.25 NS 6.58a 6.06a 0.52*

IR2035 2.34c 1.60c 0.74** 2.32c 1.71c 0.61**

Nona Bokra 5.28b 4.93b 0.35 NS 5.09b 4.31b 0.78**

Pokkali 5.14b 4.79b 0.35 NS 5.35b 4.59b 0.76**

----------------------------------------------------------------------------------------------------------------------

NS,*,** = Non-significant and significant at the 0.05 and 0.01 probability levels respectively, by LSD Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

Regardless of the cultivar used, adult longevity and fecundity increased significantly on plants salinity stressed at EC 12 than on unstressed plants(Table 11 and 12). At EC 10, female longevity increased on TN1 and IR2035 plants, but fecundity increased significantly only on IR2035 and Nona Bokra.

Insect population increased significantly on all cultivars at salinity level of EC 12, but population increased only on IR2035 at EC 10.(Table 13). Regardless of the level of salinity stress, Insect population increase was significantly higher on TN1 than on other rice cultivars.

These studies indicate that salinity induced changes in growth, biomass and chemical composition of rice plants, affected the susceptibility or resistance of rice cultivars to S. furcifera. Salinity stress at EC 12 increased N, decreased K and reduced the production of allelochemicals (quantified as steam-distilled extract) in rice plant leaf sheaths where S.furcifera feeds and survives. Increase in N was 56% in the salinity-sensitive and insect-resistant cultivar IR2035 as compared with 30% in Nona Bokra, 31% in TN1, and 35% in Pokkali.

Salinity stress of EC 12 significantly increased growth and development, male and female longevity, fecundity and population increase of S. furcifera. Regradless of the cultivar used, salinity effect was more evident at EC 12 than at EC 10, but the salinity effect on the magnitude and expression of resistance to the insect was more clear on IR2035 than on other cultivars. Reduction in the level of resistance in rice plants may be

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attributed to increase in N, reduction in K and reduction in allelochemics in rice plants. Lot of information is available which indicates that increase in N content in plants increased the population/infestation of insect pests. The effects of N and K on host-insect interactions are discussed in detail under the section of nutrients in this chapter.

Allelochemicals of plants profoundly affect plant-insect interactions. Allomones have been found to be associated with repellency, feeding deterrence, toxicity, or other adverse effects on insects. Allelochemicals also determine the susceptibility or resistance of plants to insects. The involvement of allelochemicals in various types of plant-insect interactions can determine the status of a plant either as a host or non-host or as resistant or susceptible host. Allomones are considered a major factor responsible for insect resistance in plants. Initiation and sustenance of feeding by phytophagous insects depends on the combined action and interaction of nutrients and allelochemicals. The studies conducted by Salim et al.(1990) is of paramount importance and provided the basis to explain the effects of salinity stress on host-insect interactions.

Table 11: Longevity of S. furcifera adults on salinity-stressed rice plants

Cultivar 10EC Contr Difference 12 EC Control Difference

Male longevity, days

TN1 16.4 ab † 15.8 ab 0.6 NS 16.4 a 13.8 a 2.6**

IR2035 9.4 c 5.5 c 3.9** 10.0 b 6.3 b 3.7**

Nona Bokra 16.7 a 16.3 a 0.4 NS 15.2 a 13.0a 2.2**

Pokkali 15.5 b 14.9 b 0.6 NS 15.3 a 13.3 a 2.0**

Female longevity, days

TN1 27.7.a 21.0 a 1.7* 22.9 a 19.8 a 3.0**

IR2035 10.4 c 6.9 c 1.5* 11.4 c 6.9 c 4.5**

Nona Bokra 19.5 b 18.7 b 0.8 NS 19.3 b 16.8 b 2.5**

Pokkali 19.5 b 18.7 b 0.8 NS 19.3 b 16.8 b 2.7**

NS,*,** = Non-significant and significant at the 0.05 and 0.01 probability levels

respectively, by LSD

† Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

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Table 12: Fecudity of S. furcifera females on salinity-stressed rice plants

Fecundity (no. of eggs laid by 10 female)

Cultivar Salinity Control Difference

10 EC

TN1 1089 a † 1058 a 31 NS

IR2035 192 c 118 c 74*

Nona Bokra 971 b 911 b 60*

Pokkali 1040 ab 1005 a 35 NS

12 EC

TN1 1068 a 994 a 74*

IR2035 199 c 138 c 61*

Nona Bokra 916 b 835 b 81*

Pokkali 1012 a 940 a 72*

NS, * = Non-significant and significant at the 0.05 and 0.01 probability levels respectively,

by LSD † Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

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Table 13: Population of S. furcifera on salinity-stressed rice plants at

40 days after infestation

No. † insect recovered No. † insect recovered

Cultivar 10 EC Control Difference 12 EC Control Difference

TN1 491 a 772 a 19 NS 463 a 435a 28**

IR2035 54 c 29 c 25* 60 c 27 33**

Nona Bokra 255 b 232 b 23 NS 282 b 255 b 27**

Pokkali 297 b 282 b 15 NS 281 b 252 b 29**

NS,*,** = Nonsignificant and significant at the 0.05 and 0.01 probability levels

respectively, by LSD.† Values followed by the same letter in each column are not significantly different at the 0.05 probability level by DMRT

7.3. ALUMINIUM STRESS

More than 15 per cent of the earth’s crust is made up of aluminium oxide (Al2O3) and hence aluminium (Al) is considered one of the major elements of soil constituents. In neutral and alkaline soils, its solubility is very low and hence does not pose any toxicity problem for plant growth. However, in acid soils, solubility of Al increases sharply and more than half of the cation exchange sites may be occupied by Al and hence may prove toxic for plant growth.

Aluminium toxicity is one of the major growth limiting factors for rice in many acid soils. Aluminium causes reduction in root length which consequently reduces uptake of nutrients and adversely affects the growth of shoot. Its toxicity causes various morphological and chemical changes which may profoundly influence the interactions of rice plant and insects pests. Information on this aspect was not available. Studies on the effects of Al on the plant height, tillering and chemical composition of rice plant and on the host-insect interactions were carried out (Salim and Saxena, 1992). Since it is first study of its nature, therefore, it is necessary to describe it in detail.

These studies were carried out at 29/21 oC day/night temperature and 70 per cent relative humidity. The rice plants were grown in culture solution. For the preparation of culture solution dissolved 182.8 g ammonium nitrate, 71.2g sodium phosphate monobasic monohydrate, 142.8g potassium sulphate, 234.7g calcium chloride, 648g magnesium sulphate separately in distilled water and made volume to 2L in each case and stored in glass bottles. Similarly dissolved 3.0g manganous chloride, 0.148g ammonium molybdate, 1.976g boric acid, 0.70g zinc sulphate, 0.062g cupric sulphate, 15.4g ferric chloride and 23.8g citric acid separately in 150 ml distilled water and pooled in a 5 L. glass beaker. Then added 100 ml concentrated sulphuric acid and made the volume to 2 L. with distilled water. For preparation of stock solution of Al, dissolved

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482.86g of aluminium chloride hepta hydrates in distilled water and made the volume to 2L.

For preparation of culture solution, four litre standard culture solution was prepared by adding 5ml of each stock solution viz. N.P.K.Ca.Mg. and micronutrients in 1970 ml distilled water in a plastic pan one by one. With this procedure, required quantities of standard culture solution and culture solutions with different levels of Al were prepared. The seeding of test cultivars were grown as described in case of salinity.

Addition of Al in the culture solution reduced the height of rice plants. The magnitude of reduction in plant height increased with increasing Al concentration in the culture solution and also with increase in the period of stress (Table 14). Application of Al at 30 ppm reduced plant height by 4.6-7%, 11.4-16% and 12.0-18% respectively at 12, 24 and 36 days Al-stress. The respective values at high Al level (90 ppm) in culture solution were 14.2-17%, 44.1-53% and 45.9-57%. Number of tillers/plant were significantly less in the plants grown under Al-stress than those grown in the standard culture solution without Al. The magnitude of reduction in tillering increased with increase in the concentration of Al in the growth medium and with increase in the period of Al stress. Aluminium stress significantly reduced the growth of rice roots. The reduction was comparatively more with increase in the duration of stress.

Fig. 7. Effect of Aluminium stress on growth of rice plants raised in culture solution

Likewse it was more at high level than at low level of Al. Biomass of shoot as well as root of rice plants grown under Al-stress were significantly small than the plants grown in

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culture solution without the addition of Al. Application of Al reduced P, Mg and total free sugars but increased N. Si, Zn and Al content in rice plants. Al-induced stress in the rice plant significantly reduced growth and biomass and altered its chemical composition.

In the orientation test, IR2035 plants, receiving Al, attracted significantly more number of S. furcifera female adults at 1% level by chi-square test. (Table15). Another test conducted to record the settling responses of the insect for 72 hours indicated that comparatively more number of S. furcifera adults alighted on IR2035 plants, receiving Al over the control.

Intake and assimilation of food by S. furcifera reduced significantly on rice plants which received Al (Table 16). The magnitude of reduction of both intake and assimilation of food by the insect was more at high level of Al. The amount of honey dew excreted by the insect also reduced significantly on the plants which contained high content of Al. Application of Al to rice plants had no effect on the nymphal development period of S. furcifera. Significantly more nymphs of the insect died during development on IR2035 plants which were grown in the culture solution with 90 ppm Al. Growth index of the insect was reduced significantly on plants grown under Al-stress. Reduction in the growth index was of more magnitude at high level of Al (90 ppm)

Table 14: Effect of Aluminium on rice plant growth.

Al (ppm) in culture

solution

Plant height (cm)

TN 1 IR 2035

Tillers/plant(No)

TN 1 IR 2035

Root length (cm)

TN 1 IR 2035

12 Days stress

0 30.2a 29.9a 3.6a 3.4a 14.3a 24.1a

30 28.8 27.6b 2.9b 2.9b 14.3a 24.0b

90 25.9 24.6c 1.0c 1.0c 14.3a 24.0c

24 Days stress

0 56.2 56.1a 5.7a 5.5a 17.4a 30.0a

30 49.8 47.1b 4.8b 3.7b 15.7b 25.1b

90 31.4 26.0c 2.0c 1.2c 14.9c 24.3c

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36 Days stress

0 66.5 66.3a 6.7a 6.5a 24.1a 34.0a

30 58.5 53.9b 5.3b 4.0b 21.2b 29.4b

90 36.0 25.5c 2.0c 1.2c 15.4c 26.7c

In a column, means followed by a common letter are not significantly different at the 0.05 probability level by Duncan’s multiple range test (DMRT).

15: Effect of Aluminium induced changes in rice plants on the orientation and

settling responses of S. furcifera

Treatment Chi Square

Orientation Settling

TN1 (Alo) vs TN1 (Al 30 ppm) 14.83 ns 12.21 ns

TN1 (Alo) vs TN1 (Al 90 ppm) 18.00 na 4.47 ns

IR2035 (Alo) vs IR2035 (Al 30 ppm) 44.33** 9.11 ns

IR2035 (Alo) vs IR2035 (Al 90 ppm) 46.56** 36.03 ns

X2 0.05 = 21.0, x2 0.01 = 26.2 for orientation and x2 0.05 = 36.4; x2 0.01 = 43.0 for settling responses

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Table 16: Effect of Aluminium-induced changes in rice plants on different biological

parameters of S. furcifera.

Biological parameters

TN1 IR2035

0 30 90 0 30 90

Food intake (mg/female/24h) 18.6 a 12.8 b 9.5 c 5.7 a 3.8 b 2.7 c

Food assimilation (mg/female/24h)

0.61 a 0.44 b 0.40 c 0.18 a 0.1 b 0.05 c

Honey dew (mg/female/24h) 18.6 a 12.3 b 9.1 c 5.5 a 3.7 b 2.7 c

Nymphal becoming adults (%) 92.5 a 80.0 a 60.0 b 25.0 a 22.5 a 17.5 a

Nymphal duration (days) 14.5 a 14.8 a 15.2 a 18.2 a 18.5 a 18.6 a

Growth index 6.40 a 5.41 b 3.96 c 1.37 a 1.2 a 0.9 a

Male longevity (days) 12.0 a 10.6 b 9.9 c 8.1 a 6.6 b 5.6 c

Female Longevity (days) 16.3 a 15.8 a 14.0 b 9.6 a 7.5 a 6.5 a

Fecundity/female 135 a 113.0 b

95 c 5.0 a 4.0 a 4.0 a

Population/pair of insect 152 a 66.0 b 29 c 7.0 a 5.0 a 3.0 b

In a row within a variety, means followed by a common letter are not significantly

different at probability level 0.05 by DMRT.

Adult longevity of both male and female of S. furcifera reduced significantly on plants grown under Al-stress. The adult life of the insect was reduced significantly more with increase in the application Al to host plant. Application of Al to IR2035 had no effect on the fecundity of the insect. However, it was reduced significantly on TN1 plants grown under Al-stress and magnitude of reduction increased with increase in the application of Al. Population build up of S. furcifera is considered an important criterion of assessing the suitability of rice plants because it represents the combined effects on growth and development, adult longevity and fecundity, intake and assimilation of food etc. Population of the insect was significantly low on the plants grown under Al stress. Adverse effect on the population build up of the insect was more at high level of Al. Although application of Al to rice plant had adverse effects on different biological

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parameters of the insect but, application of Al cannot be recommended to rice plant because it significantly reduced plant growth and biomass.

7.4. NUTRIENTS

A nutrient is defined as a compound required for normal growth, development and maintenance of an organism’s functions. Some of the nutrients which must be obtained from diet are called essential nutrients. The essential amino acids for insects include arginine, histidine, isoleucine, leucine, lysine, methionine, phyenyl alanine, threonine, tryptophan and valine. Carbohydrates are important components of the diet of most insects but are not necessarily essential because these can be synthesized from fats or amino acids. Sterols and polysaturated fatty acids are essential nutrients. All insects require water-soluble B-complex vitamins. Most phytophagous insects require a dietary source of ascorbic acid (Vitamin C) (Chapman, 1998). Plant as a host must be able to provide nutrients to insects that can support the growth, development and reproduction of insects. Plant species vary in their adequacy as hosts for most polyphagous insects. The success of phytophagous insects in growth, development and reproduction depends upon their ability to efficiently ingest, digest and metabolize plant N with optimum levels of leaf water (Scriber and Slansky, 1981).

The host plant may be deficient in certain nutritional elements required by the insect and hence prove unsuitable host. The nutritionally deficient plant may cause antibiotic and antixenotic effects on the insect (Arora and Dhaliwal, 2005). The chemical composition of plants is of fundamental significance in their acceptance or rejection for food and oviposition by insects.

Plant nutrients are the elements or simple inorganic compounds, indispensable for the growth of plants and are not synthesized by the plant during normal metabolic processes. All 16 elements must be present in optimum amounts and forms usable by rice plants (De Datta, 1981). Excessive application of certain nutrients such as N may not be toxic to the plant, but it may have adverse effect by producing softness in the tissue and may induce susceptibility against insect pests, diseases, lodging etc.

Innumerable interactions of one element with another are known (Hiatt and Leggett, 1974) and these interactions could be of major significance. The addition of P causes the deficiencies of certain micronutrients. Hewitt (1963) reviewed the organic constituents that change under mineral deficiencies. Fertilizers change metabolism and disrupt the relations between plants and pests (Gurevich et al., 1971). The development of poor nutrient sap offers a potent challenge to the viability of insects due to the deficiency of some of the amino acids in the plants (Boyd, 1970).

Nutritional disorders are caused by a nutrient deficiency or toxicity of an element or more elements or substances (Tanaka and Yoshida, 1970). Excessive application of nutrients or nutritional deficiencies may cause substantial changes in the chemical composition of plants which may profoundly effect host-insect interactions (Salim, 1988). Effects of nutrient deficiencies or excessive amounts in plants on host-insect interactions are briefly described.

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7.4.1. NITROGEN (N)

Nitrogen is essential for all living organisms. The synthesis of cellular proteins, amino acids, nucleic acids, purine and pyrimidine nucleotide are dependent upon N. It is the most abundant mineral element in plant tissues which is derived from soil. However, excess N may cause significant biochemical changes in plants and may lead to nutritional imbalances (Mills and Jones, 1979). Excess N supply raised the level of soluble N in the plant (Goodall and Gregory, 1947) and increased nitrate accumulation (Bonner, 1950). Carbon and soluble proteins increased with increase in the level of N in the culture solution. Silica content decreased drastically with increase in the application of N. The C:N ratio decreased considerably with increase in the quantity of N in culture solution (Salim and Saxena, 1991).

Nutrient of host plant can influence the expression of host plant resistance to insect attack (Leuck and Hammons, 1974). Excessive application of N to plants generally induce susceptibility or reduce the level of resistance in plants to insect pests by inducing succulence in plants or by changing chemical composition of plants or by both.

The importance of N in insect-plant interactions particularly plant resistance to pests has been reviewed by the scientists. In more than 115 different studies, increased plant N was found to increase insect damage, growth, fecundity or insect population, but in some instances insect populations or damage to plants decreased with increase in N application. The digestion capacity of an insect increased with increase in N level in food (Scriber, 1984) and larval developmental period was shortened with increase of N level in the food (Al-Zubaidi and Capinera, 1984). Fecundity of leafhoppers was positively correlated with soluble N in rice plant (Hinckley, 1963). Likewise, fecundity and population levels of rice leafhoppers were linked with asparagines content in the rice plant (Sogawa, 1971). Nymphal survival of S. furcifera on resistant varieties increased slightly with an increase of N application (Pablo, 1977). In contrast, high N level in the food was found to prolong instar duration in few species of leafhoppers feeding on Holcus lanatus (Prestidge, 1982). High population densities of leaf sucking insects were found associated with high concentration of total N and reducing sugars in leaves (Fritzche et al., 1957). Peak reproduction rates of grass leafhoppers, Dicranotropis hamata (Waloff and Soloman, 1973) and spruce aphid, Elatobium abietinim were correlated with amino acid levels in host plant leaves. Nitrogen stress in host plant during the reproduction phase of insect drastically affected their populations (Mc Neill and Southwood, 1978).

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Fig. 8. Effect of N on growth of rice plant Fig. 9. Effect of N on root growth

Fig. 10. Chamber to study orientational

response of planthoppers

Fig. 11. Cages to study settling response

of planthoppers

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Nitrogen fertilizers can also influence host plant resistance through changes in production of allelochemics and in interaction of nutrients and allelochemicals. Increasing plant N through fertilization lowered phenolic content and lignification (Kiraly, 1976), while decreasing N availability increased levels of phenolics (Haklbrock and Grisebach, 1979).

Salim and Saxena (1991) carried out studies on the effects of N deficiency or excessive amount of N on rice plant and host-insect interactions. These studies were conducted at 29/21o C day/night temperature. Nitrogen levels in the culture solution were 5 ppm (low), 40 ppm (optimum) and 200 ppm (high). These studies were carried out by growing rice plants in culture solution. In these studies all the parameters of host-insect interactions were studied in detail. Chemical analysis of rice plants grown at different levels of N was also done.

Different levels of N in culture solution profoundly affected the growth and biomass of both test rice varieties (TN1 and IR2035) (Table 17). Plant height, number of tillers/plant and shoot as well root biomass was significantly less in N-deficient plants of TN1 as well as IR2035. Root length significantly decreased with excessive application of N to rice plants. While shoot biomass increased with increase in the application of N in rice cultivar IR2035. Excessive application of N induced succulence in the rice plant.

Nitrogen deficiency or application of excessive amount of N caused drastic changes in the chemical composition of rice plants. Increase in N application increased more than seven times N content in TN1. Silica content decreased drastically with increase in N application. Iron content in rice plant increased with increase in N application (Table 18).

The studies revealed that significantly more S. furcifera females were attracted to and settled on plants treated with higher than lower concentrations of N on both susceptible and resistant rice plants (Table 19). Intake and assimilation of food by its females increased significantly with an increase in N concentration (Table 20). Interaction between variety and N level was significant for food intake. Fewer nymphs of the insect became adults on N-deficient plants and those reaching the adult stage required more developmental time (Table 21). Insect growth increased with increasing N concentration. Adult longevity and fecundity increased with increasing N (Table 22). The magnitude of increase was greater on resistant IR2035 than on susceptible TN1 plants. Adult longevity and fecundity of insect had a significant interaction between variety and N concentration. Population of S. furcifera increased significantly with increase in the application of N to rice plant (table 23).

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Table 17: Growth and biomass of rice plants grown in culture solution with

In culture Plant Tillers/ Root Shoot biomass (g) Root biomass (g)

Solution Height plant Length Fresh Dry Fresh Dry

N (ppm) (mm) (no.) (mm)

TN1

5 391c 3.8c 301a 11.2b 3.0b 13.1c 1.7c

40* 649b 10.1b 300a 84.7b 18.8a 40.7a 5.3a

200 719a 14.1a 145b 102.8a 18.1a 31.0b 4.4b

IR2035

5 478c 4.1c 508a 9.7c 2.5c 12.8c 1.6c

40 738b 10.1b 478b 71.1b 14.4b 43.8a 5.8a

200 793a 13.3a 202c 93.6a 16.9a 36.7b 4.8b

Within columns and cultivars, means followed by the same letter are not significantly different at the 0.05 probability level by DMRT.

Table 18: Effect of N on the chemical composition of rice plant

Constituents TN1 I R2035

5 40 200 400 5 40 200 400

N (%) 0.57 1.03 3.11 4.42 0.64 1.03 3.09 3.72

P (%) 0.29 0.29 0.23 0.33 0.39 0.31 0.30 0.40

K (%) 1.69 1.87 1.17 1.70 1.97 2.05 1.44 2.00

Mg (%)

0.14 0.20 0.15 0.15 0.21 0.23 0.20 0.20

Ca (%)

0.09 0.07 0.06 0.07 0.09 0.08 0.06 0.09

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Si (%)

3.72 2.18 1.98 0.31 4.57 2.57 2.36 0.34

C (%) 37.0 39.0 40.0 41.0 36.0 38.0 39.0 42.0

Zn (ppm)

23.0 14.0 8.00 11.0 26.0 12.0 11.0 11.0

Fe (ppm)

25.0 44.0 68.0 458.0 32.0 52.0 101.0 208.0

C: N 65.0 38.0 13.0 9.30 56.0 37.0 13.0 11.3

SP (%)

3.57 3.40 5.78 5.74 3.42 3.37 5.32 5.78

TFS (%)

3.66 3.69 2.99 3.52 2.08 3.38 2.15 2.86

SP = Soluble protein; TFS = Total free sugars

Table 19: Effect of different levels of N in the culture solution on the orientational and settling response of S. furcifera

Treatment Orientation Settling

TN1 (5 ppm) vs TN1 (40 ppm)

67.8** 164.3**

TN1 (200 ppm) vs TN1 (40 ppm)

57.7** 56.2**

IR 2035 (5 ppm) vs IR 2035 (40 ppm)

33.2** 287.2**

IR 2035 (200 ppm) vs IR 2035 (40 ppm)

161.2** 53.9**

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Table 20: Effect of different levels of N in culture solution on intake and assimilation of food by S. furcifera

N (ppm) in culture

Solution

TN1 IR2035 DIFF.

Food intake (mg)/female/24 h

5 6.0c 3.1b 2.9**

40 10.0b 4.8a 5.2**

200 11.7a 5.7a 6.0**

Food assimilated (mg)/female/24 h

5 0.45b 0.14b 0.31**

40 0.53ab 0.21ab 0.32**

200 0.55a 0.23a 0.32**

Honey dew excreted (mg)/female/24 h

5 5.6c 2.9b 2.7**

40 9.5b 4.6a 4.9**

200 11.1a 5.5a 5.6**

Means followed by the same letter do no differ significantly at 5% level by DMRT

**Significant at 1% level by LSD.

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Table 21: Effect of different levels of N in culture solution on the growth and

development of S. furcifera

N(ppm) TN1

IR2035 DIFF.

Nymphs becoming adults (%)

5 75.0b 15.0b 60.0**

40 95.0a 22.5ab 72.5**

200 97.5a 32.5a 65.0**

Nymphal duration (days)

5 15.6a 19.5a -3.9**

40 15.1ab 19.0ab -3.9**

200 14.7b 18.7b -4.0**

Growth index

5 4.83b 0.79b 4.04**

40 6.30a 1.19b 5.11**

200 6.66a 1.74a 4.92**

Means followed by the same letter do no differ significantly at 5% level by DMRT

**Significant at 1% level by LSD.

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Table 22: Effect of different levels of N in culture solution on adult longevity and

fecundity of S. furcifera

N (ppm) TN1 IR2035 DIFF.

Male longevity (days)

5 12.2c 4.3c 7.9**

40 13.9b 5.0b 8.9**

200 14.9a 8.1a 6.8**

Female longevity (days)

5 18.4c 6.6c 11.8**

40 20.5b 7.9b 12.6**

200 22.1a 13.2a 8.9**

Fecundity (no. of eggs/10 females

5 903b 19c 884**

40 1079ab 88b 991**

200 1170a 194a 976**

Means followed by the same letter do no differ significantly at 5% level by DMRT.

**Significant at 1% level by LSD`.

206

Table 23: Effect of different levels of N in culture solution on the population build

up of S. furcifera

N (ppm) TN1 IR2035 DIFF.

5 312c 6c 306**

40 1619b 21b 1598**

200 2184a 58a 2126**

Means followed by same letter do not differ significantly at 5% level by DMRT.

Significant at 1% level by LSD.

7.4.2. PHOSPHORUS (P)

Phosphours is an essential component of living matter. Second to N, it is the most limiting element in the soil. Phosphorus is one of the prime examples of interactions with other elements especially the micro nutrients. It is associated with many structural components of plants and affects metabolism of sugar phosphates, nucleic acids, nucleotides, coenzymes and phospholipids (Clark, 1982).

Phosphorus deficiency can also affect plant-insect interactions. Survival and development rates of migratory grass hopper, Melanoplus bilitucalis increased on P-deficient wheat plants (Smith, 1960). In contrast, P-deficiency adversely affected greenbug, Schizaphis graminum on wheat (Daniels, 1957). The resistant genotypes of cotton to whitefly, Bemisia tabaci showed higher concentration of P, K and Mg as compared to susceptible ones. Higher P in plant was correlated with shoot fly, Atherigona soccata susceptibility in sorghum (Khurana and Verma, 1983). Higher level of P in cotton leaves was responsible for imparting considerable resistance to cotton whitefly, Bemisia tabacci. Highly significantly negative correlation was obtained between its population and P contents in cotton leaves (Vir, 1989).

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Table 24: Effects of P on growth and biomass of rice plant (30 DAT)

P(ppm) Plant height (cm) Tiller (No.) Dry biomass (g)/plant

In Culture TN1 IR2035 TN1 IR2035 TN1 IR2035

Solution

1 50.8c 54.1c 6.3b 7.2b 8.0c 9.2c

40 57.2b 60.9b 7.5a 7.5a 15.4b 12.1b

10 62.7a 62.2a 7.6a 8.2a 17.9a 16.2a

In a column, means followed by a common letter are not significantly different at the 0.05 probability level by LSD.

Table 25: Chemical compositions of rice plants grown in culture solution at

different levels of P.

Element

P in culture solution

TN1 IR2035

1 10 40 1 10 40

Nitrogen 1.61 1.35 1.42 1.68 1.29 1.29

Phosphorus 0.03 0.39 0.83 0.03 0.41 0.87

Potassium 1.82 1.84 1.79 1.90 1.92 1.85

Magnesium 0.29 0.30 0.33 0.31 0.33 0.41

Calcium 0.10 0.11 0.10 0.13 0.12 0.12

Silica 1.70 1.07 0.98 1.87 1.36 1.49

Carbon 39.00 40.00 39.80 39.50 40.20 40.16

Zinc 15.00 10.00 9.50 11.00 8.9 13.00

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Iron 56.00 47.00 40.00 54.00 49.00 38.60

Aluminium 336.00 229.00 175.00 256.00 274.00 230.00

Table 26: Effect of different levels of P in rice plant on biology and behaviour

of S. furcifera

Biological parameters

TN1 IR2035

1 10 40 1 10 40

Food intake (mg/female/24h) 6.3c 7.7b 9.5a 3.9a 2.9ab 2.2b

Food assimilation (mg/female/24h)

0.64b 0.67ab 0.75a 0.24a 0.22a 0.23a

Hymphas becoming adults (%)

92.0a 95.0a 98.0a 30.0a 22.0a 25.0a

Nymphal duration (days) 15.0a 14.5a 14.4a 17.2b 18.3a 17.9ab

Growth index 6.1a 6.6a 6.8a 1.7a 1.2a 1.4a

Male longevity (days) 10.9b 11.0b 12.6a 7.9b 7.2b 9.5a

Female longevity (days) 19.9c 20.7b 23.2a 9.2b 8.1c 10.2a

Fecundity/female 779c 852a 945a 70.0a 60.0a 63.0a

Population/pair 391c 551ab 684a 58.0a 33.0b 47ab

In a row, means followed by a common letter are not significantly different for each rice variety at 5% level by DMRT.

Information on the effect of P on host-insect interactions was quite limited. Salim (1988) conducted studies on the effects of different levels of P in

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the culture solution on plant growth, chemical composition and the levels of resistance or susceptibility of rice plant against whitebacked planthopper, Sogatella furcifera.

Both low (1 ppm P in culture solution) as well as high level (40 ppm P in culture solution) of P decreased plant height, root length and biomass of rice plants (Table 24). Reduction in rice plant growth was of greater magnitude with high than low level of P. Application of different levels of P caused changes in the chemical composition of rice plants (Table 25). Increase in P application increased P content but reduced Si and Fe content in rice plant.

Increased P application to susceptible TN1 plants increased intake and assimilation of food by the insect (Table 26). Food intake by the insect decreased significantly on resistant IR2035 plants receiving the high dose of P. Low or high doses of P had no effect on assimilation of food. Different levels of P had no effect on growth and development of S. furcifera except at low P level. The developmental period of the insect was reduced on resistant IR2035 plants. Adult life of both male and female of the insect increased at high P contents in rice plants. At low P contents adult female longevity was decreased on susceptible TN1 plants but increased on resistant IR2035 plants. Different levels of P content in rice plant had no significant effect on the fecundity of the insect. Population of S. furcifera was comparatively greater on TN1 plants with greater concentration of P; however its populations were greater on IR2035 plants at the low concentration of P compared with the plants which received optimum amount of P. The effect of P on host-insect interation was not consistent. However, more in depth studies are required on this important aspect.

7.4.3. POTASSIUM (K)

Potassium is needed by plants for synthesis of sugars, proteins and starch. Deficiency of K increased soluble N (Van Emden, 1966) and accumulation of soluble sugars, amides and amino acids (Vaithilingam and Baskaran, 1982), while increased K enhanced deamination of amino acids and reduced sugars in the plant sap (Chaboussou, 1972). Low K application significantly reduced rice plant height, tiller number and root length. Increase in K to 200 ppm in the culture solution did not affect TN1 plants but increased the height and tiller number of IR2035 plants(Table 27). Shoot and root weights of rice plants increased with an increase in K application up to 40 ppm but not thereafter. Plant biomass decreased at low level of K in culture solution (Table 28). Potassium deficient plants had short, droopy and dark green leaves (Salim and Saxena, 1991).

Deficiencies of K predispose plants to herbivore attack. Its deficiencies in soil or nutrient solution increased N content in plants, which enhanced insect population. (Metcalfe, 1970). Increase in the application of K, decreased N content in rice plant (Table 29). At low level of K application (2 ppm) in culture solution N content in the rice plant were high (1.62-1.75 %) than those rice plants which received optimum (40 ppm) or high level of K (200 ppm) (0.83-1.10 % N).

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Potassium deficiency increased soluble carbohydrates, reducing sugars and amino acids and impaired synthesis of starch, glycogen and proteins. Increased sugars in the plant sap, favour insect growth. Increasing K level led to accumulation of more phenols, which probably contributed to increase insect resistance in some rice cultivars. Plants receiving high levels of K were less preferred. (Vaithilingam and Baskaran, 1982).

Increase in K in rice plant caused reduction in the feeding rate of brown plant hopper, Nilaparvata lugens (Vaithilingam et al., 1976) and decreased population of N. lugens and green leafhopper, Nephotettix spp. (Ittyavirah et al., 1979).

Deficiency of K in rice plant significantly increased food intake, assimilation of food and honey dew excretion by S. furcifera (table 30). Increase in K application from 40 to 200 ppm in culture solution had no significant effect on food intake and honey dew excretion. Higher level of K in rice plant depressed the assimilation of food by the insect and also decreased adult life and population build up. High level of K reduced the growth index and prolonged the developmental period of S. furcifera. Adult longevity and fecundity of the insect was reduced with increase in the K application to rice plants. Deficiency of K in rice plant enhanced population build up of S. furcifera and its high content in plants suppressed the population build up of the insect.

Applying of increasing doses of K to rapeseed mustard plants caused adverse effects on the reproduction and excretion of honey dew by mustard aphid, Lipaphis erysimi possibly due to decreased levels of amino acids (aspartic acid and glutamic acid) and amides in leaf sap. (Bhat and Sindhu, 1983). Strong antibiosis in rapeseed in 2 leaf seelings of T27 (E. sativa), EAL 9 and NIE 2 (Brassica nigra) and CE 9 and DLC 2 (B. carinata) was observed (Chander, 1993).

Critical analysis of literature on the effect of K on the host-insect interactions reveals that in majority of the cases, low level of K stimulated insect development, while in other cases a decrease in K level had either depressed development or had no effect. In some cases, high level of K either stimulated development or had no effect, while medium level of K had no effect or had depressing effect on development. It can be concluded that application of K to plants induces resistance in plants against insect pests.

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Table 27: Plant growth at different levels of K in the culture solution

K (ppm)

Plant height (cm) Tillers/hill (No) Root length (cm)

TN1 IR2035 TN1 IR2035 TN1 IR2035

10 DAT

3 36.3b 35.3b 5.4b 5.4b 13.5b 14.1b

40 42.4a 39.5a 6.1a 5.8a 14.3a 14.9a

200 42.4a 40.2b 6.3a 6.0a 14.1a 15.1a

30 DAT

3 43.0c 39.9b 6.9a 7.1b 25.7c 27.2c

40 59.2b 60.7a 7.3a 8.1a 25.5b 30.1b

200 63.5a 60.7a 7.3a 8.4a 27.1a 35.7a

50 DAT

3 63.6b 57.1c 11.2b 13.3c 37.7b 41.8b

40 71.8a 76.0b 12.4a 13.7b 38.6a 48.8a

200 71.8a 78.5a 12.6a 14.6a 38.2a 48.5a

In a column, means followed by a common letter are not significantly different at 5% level by DMRT within each age group of rice plants.

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Table 28: Biomass of rice plant grown in culture solution at different levels of K

K (ppm)

Shoot biomass (g) Root biomass (g)

TN1 IR-2035 TN1 IR-2035

Fresh biomass (g)

3 43.51c 38.00 20.68b 25.25b

40 77.49a 68.09a 38.18a 46.25a

200 67.46b 61.15b 38.17a 47.43a

Dry biomass (g)

3 11.04b 10.57b 2.63b 3.20b

40 17.20a 15.72a 4.68a 5.78a

200 15.56a 14.18a 4.93a 5.57a

In a column, means followed by common letter are not significantly different at 5% level by DMRT.

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Table 29: Chemical composition of rice plants grown in culture solution at

different levels of K

Constituents

Contents in rice plants at K (ppm) in culture solution

TN1 IR2035

3 40 200 3 40 200

N (%) 1.62 0.98 0.83 1.75 1.10 0.90

P (%) 0.31 0.26 0.22 0.40 0.34 0.30

K (%) 0.25 1.79 2.33 0.28 2.16 2.72

Mg (%) 0.28 0.19 0.10 0.27 0.25 0.17

Ca (%) 0.08 0.08 0.06 0.09 0.09 0.08

Si (%) 2.50 1.90 1.57 2.72 2.24 2.00

C (%) 40.00 39.00 39.00 39.00 39.00 38.00

Zn (ppm) 16.00 9.00 8.00 17.00 12.00 10.00

Fe (ppm) 55.00 50.00 41.00 51.00 54.00 58.00

Soluble proteins (%) 3.82 3.40 2.86 3.98 3.79 3.51

Total free sugar (%) 7.48 6.93 6.70 6.48 4.58 5.13

Allelochemicals (%) 7.15 8.55 9.35 4.90 11.75 12.80

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Table 30: Effect of K-induced changes in rice plant on the biology and behaviour of S.furcifera

Biological parameters

TN1 IR2035

3 40 200 3 40 200

Food intake (mg/female/24h)

13.1a 10.3b 9.1b 4.8a 3.9ab 3.7b

Food assimilation

(mg/female/24h)

0.62a 0.53b 0.46c 0.26a 0.20a 0.11b

Honey dew (mg/female/24h)

12.4a 9.8b 8.6b 4.5a 3.7ab 2.6b

Nymphs becoming adults

(%)

97.5a 92.5a 90.0a 27.5a 20.0a 15.0a

Nymphal duration (days)

14.3b 15.0ab 15.6a 17.5c 18.5b 19.6a

Growth index 6.82a 6.18ab 5.81b 1.56a 1.08ab 0.77b

Male longevity (days)

14.6a 13.2b 11.8c 8.4a 6.9b 6.0b

Female longevity (days)

20.7a 18.9b 16.6c 9.5a 7.9b 6.8b

Fecundity/female 134.0a 120.0ab 100.0 5.0a 4.0b 3.0c

Population/pair 133.0a 110.0ab 89.0b 10.0a 7.0b 4.0c

In a row, means followed by a common letter are not significantly different at probability 0.05 by LSD.

215

Increase in the population or S. furcifera at low level of K application may be attributed to increase in N content, total free sugars, and decrease in the quantity of K and allelochemics in the rice plant. The quantity of allelochemics was comparatively low in rice plants grown at low level of K and their quantity increased with increase in the application of K (Salim and Saxena, 1991).

Allelochemics in plants strongly influence the chemical environment of plants and hence play an important role in suppressing the population and adversely affecting the biology and behaviour of insects. Application of K to plants may play dual role: increase in plant growth, biomass and yield and increase in the expression of resistance against insect pests. Use of such resistance inducing agents as an adjunct to genetic resistance may play an important role in the management of insect pests. In general, K deficiency leads to accumulation of soluble carbohydrates, reducing sugars, and amino acids, impairs synthesis of starch, glycogen, and proteins and reduces the respiratory substrate and oxidative photophosphorylation rates. These effects predispose the plants to insects and other herbivore attack. Increased K produces better proteogenesis, a physiological phenomenon associated with deamination of amino acids and reducing sugars in the plant sap, which otherwise favour insect growth. It is believed that plants receiving high level of K accumulate more phenols and other allelochemicals. The titre of B-glucosidase, capable of oxidizing sugars into phenols is raised by higher levels of K. Peroxidase polyphenol oxidase enzymes are activated by enhanced K-ions in the plant system which plays an important role for the oxidation of phenols into quinines, which have been implicated in plant defences against insect pests.

7.4.4. IRON (FE)

Most agricultural crops require less than 0.5 ppm Fe in the soil, whereas its total level in soil is about 20000 ppm. Its deficiency may be caused by high pH of alkali soil under submerged conditions, high pH of calcareous soils under upland conditions, or inadequate soil reduction under submerged conditions. Deficiency of Fe occurs more frequently on neutral to alkaline upland soils, while its toxicity is found only on low land rice. Impaired translocation of Fe from root to shoot in high pH rooting media is one of the causes of Fe deficiency. Iron toxicity is widespread nutritional disorder in Asia, West Africa and South America and is especially frequent on heavy soils in rice growing areas. Iron toxicity symptoms become visible in rice leaves containing more than 300 ppm Fe. Nutritional disorders of rice that are associated with Fe toxicity are bronzing in Srilanka, Akagare type 1 in Japan, and Akiochi in the Republic of Korea. In case of bronzing, tiny brown spots appear on green leaves starting from the tips, followed by general browning of leaf blades and death of lower leaves. Symptoms of akagare type 1 include: turning of leaves first dark green, then small reddish-brown spots appear near the tips of older leaves, followed by browning and death of leaves. In case of akiochi, plants are vigorous initially but growth declines during heading stage, followed by death of lower leaves and discolouration of panicles. Chlorosis is typical symptom of Fe deficiency in rice(Yoshida, 1981). In

216

Pakistan in some cases Fe deficiency symptions are observed under field conditions but its toxicity does not occur.

Intense yellowing of interveinal leaf tissue was observed on rice plants which were grown at low level of Fe (0.02 ppm Fe) in culture solution. Leaf veins became bright green. The deficiency symptoms first appeared on young leaves. Iron deficient leaves appeared chlorotic and whitish. Due to application of high level of Fe (40 ppm in culture solution) the spots first appeared on the leaf tips and then spread towards the bases of leaves. The roots of affected plants were dark brown in colour. Iron is involved in various key metabolic reactions therefore, its deficiency or excess application imposes stress on rice plant. Such stress may induce strain in rice plant and can cause profound changes in the physiological and biochemical plant processes which subsequently alter the biology and behaviour of insect pests which feed on them (Salim and Saxena,1992). They carried out studies on the effects of deficiency and excessive application of Fe on rice plant and on biology and hehaviour of Sogatella furcifera. Rice plants grown at high level (40 ppm) or low level (0.02 ppm) of Fe had significantly less number of tillers/plant than grown in optimum level of Fe (2 ppm) in culture solution after 36 DAT. Root length of rice plant was less at low as well as high level of Fe (Table 31).

Fig. 12. Iron toxicity in rice

217

Iron deficiency as well as toxicity significantly reduced the biomass of rice plant (table 32). Application of different levels of Fe to rice plant caused changes in the chemical composition of rice plant (Table 33). Higher application of Fe to rice plant increased the quantity of N and reduced that of P in the leafsheath. Potassium, Ca, Si and Zn content in rice plant increased with low or high application of Fe. Both deficiency and excessive application of Fe reduced the amount of allelochemicals in rice plants.

Table 31: Effect of different levels of Fe on growth of rice plants

Fe (ppm)

Plant height (cm) Tiller/plant (No) Root length (cm)

TN1 IR2035 TN1 IR2035 TN1 IR2035

18 DAT

0.02 44.6b 46.3b 2.8a 3.2a 15.8b 19.5a

2.00 46.6a 47.8a 2.9a 3.3a 16.1a 19.9a

40.00 43.7c 45.4c 2.7a 3.0a 14.3c 18.6b

36 DAT

0.02 56.7b 56.3b 3.7b 3.7b 17.0b 28.8b

2.00 60.6a 60.3a 4.3a 4.5a 20.7a 33.1a

40.00 44.8c 54.7c 3.4b 3.5b 17.1b 24.5c

In a column, means followed by a common letter for each age group are not significantly by LSD.

218

Table 32: Effect of different levels of Fe on biomass of rice plants

Fe in culture solution(ppm)

Shoot biomass(g) Root biomass (g)

TN1 IR2035 TN1 IR2035

Fresh biomass

0.02 130.97c 157.94b 39.00c 55.77b

2.00 156.96a 189.70a 51.02a 62.69a

40.00 140.27 118.78c 42.93b 43.99c

Dry biomass

0.02 32.35c 39.86b 7.65b 8.69a

2.00 42.48a 46.52a 9.54a 9.42a

40.00 33.94b 27.50c 7.78b 7.23b

In a column, means followed by a common letter are not significantly different by least significant difference test (LSD).

219

Table 33: Effect of Fe-stress on the chemical composition of rice plant

Content

TN1 R2035

0.02 2 40 0.02 2 40

N (%) 1.61 1.03 1.60 1.35 1.42 1.65

P (%) 0.56 0.33 0.22 0.43 0.39 0.13

K (%) 2.50 1.45 2.68 2.35 2.27 2.74

Mg (%) 0.25 0.21 0.22 0.28 0.25 0.23

Ca (%) 0.11 0.07 0.16 0.10 0.09 0.11

Si (%) 0.95 0.58 0.82 0.83 0.78 1.25

Zn (ppm) 80.0 40.0 52.0 66.0 44.0 48.0

Fe (ppm) 96.0 1.05 1.61 1.19 1.64 1.94

Al (ppm) 1.46 1.40 1.28 2.30 2.23 1.06

Soluble proteins (%)

7.32 5.92 6.34 5.56 5.58 6.34

Total free sugar (%)

8.67 9.48 6.92 4.92 5.10 4.76

Allelochemicals (mg)

7.85 9.10 6.85 11.19 13.97 12.57

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Table 34: Effect of Fe-induced changes in rice plants on the biological parameters of S .furcifera

Biological parameters

TN1 IR2035

0.02 2.00 40.00 0.02 2.00 40.00

Food intake (mg/female/24h)

10.7a 10.2a 10.8a 4.9a 4.3a 4.6a

Food assimilation (mg/female/24h)

0.4a 0.3a 0.4a 0.2a 0.1a 0.1a

Honey dew (mg/female/24h)

10.3a 9.9a 10.4a 4.7a 4.2a 4.5a

Nymphs becoming adults (%)

97.5a 95.0a 97.5a 22.5a 20.0a 25.0a

Nymphal duration (days)

14.5b 15.2a 14.6b 17.2b 17.8a 7.2ab

Growth index 6.7a 6.3a 6.7a 1.3a 1.1a 1.4a

Male adult longevity (days)

15.6a 14.8a 15.2a 6.2a 6.1a 6.6a

Female adult longevity (days)

18.7a 17.3a 18.2b 8.6a 7.8b 8.4a

Fecundity/female 100a 95a 102a 7a 5b 8a

Population/pair 11.4a 108b 11.7a 17b 12c 20a

In a row, within a variety, means followed by a common letter are not significantly different at 5 % level by LSD.

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Both low and high contents of Fe in rice plant reduced the developmental period of S. furcifera nymphs on TN1 (susceptible to insect) plants but on resistant IR2035 plants high level of Fe did not affect the developmental period of the insect, while low level of Fe decreased the developmental period (table 34). Longevity of S furcifera females increased on Fe stressed plants. Its fecundity was significantly higher due to Fe stress on resistant IR2035 plants receiving low or high level of Fe. Population of the insect was significantly higher on TN1 as well as on IR2035; however, the magnitude of population increase was greater on resistant IR2035 plants than on susceptible TN1 plants. Both Fe deficiency and excessive application of Fe induced susceptibility or reduced the level of resistance in rice plants against S. furcifera.

Oviposition rate of the mustard beetle, Phaedon cochleariar declined with a decrease of Fe concentration in Nasturtium officinalis leaves (Allen and Selman, 1957). It is evident that both deficiency or excessive application of Fe in rice plants reduces the level of resistance or induces susceptibility against S. furcifera – a major pest of rice in Pakistan.

7.4.5. SILICA (SI)

No direct evidence has been found of the participation of Si in essential physiological and biochemical processes in plants. In culture solution, the addition of Si had no effect on rice growth when the Si content of green leaves was above 1.25 %. Silica content in plants influenced vulnerability to pests. Silica cellulose membrane or silica double layer conferred resistance to insect pests (Yoshida, 1981).

A high Si content contributes resistance to stem borer, Chilo partellus in maize (Sharma and Chatterjee, 1972). ). In sorghum, cultivars with high Si content in fourth and sixth leaf stage had low incidence of shoot fly, Atherigona soccata (Bothe and Pokharkar, 1985). The varieties of sorghum resistant to A. soccata, as compared to susceptible one, possessed a greater density of Si bodies in the epidermis at the base of the first, second and third leaf sheath (Blum, 1968). High Si content has been reported to be associated with stem borer resistance in sorghum (Narwal, 1973). It is evident that in maize and sorghum Si induces resistance against stem borer. Almost similar results were obtained in case of fruit fly, Dacus cucurbitae resistance in muskmelons (Chelliah, 1972). In wheat, increase of Si in plant tissues increased its resistance to Mayetiola destructor (Mc Colloh and Salmon, 1923).

High degree of negative correlation between Si cells in the abaxial epidermis of the leaf sheath of sugarcane and the shoot borer, Chilotraea infuscatellus infestation was found (Rao, 1967). High Si content in cotton proved comparatively resistant against cotton jassid, Amrasca devastans. Negative correlation between the Si content of leaves and A. devastans population was found. Resistant cotton varieties had greater amount of Si, Fe in leaves irrespective of their hair characters (Singh et al., 1972).

In rice, a highly significant negative correlation was recorded between Si content of the stem in various cultivars and the susceptibility to the Asiatic rice

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borer, Chilo suppressalis. High Si content appeared to interfere with feeding and boring of the larvae and could cause defacing of their mandibles (Hanifa et al., 1974). Negative correlation between Si content and susceptibility of rice plants to C. suppressalis and Chilo zacconius has been reported (Ukwungwa and Odebiyi, 1985).

Significantly more damaged tillers by yellow stem borer, Scirpophaga incertulas on rice plants grown in Si free culture were recorded. The penetration time by S. incertulas was shortest in rice plants lacking Si. Higher Si content could be a barrier for the rice stem borer to utilize the plant nutrients because dissolved Si inhibited the action of pepsin, trypsin, amylase and acetyl choline esterase (Kind, 1954). A negative correlation between Si content and larval weight and Si content and deadheart was observed (Subarao and Perraju, 1976). Silicated cells in the form of buliform tablets were closer and double in number per unit area in rice resistant variety Punja than in susceptible TN1 (Mishra and Misra, 1992). Higher Si content was recorded in the resistant varieties whereas the susceptible varieties to yellow stem borer had lower Si content (Mishra et al., 1990). Application of Si increased the Si content of rice plants which in turn, was found to reduce deadheart incidence caused by S. incertulas (Sawant et al., 1994). Increased Si content in resistant rice varieties has been reported to affect the mandibular development in the rice leaffolder Cnaphalocrocis medinalis (Pathak and Saxena, 1980).

Kim and Heinrichs (1982) examined the effect of Si content in rice plant on growth, survival and population increase of Sogatella furcifera. According to them the S. furcifera nymphs survived poorly on resistant rice cultivar N22 when grown at high level of Si.

Salim and Saxena (1992) carried out detailed studies on the effects of Si application in culture solution on the growth, biomass and chemical composition of rice plants and on the level of resistance/ susceptibility of rice plant to whitebacked planthopper, Sogatella furcifera. The studies were carried out at 29/21 oC day/night temperatures, minimum 70% relative humidity. Rice cultivar IR2035 (resistant to S. furcifera) and TN1 (susceptible to S. furcifera) were used in these stuides. Different levels of Si were maintained through culture solution.

Addition of Si at 100 mgL-1 in the culture solution increased plant height and tiller number, but further increase of Si to 400 mgL-1 did not improve plant growth (Table 35). Application of Si caused changes in the chemical composition of rice plants (Table 36).

Insect orientation and settling responses to plants grown in culture solution with addition of Si did not differ significantly from responses to plant grown in standard culture solution. Increase in Si level decreased food intake on susceptible (TN1) plants, but not on resistant (IR2035) plants (Table 37). At high level of Si, fewer nymphs became adults on susceptible (TN1) plants, but there was no significant difference on resistant (IR 2035) plants. The developmental period at 400 mgL-1 of Si, increased on both susceptible and resistant plants. Increase in Si supply decreased female longevity and population buid up of S.

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furcifera. It is evident from this study that Si increased the level of resistance in rice plants to S. furcifera. Soluble silicic acid in rice plants have been reported to act as a sucking inhibitor to Nilaparvata lugens. However, silicic acid was a general sucking inhibitor occurring in both susceptible and resistant rice cultivars (Yoshihara and Sogawa, 1979).

The exact mechanism of Si-induced plant resistance against insect pests is not well understood, because, direct evidence of participation of Si in essential physiological and biochemical processes is not clear. Silica contents absorbed by the plants get deposited in the tissues of several plant species especially those belonging to gramineae, cyperaceae, and palmae. Silica deposited in certain plant tissues contributes to resistance to insect pests. Increased plant toughness and reduced digestibility of plant tissue by herbivores is mainly attributed to Si content in plants. It is evident from the discussion on the effect of Si content in plants on host-insect interactions that Si can be used as resistance inducing agent in plants against insect pests.

Table 35: Effect of Si on the growth of rice plant

Si (ppm) in

Culture Solution

Plant height (cm) Tiller/plant (No) Root length (cm)

TN1 IR2035 TN1 IR2035 TN1 IR2035

0 60.5b 64.7c 6.1b 6.7b 36.7a 36.7a

100 64.6a 66.4b 6.9a 7.9a 37.2a 37.2a

400 64.5a 67.9a 6.7a 7.8a 37.2a 37.2a

In a column, means followed by a common letter are not significantly different at 5% level by LSD.

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Table 36: Effect of Si on the chemical composition of rice plant

Constituents

TN1 IR2035

0 100 400 0 100 400

N (gkg-1 ) 8.8 9.5 10.4 9.6 9.2

P (gkg-1 ) 3.3 2.9 2.9 3.7 3.1

K (gkg-1 ) 42.5 24.1 16.2 16.2 14.5

Mg (gkg-1 ) 2.1 1.9 1.6 2.4 1.9

Ca (gkg-1 ) 0.7 0.6 0.6 0.9 0.8

Si (gkg-1 ) 4.0 26.6 34.0 6.9 32.0

SP (gkg-1 ) 45.9 36.7 37.7 56.5 49.0

TFS (gkg-1 ) 72.3 105.0 89.2 64.8 56.5

Zn (mg kg-1 ) 22.0 23.0 25.0 32.0 22.0

Fe (mg kg-1 ) 100.0 152.0 145.0 139.0 123.0

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Table 37: Effect of Si on the level of resistance of rice plants to S. furcifera

Insect biological TN1 IR2035

parameters 0 100 400 0 100 400

Food intake (mg)/ 17.3a 12.8b 10.5b 4.9a 3.5a 2.9a

Female/24 hours

Food assimilation (mg)/ 0.49a 0.49a 1.43a 0.17a 0.15a 0.12a

female/24 hours

% nymphs becoming adults 90.0a 90.0ab 85.0a 28.0a 25.0a 20.0a

Nymphal duration (d) 14.7b 15.0ab 15.2a 17.7b 18.1ab 18.4a

Male longevity (d) 14.6a 12.6b 12.2b 6.8a 6.1a 5.9a

Female longevity (d) 19.7a 17.6b 16.8b 8.3a 7.0ab 6.7b

Fecundity/female 135a 127a 123a 9.0a 8.0ab 7.0b

Population/pair 113.0a 100.0a 73.0b 15.0a 11.0b 9.0c

In a row, means followed by a common letter are not significantly different at probability 0.05 by LSD.

7.4. 6. BORON (B)

Population of mites was the highest on oil palm seedlings deficient in Boron (Rajaratnam and Hock, 1975). When deficient seedlings were subsequently supplied with boron, the new leaves were less affected by mites than the older leaves.

7.4.7. METAL CHELATES

The chelated metal ions of copper and zinc with EDTA prevented the pupation of Pierus brassicae (Sell and Schmidt, 1968). Brinjal plants, Solanum melongena treated with chelated metal complexes of iron and boron caused antibiosis to shoot borer, Leucinodes orbonalis. The chelated boronzinc-oxy-tetracuycline complex (0.5 ppm) significantly reduced the infestation of yellow stem borer, Scirpophaga. Incertulas on rice. The attack of gall midge, Pachydiplosis oryzae was less on rice seedlings applied with the chelated boronzinc-oxy-tetracycline complex (Panda, 1979).

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7.5. GROWTH REGULATORS

There have been many studies on the effect of growth regulators on plants and insects. Application of maleic hydrazide to broadbean plants increased mortality and reduced the fecundity of bean aphids. The growth retardant CCC (Chlormequat) has been tested on several plant/insect systems, all with appreciable increases in resistance either due to reduced fecundity of females, or increased mortality of offspring; both resulting in an overall reduction in the populations. Occasionally, as is the case with many secondary metabolites, certain insect species are affected detrimentally by the application of a compound. Gibberellin applied to broad bean reduced fecundity of the bean aphid, but had no effect on the pea aphid (Mooney et al., 1983).

7.6. HERBICIDES

Herbicides have been tested in their effect on insects. For instance, 2-4D on barley reduced fecundity in two species of grain aphids, Rhopalosiphum padi and Macrosiphum avenae, but applied to corn increased fecundity of corn leaf aphid, Rhopalosiphum maidis suggesting an improvement in the quality of food for the insect. Other herbicides have been tested with fewer insects, and results varied. Amitrol and Zitron applied to broadbean reduced fecundity and increased mortality in the pea aphid, Acythosiphon pisi, but Banvel D, Barban or MCP applied to barley increased fecundity of the grain aphids, M. avenae, R. padi and Schizaphis graminum (Mooney et al., 1983). The effect of herbicides on host-insect interactions is not consistent, it is therefore, imperative to determine the effect of each herbicide on each major insect species. If a herbicide induces susceptibility in plants against insect pests then some alternate herbicide should be used in order to minimize losses caused by insect pests. Those herbicides should be identified which induce resistance in plants and suppress the population development of insect pests.

7.7. PLANT SPACING

Infestation of rice stem borer and gall midge was considerably higher in close than wide spacing in rice (Prakasa Rao, 1972). Close spacing (10 x 10 cm) stimulated population growth of the brown planthopper, Nilaparvata lugens than wide crop spacing (50 x50 cm). Thick vegetation in broadcast rice crop contributed to N. lugens outbreak in Kerala India (Panda, 1979). Studies carried out by the author in Pakistan indicate that close spacing encourages the population build up certain species of insects and reduces the infestation by other insect species. It is, therefore, imperative, to conduct specific studies on all major insect species and determine overall impact and recommend plant spacing or plant population accordingly to minimize the losses caused by insect pests.

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7.8. ATTACK OF INSECTS AND DISEASES

Certain relationship between insects and tree invading fungi have long been recorded (Schmidberger, 1937). Ambrosia beetles are vectors of various fungi and feed on them (Graham, 1967). In certain cases these are the only source of nutrition for them. Insects feeding on sporophores and mycelia of plant pathogenic fungi (Lawrence, 1965) or on the diseased tissue such as galls caused by rust fungi have also been recorded. Blue-stain fungi associated with bark beetles have been reported to manipulate the environment of the tree in favour of the beetles (Mathre, 1964). Colourado potato beetle, Leptinotarsa decemlineata preferred to lay eggs on the leaves of plants that were in decaying condition and those carrying virus infections probably provided a more choice of food for the pest. Bark beetles, Dendroctonus brevicornis have been reported to select severely disease infested pine, Pinus ponderosa due to certain factors related to tree physiology. The oleoresin exudation pressure, yield and rate of flow was progressively reduced in pine as severity of disease increased, but the crystallization of resin increased as the severity of disease gained momentum. Both sapwood and phloem moisture contents were lower in diseased trees. Phloem thickness in advanced diseased trees was less than 60% of that in healthy trees (Cobb et. al., 1968). Such changes in the plants may affect host-insect interactions.

Some tobacco varieties hypersensitive to tobacco mosaic virus (TMV) can be protected against attack by other organisms. Local virus infections induced a systemic protection against the fungi, Phytophthora parasitica var. nicotiana and Peronospora tabacina as well as the bacterium, Pseudomonas tabaci. In addition, the reproductive rate of the green peach aphid, Myzus persicae, was reduced about 11% when females were allowed to feed and reproduce parthenogenetically on leaves at plant apices at least 6 nodes above the site of virus infection. Furthermore, it was found that growth rates of fourth instar tobacco hornworms, Manduca sexta, were reduced to 27% when reared on tobacco leaves with local TMV lesions and 16% when reared on neighbouring leaves without external symptoms. The symptomless leaves were on plants that had other leaves symptomatic of TMV. The whole plant was systemically induced to higher levels of resistance (Mooney et al., 1983).

Feeding activities of herbivorous insects often result in physiological and morphological changes in the host plant. If these changes are in direction of the accumulation of compounds with resistance properties, the herbivores themselves act as inducers of resistance or susceptibility. The induced resistance affects the inducing species itself. Thus, for instance, feeding by the pea aphid, on alfalfa induced an increase in coumestrol that may affect additional feeding by other insects. Changes may also occur in levels of primary metabolites thus affecting the nutritional value of the leaves. For example, feeding by gypsy moth, Lymantria dispar, larvae on gray birch or black oak leaves produced physiological changes in the plants that may reduce the survival and growth rates of other larvae feeding on leaves several days after the initial attack. It is noteworthy that many changes that occur following herbivory on trees result from accumulations of phenolic compounds.

A different type of induction occurs with the striped cucumber beetle, Acalymma vittata feeding on squash. Feeding by this beetle resulted in an accumulation of cucurbitacins which are strong feeding excitants for the A. vittatata itself. However,

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cucurbitacins were deterrent at high concentrations to the squash beetle, Epilachna tredecimnotata. In fact, the squash beetle seemed to have evolved a behavioural adaptation to prevent accumulation of cucurbitacins in their feeding sites. Before starting the feeding process, they cut a circular trench encircling the leaf section upon which they will later feed. The trench prevented circulation of sap into the enclosed area that remained free of the allomonal effect of the cucurbitacins, and suitable to feeding by the beetles. It is thus apparent that a plant challenged by herbivores, plant pathogens, or otherwise stressed by specific chemicals did not remain bio-chemically indifferent. The plant responded with more or less drastic metabolic changes, some of which had a profound effect on additional herbivores or other plant pathogens.

7.9. LIGHT

Light quantity, quality and duration can have effects on physiological and biochemical processes of insect pests, their host plants or both. Such effects, therefore can profoundly alter host-pest interactions. Internodal solidness in wheat stem conferred resistance against wheat stem fly, Cephus cinctus. Light intensity caused changes in stem solidness and internodal length. Stem solidness was reduced at decreased light intensities in the resistant solid stem wheat. Relative expression of resistance in solid stem wheat varieties against C. cinctus was significantly reduced under shaded conditions. In greenhouse, supplemental fluorescent and incandescent light prevented loss in expression of resistance at intensities of 4000 foot-candles, but not at 1500 foot-candles. Solidness of lower internodes was more important than that of upper internodes in resistance and that internodes 1 to 3 were subject to the greatest reduction in solidness by shading early in plant growth. Reduced light intensity also effected expression of resistance to Myzus persicae but decreased light intensity influenced tissue levels of specific chemical resistance factors in a number of plants species. In potatoes, steroidal glycosides (glycol-alkaloids) mediated resistance to the Colourado potato beetle, Leptinotarsa decemlineata with increasing foliar concentration. Shading led to decreased concentration of total glycoalkaloids in foliage of several potato species differing in resistance to L. decemlineata. Leaf feeding resistance to European corn borer, Ostrinia nubilalis in maize was conditioned by increasing tissue concentration of benzoxazolinones and their precursor hydroxamic acids. Greater levels of benzoxazolinones in seedling tissues was found with increase in the light intensity (Tingey and Singh 1980).

7.10. pH

Rice plants grow more vigorously under acidic medium (pH 4.0 to 4.8) and because of it rendered themselves more attractive to the yellow rice borer, Scirpophaga incertulas moths for shelter and oviposition in contrast to plants grown under nearly neutral (6.6 to 7.0 pH) or alkaline (8.0 to 9.0 pH) medium. The incidence of deadhearts and whiteheads was also higher at acidic pH than at other pH levels (Prakasa Rao, 1972).

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7.11. WATER

Continuous standing water in paddy fields was favourable for the growth and development of gall midge, and stem borer, while alternate wetting and drying depressed the incidence of these pests (Anonymous, 1968 and 1970). Rice fields with standing water were found to encourage the multiplication of brown planthopper, Nilaparvate lugens population (Pathak and Dyck, 1973). In Pakistan it has been observed that the population of whitebacked planthopper, Sogatella furcifera was more than double in rice fields with stagnant water than under normal conditions (Salim, 1981). With proper water management, the population of insect pests can be suppressed in different crops.

7.12. RELATIVE HUMIDITY (RH)

Infestation of rice gall midge was comparatively high at 94-96% RH due to close spacing than at 81% RH in case of wide spacing of plants. Many insects pests thrive better under high RH than at low RH (Panda, 1979). In cotton, relative humidity had significant effect on Earias spp infestation (Abro et al, 2003). In wheat, significantly negative correlation was observed between stomatal frequency, relative humidity and Sitobion avenae population while positive correlation was found between days to heading, days to maturity, amino acid (%) and temperature (Nasir, 2001).

7.13. INSECTICIDES

Application of certain insecticides can cause resurgence of insects (Ripper, 1956), possibly because of improved nutritional quality and growth of host plants (McClure, 1977). Insecticide residues in host plants stimulated the reproduction and survival of insects, leading to pest resurgence (Locher 1958). Deltamethrin produced a favourable environment for N. lugens to alight, feed, survive and reproduce. Diazinon increased the feeding rate of planthoppers. The reproductive rate of N. lugens was higher on plant treated with deltamethrin and diazinon (Chelliah and Heinrichs, 1980a and b). Foliar sprays of insecticides proved most active in inducing resurgence of N. lugens. Its reproductive stimulation appeared to be of more significance than destruction of biocontrol agents (Heinrichs et al., 1982 and b). Raman (1984) recorded significantly higher per centage of N. lugens adults on insecticide treated plans. Deltamethrin increased tillering and number of leaves per plant which attracted more insects than untreated plants.

Salim and Heinrichs (1987) carried out studies on insecticide-induced changes on the levels of resistance of rice cultivars to the whitebacked planthopper, Sogatella furcifera. Three rice varieties IR2035 (highly resistant to S. furcifera), ARC 10239(resistant) and TN1 (susceptible) were sown in the main plots. Each main plot was divided into four subplots consisting of three insecticides applied as foliar sprays: deltamethrin (0.002%) diazinon (0.04%) and monocrotophos (0.075%) and the untreated check (control). Foliar application of insecticide was done in a spray chamber with an atomizer. Control plants were sprayed with tap water. One foliar spray was done at 40 DAT. In another experiment foliar sprays were applied 20, 30 and 40 DAT.

In case of one insecticidal application, within 2 hours after release of S. furcifera adults, the per centage that alighted on each of the three varieties differed significantly

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(Table 38), with 56% on TN1, 24.4% on ARC 10239 and 19.6% on IR2035. In the subsequent observations the number of S. furcifera adults further increased on the susceptible and decreased on resistant cultivars. On the tenth day, 83.2% of S. furcifera were on TN1, 14% on ARC 10239 and only 2.8% on IR2035.

The interaction between varieties and insecticides for the attraction of S. furcifera adults was significant. On ARC 10239 and TN1 the per centage of S.furcifera adults which alighted was significantly higher for deltamethrin than for the check (Table 39). On IR2035 the difference between insecticide treated plants and the check was non-significant.

In case of three sprays of insecticides the numbers of S.furcifera adults which alighted had decreased significantly on IR2035 and ARC10239 plants but increased considerably on TN1, 24 hours after release (Table 40). During all observations, TN1 attracted the most and IR2035 the least S. furcifera adults; ARC10239 was intermediate.

The interaction between varieties and insecticide applications was highly significant. Although the level of resistance was the major determinant of the degree of preference, insecticides also had a significant effect (Table 41). Foliar application of deltamerthin attracted significantly higher per centage of S. furcifera than the check.

Table 38: Effect of insecticides on rice cultivars sprayed with insecticides on the orientational and settling response of S. furcifera.

Days/hours after infestation*

adults alighted (%)

IR2035 ARC10239 TN1

0.02 (0.5h) 31.1 a (ab) 30.0 a (b) 38.9 c (a)

0.08 (2.0h) 19.6 b (c ) 24.4 b (b) 56.0 d (a)

0.17 (4.0h) 13.6 c (c ) 22.4 bc (b) 64.0 cd (a)

0.33 (8.0 h) 8.4 d (c ) 20.8 bcd (b) 70.8 bc (a)

1.00 (24 h) 4.8 e (c ) 20.4 bcd (b) 74.8 ab (a)

10.00 (d) 2.8 f (c ) 14.0 e (b) 83.2 a (a)

In a column and in a row (in parenthesis), means followed by a common letter are not significantly different at 5% level by DMRT.

* One foliar application of insecticides was applied and insects were released on rice plants 24 hours after insecticide application.

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Table 39: Effect of one foliar application of insecticides on the orientational and settling

response of S. furcifera. on rice cultivars with different levels of resistance.

Insecticide*

Adults alighted (%)

IR2035 ARC10239 TN1

Monocrotophos 1.8 a (c ) 5.1 ab (b) 16.6 b (a)

Diazinon 2.1 a (c ) 4.6 b (b) 16.8 bs (a)

Deltamethrin 2.7 a (c ) 5.7 a (b) 21.7 a (a)

Check 1.8 a (c ) 4.5 b (b) 16.6 b (a)

In a column, means followed by a common letter are not significantly different at 5% level by DMRT. Likewise, in a row, means in parenthesis followed by a common letter are not significantly different at 5% level by DMRT.

* As table 38

Table 40: Effect of insecticides on the orientational and settling response of S. furcifera.

Days/hours after infestation*

S. furcifera adults alighted (%)

IR2035 ARC10239 TN1

0.02 (0.5h) 26.3 a (c) 32.7 a (b) 41.0 d (a)

0.08 (2.0h) 10.6 b (c ) 25.4 b (b) 64.0 c (a)

0.17 (4.0h) 7.6 bc (c ) 22.4 b (b) 70.0 bc (a)

0.33 (8.0 h) 5.6 cd (c ) 21.1 b (b) 73.2 b (a)

1.00 (24 h) 3.8 de (c ) 9.8 cd (b) 87.3 a (a)

10.00 2.8 e (c ) 7.6 d (b) 89.6 a (a)

In a column, means followed by a common letter are not significantly different at 5% level by DMRT. Likewise, in a row, means in parenthesis followed by a common letter are not significantly different at 5% level by DMRT.

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Table 41: Effects of levels of host plant resistance and insecticides on the orientational

and settling response of S. furcifera.

Insecticide*

Adults alighted (%)

IR2035-117-3 ARC10239 TN1

Monocrotophos 1.3 ab (c ) 3.4 b (b) 20.7 b (a)

Diazinon 1.3 ab (c ) 4.0 a (b) 18.4 c (a)

Deltamethrin 1.8 a (c ) 4.5 a (b) 29.9 a (a)

Check 1.0 b (c ) 2.6 c (b) 18.1 c (a)

In a column, means followed by a common letter are not significantly different at 5% level by DMRT. Likewise, in a row, means in parenthesis followed by a common letter are not significantly different at 5% level by DMRT

* As table 40.

Table 42: Effect of foliar application of one insecticides on the population of S. furcifera

on rice cultivars with different levels of resistance.

Insecticide

No. of insects/cage

IR2035 ARC10239 TN1

Monocrotophos 53 b (c ) 121 a (b) 277 b (a)

Diazinon 55 b (c ) 107 b (b) 246 c (a)

Deltamethrin 66 a (c ) 126 a (b) 300 a (a)

Check 49 b (c ) 103 b (b) 243 b (a)

In a column, means followed by a common letter are not significantly different at 5% level by DMRT. Likewise, in a row, means in parenthesis followed by a common letter are not significantly different at 5% level by DMRT.

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Table 43: Impact of foliar application of insecticides and resistance on the growth index of

S. furcifera.

Insecticide

Growth index on*

IR2035 ARC10239 TN

Monocrotophos 3.51a (c ) 5.05a (b) 7.13 ab (a)

Diazinon 3.65a (c ) 5.00 a (b) 6.95 ab (a)

Deltamethrin 3.83a (c ) 5.35 a (b) 7.43 a (a)

Check 3.65a (c ) 4.84 a (b) 6.68 b (a)

*In a column and in a row (in parentheses), means followed by a common letter are not significantly different at the 5% level by DMRT.

Table 44: Effect of insecticidal applications on the feeding rate of S .furcifera on rice

cultivars with different levels of resistance.

Insecticide

Area of honeydew (mm2 )*

IR2035 ARC10239 TN1 Insecticide means

Monocrotophos 17 ab (c ) 61 a (b) 252 a (a) 110 b

Diazinon 18 ab (c ) 54 a (b) 259 a (a) 110 b

Deltamethrin 21 a (c ) 65 a (b) 269 a (a) 118 a

Check 16 b (c ) 53 a (b) 226 a (a) 98 bc

In a column and in a row (in parenthesis), means followed by a common letter are not significantly different at the 5% level by DMRT.

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The population of S. furcifera on IR2035 sprayed with deltamethrin was significantly higher than those on plants treated with diazinon or monocrotophos, or on the check (Table 42). Plants of ARC10239 and TN1 that had been treated with monocrotophos or deltamethrin had higher S. furcifera population than diazinon treated plants or population build up was greatest on TN1 (243-300), intermediate on ARC10239 (103-126) and least on IR2035 (49-66). The population of S. furcifera increased by 8.2-11.0 and 40.5-50.0 times on IR2035 and on TN1, respectively.

Nymphal survival was about 60% on IR2035, 75% on ARC10239 and 90% on TN1. Insecticides had no significant effect on S.furcifera survival except that of deltamethrin treatment which increased survival on TN1 from 88% (check) to 95%. The low survival rate of S. furcifera on resistant cultivars indicated the presence of an antibiosis factor.

Foliar application of insecticides did not have a significant effect on the nymphal duration of S. furcifera. Nymphal duration of S. furcifera was longer on IR2035 (17 days), intermediate on ARC10239 (15 days) and shorter on TN1 (13 days), the differences between cultivars being significant.

The higher the growth index, the more suitable is the cultivar for S. furcifera development. The growth indices of S. furcifera were about 3.7, 4.8 and 6.7 for IR2035, ARC10239 and TN1, respectively and these differences were statistically significant (Table 43). The decrease in growth index of S. furcifera due to resistance was attributable to a decrease in the nymphal survival and an increase in the nymphal duration. Foliar applications of deltamethrin on TN1 caused a significant increase in the growth index of S. furcifera compared with the check, but insecticides had no significant effect on the other cultivars, IR2035 and ARC10239.

There was a significant difference in the amount of honeydew excreted by S. furcifera on the three cultivars (Table 44), with 12 times as much excreted on TN1 as on IR2035. With an increase in the level of resistance, there was a corresponding decrease in the feeding rate. The increase in feeding rate of S. furcifera after the foliar application of deltamethrin was statistically significant only on IR2035. On the basis of the insecticide means, deltamethrin significantly increased the feeding rate over that observed on the check plants. This study indicated that foliar application of insecticides, especially of deltamethrin, attracted more adults and increased nymphal survival, growth index, and population development of S. furcifera. The effect of the insecticides, in altering susceptibility of plants to S. furcifera, occurred in both susceptible and resistant cultivars.

7.14. MISCELLANEOUS

Rice varieties which were resistant to brown planthopper, Nilaparvata lugens had high content of K,. Fe and Zn than susceptible varieties (`Raman, 1984). Higher amount of N, P, K and low Si and Fe content were found in maize borer susceptible maize varieties. Silica and Fe contents contributed to resistance, but the content of N and P were lower in the resistant genotypes than susceptible ones (Sharma and Chatterjee, 1971). Potassium contents were significantly lower in stem borer susceptible genotypes, but P content was relatively higher (Kabre and Ghorpade, 1998). Shoot fly reisstnat varieties in maize possessed low contents of chlorophyll a, b and totals chlorophyll than

235

susceptible varieties (Rao and Panwar, 1995). Resistant cotton cultivars had greater amount of Si, total minerals, Fe, P and Mg in cotton leaves (Singh et al., 1972). Higher application of N,P and K enhanced the incidence of jassid and bollworms on cotton (Jayaswal and Sundaramurthy, 1992; Purohit and Deshpande, 1991 and 1994; Sivasubramanian and Uthamasamy, 1992) and whitefly (Rote and Puri, 1992). Induction of antibiosis under the influence of S nutrition has been recorded in Brassica alba, B. compestris and B. juncea (Rani, 1981). Further information is available in Table-45.

Table 45: Resistance inducing agents in crop plants against insects pests

Crop Insect pest Nutrient Reference

Chickpea Helicoverpa armigera Non-reducing sugars

Chhabra et al., 1990

Cotton Bemisia tabaci

Amrasca devastans

K

Silica (Si)

Fe

P

Mg

Rao et. al., 1990

Singh 1970

Singh et. al., 1972

Singh et. al., 1972

Singh et. al., 1972

Guava Bactrocera correcta Ascorbic acid Arora et. al., 2000

Maize Spodoptera frugiperda

Chilo partellus

Aspartic acid

K

K

Silica (Si)

Hedin et. al., 1990

Hedin et. al. 1990

Kabre &Ghorpade,1998

Sharma &Chatterjee,1971

Mungbean B. tabaci Amino acids Chhabra et. al., 1988

Oats Rhopalosiphum padi Aspartate Weibull, 1988

Okra Amrasca biguttula Total sugars Singh and Agarwal, 1988

Pigeonpea Melanagromyza obtsua

Total sugars

Sithanan et. al., 1983

236

Helicoverpa armigera Total soluble sugars

Total amino acids

Dodia et. al., 1998

Dodia et. al., 1998

Rice Nilaparvata lugens

Orseolia oryzae

N. lugens

Sogatella furcifera

Gall midge

Scirpophaga incertulas

Aspartic acid

Free amino acids

K

Protein content

Solicic acid

Oxalic acid

K

Si

Amino acids

Silica (Si)

Sogawa and Pathak, 1970

CRRI, 1985

CCRI, 1985

CCRI, 1985

Yoshihara et. al., 1979,

Salim and Saxena, 1991

Salim and Saxena, 1992

Kind, 1954; Subbarao and Perraju, 1976

Mishra and Misra, 1992

Swant et al. 1994

Sorghum Atherigona soccata

Chilo partellus

Total amino acid

Silica (Si)

Khurana and Verma, 1982

Narwal, 1973

Sugarcane Chilo infuscatellus

K

Rao, 1967

Brassica Brevicoryne brassicae Total sugar & S

Total sugars & Fe

Bakhetia et. al., 1982

Narang, 1982

Chickpea H. armigera Malic acid

Oxalic acid

Rembold, 1981;

Yoshida et. al., 1995

237

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CHAPTER-8 DEVELOPMENT OF INSECT RESISTANT VARIETIES

The concept of host plant resistance is very old. Unintentional selection for plant resistance against herbivore attacks probably occurred in the early stage of agriculture. The process of natural selection for resistant plants continued until man started to interfere actively with the process by favouring certain plant species of desired qualities. Improvement of the desired product frequently involved the deterministic reduction of factors that were coincidently involved in resistance mechanisms. The first extensive search for insect resistant plant sources was made in California over a period of ten years beginning in 1891. It was during 1920s and 1930s that the development of plant breeding techniques made it possible to apply this information fruitfully by incorporating genetic resistance factors into a satisfactory wheat variety. Probably the earliest studies on the inheritance of pest resistance can be traced in 1916, which involved the resistance in cotton. Studies on Hessian fly, Mayetiola destructor resistance to wheat were commenced by the Kansas State Agricultural Station in 1914 and have continued without interruption to the present time. Likewise, studies on host plant resistance were initiated on different crops and insect pests and large number of resistant varieties have been developed and are being cultivated on large areas throughout the world. The development and use of insect resistant crop varieties offers promising possibility and becomes inevitable under certain situations. It is necessary to develop resistant varieties and manage the resistance effectively for enhanced, durable and broad spectrum resistance.

8.1. NEED FOR RESISTANT VARIETIES

Insect pests cause substantial qualitative and quantitative yield losses. In Asia, yield losses due to insects have been estimated at about 25% (Cramer, 1967). In most of the cases insect pests were being controlled with the use of insecticides which have posed multifarious and multifacet complicated problems. Pesticide-induced problems include development of resistance to pesticides, destruction of bio-control agents and other beneficial organisms, disturbance in natural balance, accumulation of undesirable insecticide residues, environmental pollution, resurgence of target pests, outbreak of secondary pests and health problems in human beings and livestock and other organisms at different trophic levels.

Planting of resistant varieties is an eco-friendly strategy of pest management and can provide a foundation on which to build a good IPM system. Extensive use of chemical insecticides is not a viable strategy. It only provides ephemeral benefits and terminates into worsening of the farmers overall insect pest problems. Therefore, development of insect resistant varieties by exploiting host plant resistance offers a better alternative. In resistant varieties, the pest control is ensured without incurring any extra expenditure. Further, the control is eco-friendly, stable and durable.

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Host plant resistance is an excellent pest suppressing method and, when integrated with other methods of insect control, offers a sound approach to deal with insect pest problems. Developing insect pest resistant crop varieties is of paramount importance, because such approach is environmentally safe and more stable. The most important advantage being its compatibility with other methods of pest management. Moreover, it has no adverse effects on ecosystem, besides being easy for adoption by the growers, since no extra cost is involved. The best way to manage the insect pests is by growing resistant varieties. Varietal resistance is the most economical, least complicated and environmentally sound approach for protecting the crops from insect damage. Even a variety with moderate level of resistence is much importance as it requires less insecticidal applications.

Traditional varieties which had some resistance for insects pests have, in some cases, been replaced by higher yielding but more susceptible varieties. Insecticides replaced the traditional insect pests control methods and in many cases increased yields. However, yield stability has not been achieved with the repeated use of insecticides alone. The incorporation of insect resistance into modern varieties is a major objective of most breeding programmemes of almost all crops in different countries. Some predators are more effective against insect pests on resistant varieties. Resistant varieties increase the susceptibility of insect pests to insecticides. Resistance varieties are essential to minimize insecticide-induced problems, reduce cost of production and achieve stability and sustainability in the production systems.

8.2. SOURCES OF RESISTANCE

Genetic diversity supplies the raw materials for the development of varieties resistant to insect pests. Most of the major crops have been grown over extensive geographical regions and in numerous ecological environments for long time and have generated enormous reserves of genetic diversity. Resistance sources are available even within populations that are largely susceptible, among cultivars, wild races, related species and related genera. If cultivars resistant to an introduced insect species are sought, it may be desirable to obtain germplasm from the insects native home, where natural resistance may have developed through many years of selection. Another source, from which to secure plant varieties is the region of maximum variability, where the prevalence of wide differences in physiological characters could be the basis of resistance. Resistance within the species, modern breeding refinements afforded by chromosomal substitution, induced translocation or induced crossing over between chromosomes have made it possible to provide sources of genes for resistance from related species or even genera. Interspecific crosses have been successfully used and are still in progress in breeding for resistance. Many plant mutations have been induced by radiation or chemicals but very few have been useful in the development of insect resistant cultivars.

Genetic sources can be classified into primary, secondary and tertiary gene pools. Primary gene pool consists of improved and unimproved varieties, land races, and wild species that readily hybridize with the cultivated germplasm, produce viable hybrids and have normal chromosome pairing and combination. Introgressing these sources into the cultivated germplasm serve as donor for insect resistance and increase the genetic

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diversity of the cultivated crop germplasm pool. The secondary gene pool consists of wild species that are difficult to cross with the cultivated species because of ploidy and other differences. The wild forms and non-domesticated crop relatives are extremely variable and continue to evolve in response to biotic and abiotic stresses. This is a valuable source of insect resistance genes. However, the introgression of resistance genes from these sources into elite genetic background is often slow and difficult. The tertiary gene pool consists of even more distantly related species belonging to a different subgenus or related genera and are more difficult to hybridize. The recent developments in the field of molecular biology have opened up new avenues and possibilities of transferring alien genes from totally unrelated sources into plant system and provide the opportunity to develop transgenic crops with novel genes (from bacteria, viruses, animals and unrelated plants) for insect resistance. These novel genes either alter the existing function(s) or add new character into transgenic plants. The foreign gene(s) must be integrated into the genome of the recipient species and must express itself in the new genetic background. The introduction of foreign genes can be achieved through agrobacterium mediated DNA transformation, ballistic method of transformation, electroporation, polyethylene glycol mediated transformation etc. (Singh and Yerulkar, 2005).

To identify insect resistance, it is important to have many genetically diverse crop strains available. Seed can be obtained from various germplasm collections, which contain the traditional centuries tested varieties. National germplasm collections are maintained in many countries. The size of the collections may vary from few hundreds to several thousand accessions. For example, IRRI maintains about 60,000 accessions of Oryza sativa, 2614 strains of O. glaberrina and 1100 wild rice. Similarly gene banks have been established for other crops. The germplasm collections consist primarily of traditional varieties. When beginning a screening programmeme for a particular insect, it is best to first evaluate advanced breeding lines and released varieties developed by a research institute and related stations. These plants make good donors if resistance is found. Seed of released varieties and of resistant lines bred at other institutes/stations should then be obtained and tested against the local population of the particular insect. Next, obtain seeds of traditional varieties from the germplasm collection of national gene bank or from other local sources. Also try to get resistance sources from national programmes of different countries and from International Research Institutes/Centres. Where insect resistance cannot be found in cultivated plant varieties, wild relatives of crop plants should be screened to get resistance sources. Scheme for germplasm flow of rice at International Rice Research Institute (IRRI) is given in Fig1 and generalized mechanism for germplasm flow in Pakistan is explained in Fig 2. Almost similar approaches are used for other crops and other countries.

8.3. MULTIDISCIPLINARY TEAM APPROACH

Development of varieties resistant to insect pests involves a lot of studies on different aspects, therefore, multidisciplinary team approach is necessary for the success of the programmeme. The composition of a team changes from simple to complex. It is not sufficient to involve scientists belonging to different disciplines such as plant breeded, entomologist, plant physiologist, plant pathologist, agronomist, biochemist, soil scientist etc. but they should be of right type and competent scientists with urge of working in close collaboration with each other. The entomologist should have special training in host

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plant resistance. The breeder should be cognizant of the difficulties and problems in handling the insect, providing insects of different instars and stages at specified time periods. Likewise, the entomologist should develop a genetic view point in looking at procedures and explanations of developments. It is also necessary for the entomologist to know breeding procedures and materials in the crop under study, important varieties and specific problems that the plant breeder faces. It is also imperative for the entomologist to

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Fig 1. Germplasm flow for the development of resistant varieties at IRRI, Philippines

Source: Heinrichs et al. 1985.

Initial screening for insect resistance (entomologist)

Retesting of selected entries (entomologist)

Hybridization (plant breeder)

Single and multiple crosses (plant breeder)

F2 populations in field under unprotected condition(plant breeder)

Pedigree nurseries in the field under unprotected condition (plant breeder)

Varieties released by national programmeme

Improved materials from IRRI and national programmeme

IRRI germplasm collection

Screening of replicated yield trial (entomologist) entries for insect resistance in field and greenhouse (entomologist)

Resistance mechanisms and gene identification (entomologist and plant breeder)

Evaluation by national programmeme

International Rice Testing Programmeme entries for insect resistance in field and greenhouse (entomologist)

Release of a variety

Other screening programmemes (problem area

scientist)

Screening pedigree nurseries for insect

resistance (entomologist)

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Fig-2: GERMPLASM FLOW FOR THE DEVELOPMENT OF CROP VARIETIES IN PAKISTAN.

Source: Crop Sciences Institute (CSI), NARC, Islamabad

INTERNATIONAL CG RESEARCH

CENTRES

ADAPTATION

PROVINCIAL AND FEDERAL BREEDERS

CANDIDATE VARIETY

RELEASED/ RECOMMENDED FOR DIFFERENT

ECOLOGIES

RECOMMENDATION NS TO VARIETY

EVALUATION COMMITTEE (VEC)

DATA ANALYSES AND

COMPILATION

SEED COUNCIL

COORDINATED COMMODITY RESEARCH

PROGRAMMEME

INSECT RESISTANCE

SEED AGENCIES

QUALITY

FARMERS

FSC & RD

- MATERIAL DEVELOPED THROUGH HYBRIDIZATION - TESTING OF INTERNATIONAL MATERIAL

PRELIMINARY AND ADVANCE TESTING

BEST LINES SELECTED

INSECT NURSERIES

NUYT

DISEASE NURSERIES

DISEASE

RESISTANC

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possess necessary knowledge of bio-ecological parameters, nature of damage, population build up potential and rearing techniques of insects on plants and on artificial diet. In the development of resistant varieties, a considerable crossing and selection is necessary to transfer resistant genes from exotic or wild species into promising germplasm and varieties. It is therefore, very important to involve entomologist and agronomist in the programme. If resistance becomes attributable to chemical factors at early stage of the programme, then it becomes necessary to carry out precise chemical tests at the same stage by a biochemist.

In general at the beginning of the programme, a plant breeder and an entomologist may be sufficient. An initial step for entomologist is to identify sources for resistance and provide these sources to the breeder for use in the hybridization programme. As the programme develops, scientists from other disciplines should be involved. The role of entomologist is to develop efficient screening methods, identify resistant donors, screen breeding lines, determine the nature and causes of resistance, select for biotypes to be used in the screening programme, determine the rate of biotype selection and collaborate with the breeder in identifying genes for insect resistance. For more information on this aspect readers are advised to consult Panda (1979) and Heinrichs et al., (1985).

8.4. SETTING PRIORITIES FOR THE DEVELOPMENT OF INSECT RESISTANT VARIETIES

Following criteria may be taken into consideration for setting priorities for the development of insect resistant varieties:

i. Potential damage: Those insects should be selected for studies of host plant resistance which cause high yield loss.

ii. Distribution of insect pests: Wider the distribution more serious the pests are considered. A pest with low damage potential but with wider distribution may cause more financial loss than a pest which has a higher damage potential but occurs only in isolated areas. Studies on host plant resistance should be carried out for those insect pests which are widely distributed.

iii. Mode of damage: An insect which causes direct damage by feeding and in addition, transmits a virus disease would have higher priority than the one which causes only feeding damage.

iv. Availability of resistance sources: Screening should be undertaken against insects for which there is a good possibility of success in selecting resistant sources and transferring resistance into good agronomic types. It is better to screen varieties which have resistance to several insect species.

8.5. SCREENING AND EVALUATION OF GERMPLASM FOR INSECT RESISTANCE

A number of criteria are employed for resistance evaluation. Measurements are through the effect of the exposure of insects on the plant or plant parts of a known resistant or susceptible cultivar. This effect can be evaluated in terms of the plant as the

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per centage of damage to the foliage or to fruiting parts, reduction of the stand, per cent yield reduction and general vigour of plants. Resistance can also be measured in relation to various parameters of the insect such as fecundity, food preference, population development, food utilization, duration of developmental periods, mortality and adult longevity. In mass screening of large plant populations, a visual rating system is usually used. A more accurate assessment is necessary in the study of resistance mechanisms and in the investigations on chemical factors relating to antibiosis. In these studies, specific biossays are developed to monitor the effects of particular chemicals on behavioural and metabolic processes related to host plant finding, acceptance and suitability.

Techniques employed for evaluating germplasm for resistance depend on the insect and crop. Some broad, basic evaluation techniques and breeding methods for developing insect resistant plants, three fundamental factors must be considered irrespective of the crop or insect involved: (i) what are the best methods for identifying the gene or genes for resistance to the major insect pests of the crop under consideration? (ii) how can the genes for resistance to an insect be incorporated into commercial cultivars, inbreds, synthetics, or composites without introducing adverse effects ?(iii) how can insect resistance be incorporated so that the cultivar remains resistant for long period (Dahms, 1972).

Breeding programme for host plant resistance to insects must begin with an extensive, working Germplasm Bank. Because breeding plants resistant to insects involves an interaction between the plant and the insect, studies on the insect life cycle, its population fluctuations, ovipositional habits and seasonal occurrence may be needed before resistant studies commence. Techniques for evaluating large amounts of genetic material for pest resistance will differ for each insect species. However, there are several standard procedures that must be considered when a germplasm evaluation programme is initiated (Bellotti and Kawani, 1979):

Evaluation of programme must ensure large and uniform insect populations to guarantee adequate selective pressure. This is of paramount importance for screening of germplasm under field conditions using natural populations.

If field populations are inadequate, two alternatives can be implemented: (i) varieties grown in the field can be infested manually with the insect to ensure that every plant will receive an initial population of the pest being screened in addition to whatever natural population may occur, (ii) germplasm can be initially screened under controlled conditions with artificial infestations.

The initial germplasm screening programme can be geared to eliminate susceptible germplasm to reduce massive amounts of germplasm to a manageable number of lines or accessions. The selected lines are studied in details by exerting heavy selection pressure through maintaining a higher than normal insect populations.

When field screening is practised, pest releases or infestations should coincide with the season or climatic conditions that favour the development of the pest to ensure adequate selective pressure on the test material.

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When natural populations are used in a field screening, and no artificial infestation is supplemented, numerous rows of susceptible varieties should be planted throughout the varieties being screened to produce a more uniform pest population.

A host reaction scale should be developed in a screening programme to describe accurately the damage levels. This scale should define highly resistant, intermediate, and susceptible plants in terms of plant damage.

Scales should be devised to take into account the existing insect population, when symptoms are not sufficiently pronounced to evaluate resistance accurately.

Methods should be designed to evaluate rapidly large number of plants for insect damage or infestation. This can be done in the field or greenhouse. Mass screening of seedlings offers a useful time saving tool. However, resistance in the seedling should be correlated to that in the older plants.

Procedures used to screen crops for resistance to insects are based on the nature of damage and resulting injury symptoms caused by the pests. Progress in selection for resistant plant types depends on uniform and sufficient selection pressure. Although selection can be done under natural infestations, the pest population and/or crop often be manipulated to create selection pressure and to ensure that susceptible plant types do not escape infestation. Modification of existing screening and evaluation procedures is often required. Resistance in plants can be evaluated successfully under natural pest infestations, provided populations remain uniform and reach high levels at appropriate times.

Success in breeding for resistance to insect and mite pests requires efficient rearing and evaluation techniques. Populations of insects must be maintained in large numbers either in field or in laboratory preferably both, and methods should be developed to evaluate plants for resistance, both in the seedling and mature stages. Insects can be reared in the laboratory both on artificial media or host plants. Artificial media leads to more refined studies, but naturally occurring hosts are better selectors for insect biotypes. In the laboratory, insect rearing can be done through individual progenies from single pair mating or by mass rearing (Everson and Gallun, 1979).

In the initial screening, entries are non-replicated. In retesting, entries are replicated from 3-10 times depending on the insect species and other important factors. Entries in the replicated yield trial are screened in either the field or the greenhouse or both in the greenhouse and in the field. To establish a screening method, it is imperative to understand the biology of the insect and its potential plant damage. Developing an efficient rating method is extremely important. Ratings used in insect screening programme are based on degree of plant damage or insect numbers. To compare entries, a numerical rating system should be used. The screening method should give distinctly different reactions between susceptible and resistant entries. Plant reaction and subsequent damage rating depends on number of insects per plant, plant vigur, plant age and environmental factors.

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It is not feasible to describe screening techniques for all crops and even all insects of one particular crop in this book. Screening techniques of few insect species are given. These techniques can be used for other insects with minor modification based on the biology and behaviour of the insect and the nature of the crop.

8.5.1. WHITEBACKED PLANTHOPPER, SOGATELLA FURCIFERA

Germplasm screening for S. furcifera resistance can be done in greenhouse or in the field. Greenhouse screening is the rapid method for evaluating a large number of breeding lines, but the resistant entries should also be screened in the field before releasing them as varieties. Greenhouse screening can be done by conventional seed box test or by the modified seed box test.

The conventional seedbox test is a rapid method of screening large volumes of material for qualitative resistance. However, it usually cannot detect moderate resistant varieties and rate them as susceptible. In this method, two types of seedboxes are used: (i) the standard seedbox, (ii) the compartment seedbox. The standard seedbox measures 60 x 40 x 10 cm. Thirty nine test entries can be screened in a box. In the compartment seedbox, the compartments serve as a guide in sowing the seeds of test entries and prevent dispersing of seed while watering. The compartment seedbox measures 114 x 69 x 70 cm and 180 test entries can be screened per seedbox. If the greenhouse space is limited use the compartment seedbox.

At 3-leaf stage, release second to third instar nymphs of insect @ 8 nymphs per seedling in the seedbox. If older test seedlings are used, increase the number of insects per seedling. Grade the entries in each seedbox when the susceptible check seedlings are about 90% dead. Grade for plant damage using the Standard Evaluation System (SES) for rice as follows:

Scale Damage

0 None

1 Very slight damage

3 First and second leaves with orange tips, slight stunting

5 More than half the leaves with yellow orange tips; pronounced stunting

7 More than half the plants dead; remaining plants severely stunted and wilted.

9 All plants dead.

GREENHOUSE SCREENING BY THE MODIFIED SEEDBOX TEST

The plants are older at the time of infestation and fewer hoppers per seedlings are placed on the plants. In this method, the progeny rather than the initial source of infestation of the insects that cause the plant damage is taken into consideration.

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FIELD SCREENING

Breeding lines identified as resistant in greenhouse should be tested in the field before their release as varieties. Select a screening site and time, when whitebacked planthopper are abundant. If the population is low or unevenly distributed, then increase the population of the insect through application of resurgence inducing insecticides. In preliminary testing, entries are not replicated. In retesting, replicate each entry four times. Each replication consists of 2 rows, 2.4 m long for each test entry. Provide border rows around each replication, by planting a susceptible variety in 1.4 m strips of 7 rows each. When plants in susceptible check start wilting, begin grading. Grade the entries on a row basis. Use the SES (Standard Evaluation System) scale for damage rating.

SCALE DAMAGE

0 None

1 Slight yellowing of a few plants.

3 Leaves partially yellow but with no hopperburn

5 Leaves with pronounced yellowing and some stunting or wilting and 10- 25% of plants with hopperburn, remaining plants severely stunted.

7 More than half the plants wilting or with hopperburn, remaining plants severely stunted

9 All plants dead.

These screening and evaluation techniques can be used for brown planthopper, Nilaparvata lugens, and other species of planthoppers and leafhoppers with minor modifications. This can also be used as guideline for this type of pests on other crops.

8.5.2. YELLOW STEM BORER, SCIRPOPHAGA INCERTULAS AND WHITE STEM BORER, SCIRPOPHAGA INNOTATA

GREENHOUSE SCREENING

In case of greenhouse or screenhouse screening, transplant about two weeks old seedlings in the beds (25m x 5m) at 20cm x 20cm spacing. Plant one row for each entry. For every 10 rows of test entries, plant a row each of the susceptible and the resistant check. Plant one replication for initial screening and four replications for retesting. About 14 days after transplanting, infest the test plants with newly hatched larvae (10 larvae/hill). Using a camel hair brush, place the larvae on the youngest leaf auricle. Infestation of plants can also be done with inoculators. Use of inoculators is about 10 times faster than camel hair brush technique.

Count the number of ‘deadhearts’ about 4 weeks after infestation with larvae. The test is considered valid when per centage of deadhearts on the susceptible

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check is at least 25. Compute the per centage of ‘deadhearts’ for each entry. Convert per centage of ‘deadhearts’ to D to correct the level of infestation.

% deadhearts in the entry

D = --------------------------------------------------- x 100

% deadhearts in susceptible check

Transform the converted figure to a 0-9 scale:

SCALE % DEADHEARTS (D)

0 None

1 1-20

3 21-40

5 41-60

7 61-80

9 81-100

FIELD SCREENING

Field screening of rice germplasm for resistance against stem borers can be done under natural infestation. However, infestation with first instar larvae can be done if natural infestation is low. In field screening, it is imperative to select an area where the population level is high (hot spot). Select a time of the year when populations are high and try to coincide the susceptible stage of the plant with high populations of the insect. Transplant 20-day old seedlings in 5 m rows and 25x25 cm distance (20 hills/row). For initial screening, plant one row (replication) per entry but for retesting, plant 4 replications. After every 10 rows of test entries, plant a row each of susceptible check and a resistant check.

Observe for ‘deadhearts’ 30 and 50 days after transplanting (DAT) on 16 plants in each row. Compute per centage of ‘deadhearts’ and convert to D and 0-9 scale as in screenhouse.

Count ‘whiteheads’ before harvest. Compute per centage of ‘whiteheads’. The test is considered valid when ‘whiteheads’ in the susceptible check average at least 60%. Convert 0-9 scale as follows:

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SCALE % WHITEHEADS (D)

0 None

1 1-10

3 11-25

5 26-40

7 41-60

9 61-100

8.5.3. Leaffolder, Cnaphalocrocis medinalis

Greenhouse screening

In greenhouse screening, seedlings are grown in clay pots and are infested with moths which lay eggs on the seedlings. In initial tests, entries are not replicated. Entries with a rating of 0-5 in the initial screening are retested using 10 replications.

INITIAL SCREENING

Sow 10 seeds/plot and place the pots in a metal tray with water. Do thinning of seedlings and maintain 5 seedlings/pot at 10 days after seeding (DAS). At 14 DAS, enclose potted plants with a fiberglass net cage and introduce adults for oviposition. Instal one honey feeder in each of the four corners of the cage. Seven days after infestation, remove the honey feeders. When the moths are dead, remove the cage to allow more sunlight for better growth of seedlings. At 21 days after infestation, evaluate the damage caused by the insect larvae. For the test to be valid, at least 60% of the leaves of the susceptible check should be damaged. Consider the extent of damage on each leaf. For each entry first examine all the leaves and rate each one, 0-3 based on the extent of damage as follows:

GRADE DAMAGE

0 None

1 Upto 1/3 of leaf area scraped

2 > 1/3 to ½ of leaf area scraped

3 > ½ of leaf area scraped

Based on the number of leaves with each damage grade, compute the damage rating ® as follows:

A) = (No. of leaves with damage grade of 1 x 100) 1

Total no. of leaves observed

B)= (No. of leaves with damage grade of 2 x 100) 2

Total no. of leaves observed

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C) = (No. of leaves with damage grade of 3 x 100) 3

Total no. of leaves observed

R = A + B + C ÷ 6

Calculate damage rating ® for each test entry and the susceptible check. Then determine the adjusted damage rating (D) for each test entry as based on extent of damage in susceptible check by:

R of test entry

D = ----------------------------------------------------x 100

R of susceptible check

Convert D to a 0-9 scale

SCALE % ADJUSTED DAMAGE RATING (D)

0 0

1 1-10

3 11-30

5 31-50

7 51-75

9 More than 75

RETESTING

Entries with ratings of 0-5 in initial screening are retested. Entries are replicated 10 times. Seedlings are thinned to 5/plot and tillers pruned to 1 seedling/plot at 21 DAS. Two, first instar larvae are placed on each tiller with a camel hair brush. At 17 days after infestation, entries are evaluated for damage and damage ratings are determined as described earlier.

FIELD SCREENING

In field screening, a test is considered valid when infestation of leaves in the susceptible check is at least 50%. To increase population/infestation, select the proper planting date and area (hot spot). In Pakistan infestation of the insect is substantially higher on late rice crop than early sown or sown at recommended time. Plant, rows of a susceptible variety at the border of each plot or replication at close spacing to increase population or infestation of insect. Use higher doses of nitrogenous fertilizers. Apply resurgence causing insecticides to the susceptible border row plants or those insecticides which are comparatively more toxic to natural enemies than the pest.

After transplanting the test entries, release laboratory reared or field collected moths on the susceptible border plants to increase the level of infestation. When

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the susceptible check has at least 50% infestation of the leaves, make the damage grading. Count the damaged and undamaged leaves in 20 hills selected at random in each entry. Sum up the total leaves damaged and undamaged in each entry and compute the per centage of damaged leaves. Convert the per centage of damaged leaves as follows:

% damaged leaves in test entry

D = -------------------------------------------------------------- x 100

% damaged leaves in the susceptible check.

Convert D to a 0-9 scale

SCALE % ADJUSTED DAMAGE RATING (D|)

0 No damage

1 1-20 %

3 21-40 %

5 41-60 %

7 61-80 %

9 81-100 %

8.6. MAINTENANCE OF INSECT POPULATION

It is imperative to maintain an adequate insect infestation for getting reliable results. If infestations are too low, separation of resistant and susceptible strains and selection of plants in segregating generations cannot be done and much susceptible material is accumulated. While, too severe infestation may eliminate the material with a moderate degree of resistance. It is therefore, necessary to regulate insect populations so that they give the maximum demarcation between the resistant and susceptible types and provide some choice among the more resistant segregates from a cross. In certain areas insect populations are more than other areas. It is therefore, imperative to undertake varietals screening programme in such endemic areas or ‘hot spot’ locations. In most of the cases natural infestations have to be increased by suitable methods. Populations of some insects can be kept at a satisfactory level by establishing the test plot in a larger area grown with susceptible host plants or by interplanting rows of susceptible varieties among those being tested. Trap crops may also be used in or around the evaluation nursery to attract insects into selected area. Specific attractants may also be used to bring the insects into the test material. Infesation/population of test insect pests can be increased in a number of ways keeping in view the biology and behaviour of insect. Techniques for increasing infestation or population of few insect pests are described.

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8.6.1. INCREASING INFESTATION OF SORGHUM SHOOT FLY, ATHERIGONA SOCCATA

The insect population can be increased in the field by “spreader rows method” which consists of surrounding the experimental plot with early sown border rows. The test material can be sown in such a way that the plants are at a susceptible stage when the adult flies emerge from the border row plants (Blum, 1967). In India, in kharif season, early plantings suffer more from shoot fly damage in areas where kharif and rabi sorghum overlap. Planting of sorghum at proper time provides an effective and inexpensive way of maintaining shoot fly population for conducting screening trials under field conditions (Vidyabhushanam 1972).

Shoot fly infestation in experimental plots is ensured in Uganda by planting susceptible sorghum variety about two, weeks earlier than the test plants as border and in between the trial blocks. The shoot fly built up on susceptible variety and as soon as the varieties under test reached a susceptible stage, fish meal or meat meal is spread between the rows to attract the flies (Doggett, et al., 1970).

8.6.2. INCREASING FIELD INFESTATION OF RICE GALL MIDGE, PACHYDIPLOSIS ORYZAE

Infestation of the insect in field can be increased by the following procedure (Prakasa Rao, 1975):

Synchronize vulnerable stage of rice seedling with the peak period of gall midge emergence.

Maintain standing water during tillering stage.

Use higher doses of nitrogenous fertilizers.

Maintain closer spacing (15cm x 15 cm or 10 cm x 10 cm) of hills.

Use of light traps when the crop is 40 days old to attract insects.

8.6.3. INCREASING POPULATION OF RICE BROWN PLANTHOPPER, NILAPARVATA LUGENS

Select a screening site and time of the year when population of N. lugens are abundant. Sometimes the populations of the insect are too low or unevenly distributed and not suitable for reliable field screening. These handicaps to field screening can be overcome by causing N. lugens population increase through the application of resurgence inducing insecticides. Insects from culture can also be used to supplement natural field populations. About two months before the scheduled day of transplanting, place the insect on caged 80-day-old TNI plants in the greenhouse. Rear them on the plants, adding fresh plants as needed within two generations (about two months). The progeny from each female is sufficient to infest a 25 m long strip of susceptible border rows 1.4 m wide. This technique can also be used for increasing the population of whitebacked

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planthopper, Sogatella furcifera and other species of plant hoppers. Microplot technique can also be used to increase the population of the insect. A cage placed over a small plot consisting of one entry, induces the build up of artificially infested N. lugens populations by preventing natural enemies from entering the plots and preventing the insect from escape. In the microplot technique, 1.5 x 1.5 x 2 m cages are placed either over small plot in a wetland field or in specially prepared plots around the greenhouse or in a dryland field. One cage accommodate one entry (Heinrichs et al., 1985).

To increase the population of N. lugens, a polyethylene sheet is placed around small field plots to prevent movement of nymphs outside the plot and to prevent predators from entering the plot. This technique is called polyethylene barrier technique (Kalode et al., 1982).

8.7. BREEDING METHODS FOR HOST PLANT RESISTANCE

Most species of crop plants evolved during the period of very primitive agriculture, at first was by natural selection and then by deliberate selection by farmers for desirable characteristics. Such selections played a significant role in enhancing productivity and in improving quality (Russell, 1978).Testing and selection procedures in breeding for pest resistance depends on breeding system of the host plant, type and genetics of the resistance involved, and biology of the pest. In cross-pollinated crops, individual plants in a population usually have different genotypes and there is considerable plant to plant variation in resistance to a pest. It is, therefore, often desirable to select for improved resistance on individual plant basis in these crops. It is a common practice to self-pollinate selected plants, or cross them in pairs or in small groups. The progenies of selections are then tested for resistance and further selections are made. These re-selection can be inter-pollinated in larger groups to produce ‘breeding lines’ or crossed with progenies of unselected plants to maintain a high level of genetic variability. Alternatively, selected plants can be self pollinated in successive generations for re-selection so that resistant inbred lines are produced. This technique has certain drawbacks especially inbreeding depression. It is, therefore, desirable to avoid close inbreeding. Recurrent selection for resistance, in population derived by inter pollinating selected plants, is widely practiced in out breeding crops. This procedure can improve the level of resistance in the populations while maintaining an acceptable level of heterozygosity.

In self-pollinated crops, the progeny of a plant that has been selected for resistance to a pest, consists of genetically identical individuals, but segregation for resistance will occur in later generations. True resistance breeding lines can be selected for pest resistance in later generations, using the pedigree or bulk method. After several generations of selfing, each plant will be homozygous and no further selection for resistance will then be possible.

In self-pollinated crops, screening for resistance to a pest usually involves testing a number of individual, genetically stable genotypes and retaining the more promising genotypes for retesting and multiplication. The level of resistance can be further improved only by crossing some of these genotypes and seeking genotypes that express greater resistance among the F2 and later generations. In cross-pollinated crops, populations of

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genetically distinct plants are screened for resistance to a pest and the most resistant plants are selected and usually inter-pollinated. The progenies of selected plants are then tested for resistance and reselections are made. In this way, populations of plants with a higher level of pest resistance are developed by recurrent selection.

Methods to get resistant varieties include introduction of varieties from other countries or areas or ecologies, selection from available germplasm at national and international level, hybridization, mutation breeding and innovative approaches. These methods/approaches are briefly discussed.

8.7.1. INTRODUCTION OF VARIETIES

Introduction may be of new crops, new varieties of cultivated crops or varieties/crops with new or improved characters. Introductions should be made from those areas where the climatic conditions are similar to the area where the plants are to be introduced. Introductions are tested under local conditions and if prove superior to other cultivars then can be planted on commercial scale without further work. Moreover, these may be a source of valuable parental material in a selection programme or as parents in a hybridization programme.

8.7.2. SELECTION

Selection is one of the oldest breeding procedures and forms the basis of crop improvement. Selection (natural or artificial) is a process by which individual plants or groups of plants are sorted out from mixed populations. Generally individual resistant plants are selected from otherwise susceptible cultivars and are multiplied in isolation to form a new variety. Large numbers of pest resistant varieties have been developed through selection.

In plant or line selections, the initial number of plants or lines used is very large. This number is gradually reduced and only the desired number of superior plants or lines are selected. A variety is then synthesized from the surviving plants or lines.

8.7.3. HYBRIDIZATION

Hybridization can be defined as “biochemical mixing of different genotypes to get desirable combinations for human use/welfare. It usually involves improving a number of characters simultaneously while retaining other acceptable characters. The transfer of resistant characters to otherwise promising material is achieved by hybridization. The seeds as well as the progeny resulting from the hybridization are known as hybrid or F1. The progeny of F1 obtained by selfing or intermating of F1 plants, and the subsequent generation are termed as segregating generations. The chief objective of hybridization is to create genetic variation. When two genetically different plants are crossed, the genes from both the plants are brought together in F1. Segregation and recombination produce many new combinations in F2 and subsequent generations. In hybridization, different techniques are used.

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8.7.3.1. PEDIGREE SELECTION

Pedigree selection is also called “ear to row” selection. It means an ear selected from a plant will be used to plant on row in the next generation. After selection of desirable genotypes/plants, these are crossed with susceptible plants. The purpose of cross is to incorporate pest resistance character to the susceptible. All the F1 plants will be resistant to the insect pest if the resistant gene is dominant. All the F1 will be forwarded to next generation without selection. In F2, as maximum segregation is available, individual plants are selected and forwarded to next generation in “ear to row” fashion. In pedigree method, superior types are selected in successive segregating generations and a record is maintained of all parent progeny relationships. Selection begins in the F2 generation when the individuals are selected which will produce the best progeny. F3 progeny rows are grown from selected F2 plants. Resistant selections having agronomically desirable characters are made within F3 lines. Progeny row selections are continued through subsequent generations till the best resistant selected types become homozygous. Generally, by F5 - F6 generations, most of the lines become homozygous, hence further selection within lines is not required and emphasis shifts to selection among lines. The pedigree record at this point makes possible the elimination of all except one member of such closely related lines.

Pedigree method is effective for vertical resistance and variety produced contains high resistance. However, it is very laborious. All the selections are on phenotypic basis, therefore, highly skilled personnel is required for selection.

8.7.3.2. BULK SELECTION

Bulk selection is easy in practice and requires less labour. The variety developed through this method has more adaptability to the environment than other methods. Genetic purity is comparatively less as compared to pedigree or pure line methods.

In bulk breeding method, the F2 generation is planted in a plot large enough to accommodate several hundreds or even several thousands of plants. On maturity, the plot is harvested in bulk and the seeds are used to plant a similar plot in the following season. This process is repeated many times as resistant selections having agronomically desirable characters are made. Progeny row selections are continued through subsequent generations until the best resistant types are selected and all plants within a progeny row appear similar. During the period of bulk propagation, natural selection presumably plays a role in shifting gene frequencies in the bulk population. It is not desirable to rely exclusively on natural resistance in bulk population breeding. Artificial selection may aid natural selection by faster elimination of susceptible individuals from the bulk plot and increasing

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the frequency of resistant phenotypes. Subsequently single plant selections are made and evaluated in the same way as in the pedigree method of breeding, permitting the selection of completely self-fertilized, near homozygous lines in F6 and F7 generations.

8.7.3.3. BACKCROSS METHOD

The backcross method of breeding is used to transfer one desirable or needed characteristic found in an undesirable or un-adapted genetic background into a cultivar that is well adapted to a particular area. In other words, it provides a mean of improving varieties that have large number of good characteristics but are deficient in a few characteristics. In this method, an original cross is made between the adapted (recurrent) parent and un-adapted (non-recurrent) parent. It makes use of a series of backcrosses to the variety to be improved during which the desired characteristics are maintained by selection. At the end of backcrossing, the genes being transferred will be heterozygous. Selfing after the last backcross produces homozygosity for this gene pair and coupled with selection, will result in a variety with exact adaptation, yielding ability and quality characteristics of the recurrent parent but superior to that parent in that characteristics for which the improvement programme was undertaken.

If the character being transferred is dominant and easily identified, the plants with the desirable characteristic are backcrossed to the recurrent parent. Progeny with the desired characteristic from this cross are again backcrossed to the recurrent parent. Crossing with the recurrent parent is continued until a sufficient amount of germplasm of the recurrent parent has been recovered. One or two cycles of self pollination of phenotyhpic recurrent selection can be done after crossing in order to intensify or increase the gene frequency of the desired character. If the character being transferred is recessive, one generation of selfing may be required after each cross in order to recognize the phenotype of the desired character.

Backcross method provides the plant breeder with a high degree of genetic control in the populations. It is probably the most easy, efficient and short method. No testing is required if gene is incorporated to a cultivated variety. However, gene transfer becomes difficult if the desirable character is polygenic or recessive.

8.7.4. MUTATION BREEDING

Mutation is a sudden heritable change in a trait of an organism. Mutations produced by changes in the base sequence of genes are known as point mutations. Mutations occur in natural populations at a low rate. These are known as spontaneous mutations. It is estimated that the frequency of natural mutations is generally one in one million. Mutations may be artificially induced (induced mutations) by a treatment with certain physical (Alpha, beta and gamma

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rays, neutrons, x-rays, ultra violet light) or chemical [Ethyl methane sulphonate (EMS), methyl methane sulphonate (MMS), ethidium bromide, acriflavine proflavine, 5-bromoural, nitrous acid, sodium azide etc.] agents (mutagens). The utilization of induced mutations for crop improvement is known as mutation breeding. Treating a biological material with a mutagen in order to induce mutation is known as mutagenesis. Exposure of a biological material to radiation is known as irriadiation. Mutation breeding is an effective and quick tool for genetic improvement. Less time is required to bring variation through this approach.

8.7.5. METHODS OF PRODUCING TRANSGENIC PLANTS

Breeding for host plant resistance to insects due to the presence of natural defence mechanisms have been painstakingly slow, less efficacious and less productive. Development of transgenic crops by transferring the genes for insect resistance from microbes or other plants or synthesis of new insecticidal genes is a promising approach for effective pest control. Bacillus thuringiensis (Bt) subsp. Kurstaki is considered the most effective bacterial insecticide against lepidopterous insect pests. It produces parasporal crystalline (Cry) proteinaceous endotoxins, which cause toxicity in plants against lepidopteran insects. Bt is also effective against coleopteran and dipteran pests. Insecticidal cry genes from Bt and protease inhibitor (PI) and lectin genes from plants have been used for developing pest resistant transgenic crops. Of these, Bt transgenic crops have made it possible to express cry genes in all parts of crop plants throughout plant growth duration. Cultivation of transgenic crops has been helpful in eliminating or at least reducing insecticide use.

Most of the important crop species have been successfully transformed. In general, plant transformation employ a technique for delivering the DNA into the cell without regard to its ultimate intracellular location. Once, inside the appropriate cellular location by a chance process, the DNA is integrated into the chromosomal (or organellar) DNA, usually by a nonspecific recombination process. In plant transformation, two physical barriers prevent the entry of DNA into a nucleus, namely the cell wall and the plasma membrane. Most plant transformation methods overcome these barriers. Genetic transformation of plants requires efficient transfer of genes into cells, which are competent both for integrative transformation of transgenes and regeneration into plants. Several techniques have been developed for the delivery of genes into plants. Transgenic plants can be produced by number of methods such as agrobacterium mediorted gene transfer, direct gene transfer, polyethylene glycol (PEG) mediated gene transfer, electroporation and microprojectile bombardment.

8.7.5.1. AGROBACTERIUM MEDIATED GENE TRANSFER

This approach involves various strains of Agrobacterium tumefaciens such as LBA4404, EHA 105 and C 58 carrying a T-DNA vector. Introduction of foreign genes into plants is based on the natural DNA transfer capability of Agrobacterium tumefaciens. It infects plants causing the crown gall disease. The T-DNA of the tumor inducing Ti

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plasmid is transferred and integrated into the genome of the explant. The border sequences of T-DNA consisting of direct repeats of 24 or 25 base pairs flanking the T-DNA, are required for DNA transfer. Transformed plants are regenerated from the plant cells after brief exposure to the Agrobacterium suspension via tissue culture by manipulating and optimizing nutritional and environmental conditions for plant growth in vitro. Selection markers are used to select only those transformed cells, that have acquired the selection marker trait by virtue of integration of the T-DNA carrying the selection marker or reporter gene, as well as the foreign gene of interest into their genome.

Agrobacterium-mediated transformation has been successful in rice, maize, sorghum etc. However, development of reliable and efficient protocols are needed to improve the efficiency and range in transforming crop plants.

8.7.5.2. DIRECT GENE TRANSFER

Direct gene transfer makes use of chemical, physical or electrical methods to introduce DNA into the cells. These techniques include polyethylene glycol (PEG) mediated gene transfer, electroporation, microinjection and microprojectile bombardment. DNA can be transferred into protoplasts by chemical treatment with PEG at high pH in the presence of divalent cations. Upon treatment with PEG, the permeability of the plasma membrane of protoplasts is increased, leading to transient formation of pores, thus facilitating the uptake of DNA by protoplasts.

Application of pulses of high voltage (electroporation) results in transient opening of plasma membrane and uptake of DNA to protoplasts. Microinjection of DNA into cells is done using microcapillaries and microscopic devices. It is a precise method of delivery of DNA into specific cell compartments. Microinjection of DNA into the nucleus can give high efficiency of transformation. In case of particle bombardment or biolistics, DNA is shot at the tissue at a very high voltage such that the plant cell membrane opens up transiently and DNA is taken up by the cells. This method has been used for plants which are neither amenable to agrobacterium mediated transformation nor to protoplast culture. Metal particles, usually tungsten or gold are shot at the target cells at a high acceleration employing electrical discharge firing under vacuum or compressed helium discharge. For further details readers are advised to consult Shillito et al. (1985), Newhaus et al. (1987), Klein et al. (1987), Webb and Morris (1992), Bechtold et al. (1994), Hiei et al. (1994), Bent et al. (1994), Zupan and Zambryski (1995), Ishida et al. (1996), Galun and Breiman (1997), Ramaiah and Skinner (1997), Cheng et al. (1997), Barcelo and Lazzeri (1998), Gheysen et al. (1998), Kado (1998), Veluthambi et al. (2003), Chandra and Pental (2003), Skinner et al. (2004), Kaur and Gujar (2005),

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8.8. LEVEL OF RESISTANCE: EXAMPLES FROM PAKISTAN

Incorporation of insect resistance in crop plants is one of the most important priority areas in the development of new varieties of major crops in Pakistan and elsewhere. No new variety of major crops in Pakistan is approved for general cultivation which is susceptible to insect pests and diseases. Due to the development of new biotypes of insect pests host plant resistance may breakdown or resistance level may be reducd due to certain physiochemical plant stresses. Such situations necessitate replacement of existing varieties. Development of insect resistant varieties is, therefore, is a continuous process to save growers from loss and achieve sustainability and stability in the crop production systems.

It is the obligation of scientists to carry out studies on insect-host interactions on sustainable basis to identify resistance sources against major as well as potential insect pests. Such resistance sources are precious assets for food security. Resistant sources should always be available in the gene banks to cope with the ever changing scenario and to be used at the time of need and for disaster management. The development and use of insect resistant crop varieties offers a promising option in most of the cases and becomes inevitable under certain situations.

Although, in Pakistan some emphasis is laid on the development of new varieties which have resistance against insect pests, but the overall system have number of weaknesses. There is shortage of trained manpower and financial resources. Coordination among breeders, entomologists and other relevant scientists is lacking. In most of the cases scientists rely on field screening and evaluation for resistance against insect pests. Standardised techniques and procedures for such studies are generally not followed. Such approach fails to differentiate between pseudo-resistance and actual resistance. It is, therefore, imperative to remove all such weaknesses in the system.

The purpose here is not to review all the work done for development of new varieties of different crops, but give some examples only.

8.8.1. COTTON

Comparison of a high yielding, early maturing and heat tolerant cotton variety CRIS-134 was done with NIAB-78, CRIS-9 and CIM-448 for resistance against sucking insect pests and bollworms. Minimum population of Bemisia tabaci and Amrasca devastans was noted on CRIS-134. While, minimum population of B. tabaci was recorded on this variety only during 1997. Minimum population of A. devastans and maximum of B. tabaci was observed on CRIS-134. Considering comparatively high level of resistance of CRIS-134 against Thrips. tabaci, A. devastans and bollworms, this variety can be grown without the use of insecticides (Soomro et al., 2002).

Hair density on midrib and leaf lamina of cotton were found to contribute resistance against aphid, mite and bollworms in cotton varieties and advance lines SP16, Bl6, B557 and B75 (Javed et al., 1998).

Cotton variety CRIS-7A proved resistant against A. devastans but susceptible to B. tabaci. The susceptibility of this variety against B. tabaci

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indicated the positive correlation of hairiness and B. tabaci attack. Moreover CRIS-7A was comparatively resistant to bollworm than NIAB-78 and CRIS-9 (Soomro et al., 2000)

Eight genotypes of transgenic and conventional cotton were studied against bollworms. IRFH-901 was comparatively resistant to bollworms. Overall infestation of bollworm complex was high on H-900 as compared to genotypes of transgenic and conventional cotton and it was more susceptible to cotton pest complex (Sarfraz et al., 2005).

Ten genotypes of cotton were evaluated for resistance against sucking insect pests including A. devastans, and T. tabaci. CIM-70 and FH-87 showed maximum population of A. devanstans (1.69/leaf) and T. tabaci (5.3/leaf). P-43/60 and S-20 had minimum population of A. devastans (0.71/leaf) and T. tabaci (2.7/leaf). These differences in the level of resistance were attributed to differences in the chemical composition of different varieties/advance lines (Ali et al., 1995a).

Four varieties of cotton (CIM-70, FH-87, NIAB-86 and MNH-93) and 6 advance lines (P-2/2, P-43/13, P-43/51-2, P-43/60, S-9/1 and S-20) were tested for resistance against A. devastans and T. tabaci. The population of A. devastans and T. tabaci differed significantly on different entries. Higher quality of N. protein and Ca in top leaves; total lipids reducing sugars Ca and Zn in middle and bottom leaves and lower amount of total minerals, copper (Cu) and Zn in top leaves contributed resistance against T. tabaci. Total lipids, reducing sugars, magnesium (Mg) and Ca in middle + bottom leaves showed negative and significant correlation with A. devastans population while total minerals played positive and significant role for resistance to the insect (Ali at al., 1995b).

Ten varieties/advance lines (CIM-70, FH-87, MNH-93, NIAB-86, F-2/2, P-43/13, P-43/51-2, P-43/60, S-9/1 and S-20) were tested for resistance against A. devastans and T. tabaci. CIM-70 and FH-87 showed maximum population of A. devastans (1.69/leaf) and T. tabaci (5.3/leaf). P-43/60 and S-20 had minimum population of A. devastans (0.71/leaf) and T.tabaci (2.7/leaf). These differences in the level of resistance were attributed to differences in the chemical composition of different varieties/advance lines (Ali et al., 1995c).

Nine genotypes of cotton (SLS-1, N-26-S, N-92, Karishma, MNH-329, N-86-S, BH-89, N-86-Pb and BH-36) were evaluated for resistance against sucking insect pests including A. devastans, B. tabaci, and T. tabaci. N-86-Pb, N-92 and Bh-89 respectively showed the maximum per leaf populations of A. devastans (3.96), B. tabaci (3.30) and T. tabaci (0.95), while the minimum population of A. devastans (1.66), B. tabaci (1.89) and T. tabaci (0.34) per leaf was found on N-92, BH-86 and MNH-329, respectively. Hair density on midrib and lamina and gossypol glands on midrib showed significantly negative correlation with A. devastans population. (Aheer et al., 1999).

Five cotton varieties (FH-643, FH-634, FH-685, CIM-448 and NIAB-Karishma) were tested for resistance against A. devastans, and B. tabaci. A negative correlation was recorded between A. devastans attack and number of

275

hair per unit leaf area. A positive correlation between B. tabaci population and leaf hair density was observed (Mansoor-ul-Hasan et al., 1999).

Twenty cotton genotypes were screened for resistance against B. tabaci. The comparative resistance against the pest in different genotypes varied. NEH-43/30 variety was found to be most resistant. The variation in resistance level was mainly associated with changes in thickness of leaf lamina and number of gossypol glands of mid rib. The former factor was positively correlated, while later negatively correlated with population development of the insect (Murtaza et al., 1999).

Studies were carried out to evaluate different genotypes of cotton for resistance against different species of insect pests. CRIS-134 showed tolerance against bollworm, followed by CRIS-133. Maximum seed cotton yield was obtained from CRIS-120, followed by NIAB-78 and CRIS-126 (Leghari et al., 2001).

Seventeen genotypes of cotton (CIM-1100, CIM-435, MS-84, DNH-40, CIM-240, CIM-443, CIM-109, FH-87, S-12, CIM-436, MNH-93, Ravi, NIAB-78, Shaheen, Rehmani, MNH-147 and Gomal-93) were tested for resistance against sucking pests. (B. tabaci; A. devastans; T. tabaci and mite, Tetranychus telarius) and bollworms (pink bollworm, Pectinophora gossypiella; spotted bollworms, Earias insulana, and E. vittella and American bollworm, Heliothis armigera). Cultivars CIM-443, MNH-147, CIM-436 etc. were susceptible to these pests. Cultivar Ravi was found most resistant by exhibiting minimum average population of B,. tabaci (2.98/leaf), A. devastan (1.27/leaf), T. tabaci (1.84/leaf), mites (1.09/leaf) and lowest bollworm damage (12.57%) (Khan et al., 2003).

Studies were carried out on varietal resistance of cotton against Earias spp. The minimum and the maximum infestation of 1.79% and 2.38% were recorded on Red Okra-VI and Green Red Okra varieties of cotton, respectively. Twelve cotton varieties were evaluated but not significant differences in the infestation were found (Abro et al., 2003).

Hussain (2001) evaluated 5 cotton cultivars (FH 634, FII-910, F14-919, CIM-473 and MNII-552) for resistance against cotton whiteflies, B. tabaci; cotton jassid, A. devastans; cotton thrips and cotton aphids. CIM-473 was found to be the most resistant to test insects. Based on the results it is suggested that the cultivation of CIM-473 should be encouraged and that of FH-634 which proved highly susceptible to the sucking pests complex should be discouraged for cultivation.

Four cotton cultivars (NIAB-Krishma (vi), FH 900, CIM 446 and BH 118) were tested for resistance against American bollworm, Helicoverpa armigera. Mean population and damage caused by the larvae of H. armigera was non-significant for all the cultivars (Mustafa et al., 2004).

Twenty two cotton genotypes (BH 121, B 1-1-125, BH 147, CIM 473, CIM 482, CIM 499, CIM 511GE, CIM 707, CRIS 467, CRIS 468, DNH 157, FNH 945, FNH 1000, MNH 633, MNH 635, MNH 636, NIBGE 1, RH 510, SLH 244,

276

SLH 257, VH 141 VH 142) were evaluated for resistance against B. tabaci, A. devastans, and T. tabaci. The maximum mean seasonal population was 1.3, 1.7 and 3.1 of B. tabaci (adult and nymphs), respectively on genotypes BH 147 and FNH 945. The minimum mean seasonal per leaf population of B. tabaci was 0.5/leaf on genotype BH 121 and CRIS 467, A. devastans 0.6/leaf on MNH 635 and T. tabaci 0.8/leaf on CIM 499. Leaf hair density and length were important plant traits contributing resistance against sucking insect pests. Leaf hair densities on leaf midrib vein, lamina and leaf hair length were positively correlated with the population of B. tabaci and T. tabaci. These plant traits were negatively correlated with A. devastans (Aslam et al. 2004).

Five cotton cultivars (FH 634, F11-910, F14-919, CIM 473 and MN11-552) were tested for resistance against A. devastans, B. tabaci, T. tabaci and cotton aphids. CIM 473 was found most resistant to these insect pests (Hussain, 2001).

The differences among infestation of spotted bollworms (E. insulana and E. vittella) on G. arboretum and G. hirsutum varieties were non-significant. Its maximum infestation was observed on variety 1174 (11.8%) and the lowest on Desi cotton (6.2%). While infestation of pink bollworm (P. gossypiella) was maximum on Desi and lowest on 1174 and Qalandri. Qalandri variety proved best (31.1 maunds/acre) yield, followed by 1174 variety (Yashkun, 2000)

8.8.2. SUGARCANE

With the advent of genetic transformation techniques it has become possible to clone insect genes into the crop plants to confer resistance to insect pests. Resistance to insects has been demonstrated in ransgenic plants expressing genes for delta endotoxin from Bt, protease inhibitors, enzymes and plant lectins. Investigations were carried out to generate insect resistant sugarcane plants (Khan, 2001).

Five sugarcane genotypes (BF-162, SPSG-26, L-118, CP-43/33 and CP-72/2086) were tested for resistance against top borer, Scirpophaga nivella at tillering stage. FB-162 had comparatively high level of resistance and CP-43/33 was susceptible against the insect. It was concluded from the data that N, K, Ca, Mg and Fe contents in sugarcane plant played a significant positive role, while P, carbohydrates, fats and Zn content played a significant and `negative effect on pest infestation at tillering stage (Ashfaq et al., 2003).

Of 24 varieties/mutants evaluated in the field for resistance to S. nivella, Chilo infuscatellus and Emmalocera depressella, the variety CP67-412 and the mutants AEARC-1002 and AEARC-2001 were comparatively resistant (Ashraf et al., 1986). Sugarcane cultivars SPSG-26, BF-162, CP-72/2086, CP-43/33 and L-118 were tested for resistance against top borer, S. nivella. BF-116 was found comparatively resistant to the insect. Plant height, cane girth, leaf area, and leaf sheath hairiness showed negative and significant correlation with pest infestation at tillering stage (Khaliq et al., 2003). Information on the level of resistance of approved sugarcane varieties against major insect pests is given in table 1.

277

8.8.3. PULSES

Resistance of seven strains of chickpea against Callosobruchus chinensis was investigated. The varieties/ lines of chickpea included: NCS-96002, NCS-950004, NCS-950012, 92CC-076, 92CC-079, NCS-950183, NCS-960003. Chickpea variety Paidar-91 was used as check. NCS-960003 was comparatively more resistant than check (Riaz et al., 2000).

Table 1. Insect resistance status of approved sugarcane varieties in Pakistan

Varieties Top borer Stem borer Root borer Gurdaspur borer Pyrilla

BF-162 MS MS MS MS R

SPSG-26 MS MR MR MR MR

BF-129 MS MS MS MS MS

CP-43-33 R R R R R

CP-72-2086 MS MS MS MS MS

CP-77-400 MR MR MR MR MS

CPF-237 MR MR MR MR MS

SPF-213 R R R R R

COJ-84 MS MS MS MS MS

HSF-240 R R R R R

SPF-234 MS MR MR MR MR

SPF-245 MR MR MR MR MS

HSF-242 MR MS MR MR MS

Ghulabi-295 MS MS MS MS R

NIA-98 MR MR MR MR R

Thatta-10 MR MR MR MR R

NIA-2004 MR MR MR MR R

LRK-2001 MR MR MS MS MR

CPM-13 MS MS MS MR MS

CO-1321 MS MS MS MS MS

Mardan-2005 MR MR MR MR MR

Mardan-92 MS MS MR MR MS

Mardan-93 MS MS MS MR MS

NSG-311 MR MR MS MR MS

NSG-555 MR MS MS MR MS

MCP85-1491 MR MR MR MR MS

278

MCP80-1827 MR MR MR MR MR

S-2002-US-560 R R R MR MR

S-2002-US-637 R R R R R

S-2002-US-640 MR MR MR MR MR

S-2000-CPSG-449 MR MR MR MR MR

S-2000-CPSG-1550 MR MR MR MR MR

LRK-2003 MR MR MR MR MS

LRK-2004 MR MR MR MR MS

Ganjbakhsh MR MR MR MR MR

GT-11 MS MS MS MS MS

CPNIA82-1026 MR MS MR MR R

HoTh-127 R R R MR MR

HoTh-300 R R R MR MR

HoTh-326 MR MR MR MR MR

CPD-01-245 R R R MR MR

CPD-01-354 R R R MR MR

CPD-01-335 R R R MR MR

R= Resistant. MR= Moderately Resistant MS= Moderately Susceptible

Source: National Coordinated Sugar Crops Research Programmeme, NARC, Islamabad.

Eighteen chickpea, genotypes were evaluated for resistance to Callosobruchus maculatus by taking into account the number of undamaged seeds, number of eggs oviposited and number of emergence holes. The number of emergence holes was a better indicator of seed resistance (Ahmad et al., 1989).

Out of 20 chickpea lines tested for resistance against pod borer, Helicoverpa armigera, cultivars 549 and 600 were resistant to the insect. Four cultivars (620, 662, C-727 and 905) were moderately resistant to the pest, while rest of the cultivars were moderately susceptible (Pervez et al., 1996a). Out of 20 lines of chickpea tested for resistance against H. armigera, genotypes 1230 was resistant. Genotypes 932, 1084 and 4001 were moderately resistant, while the remaining cultivars were moderately susceptible (Pervez et al., 1996b).

Varieties of mung (321, 6601, 141, No.1 and 702) mash (80, 48, 59 and 118 and AARIM 117) and chickpea (86208, CM-72, 86221, 86037 and C44) were tested for their relative resistance against dhora, Callosobruchus chinensis under controlled laboratory conditions (25-30 oC). Differences regarding oviposition, per cent adult emergence, per cent number of grains bored and per cent weight loss caused by C. chinensis were significant among varieties. Mung No.1, Mash 59 and Gram 86037 were comparatively resistant (Ashraf et al., 1991).

279

8.8.4. RICE

Out of forty four rice cultivars tested for resistance against yellow stem borer, Scirpophaga incertulas IRI-3639-34, BG-276-5 and DR-83 were moderately resistant and others were susceptible (Mahar and Bhatti, 1985).

Studies were carried out to screen 18 coarse and 23 fine grain rice genotypes for resistance against yellow stem borer, Scirpophaga incertulas and pink stem borer, Sesamia inferens. Coarse rice genotypes IR8-151, IR-20, B/94, IR6-25-1 and IR6-2513/94 were comparatively resistant to borer attack and produced high yield. Among the fine grain genotypes Basmati 15-14/93 had significantly low borer damage and gave the highest paddy yield (Shafiq et al., 2000).

Sixteen cultivars/ lines were screened for resistance to S. incertulas. TKM 6, Basmati 370 and Basmati 4321 had the lowest deadhearts and whiteheads. TKM 6 and Basmati 385 exhibited non-preference and antibiosis, whereas line 4321 showed preference and antibiosis for S. incertulas. Basmati 198 and line 4439 were susceptible, while other test entries were moderately resistant (Riaz et al., 1993).

Studies on the screening of rice germplasm for resistance against leaffolder, Cnaphalocrosis medinalis have been carried out by Hassain (1986), Arshad (1987), Ahmad et al., (2000) and Rehman and Salim (2003). Rehman and Salim (2003) screened and evaluated about one thousand rice accessions under field condition for resistance against C. medinalis. Of these 4, 98, 210 and 688 were resistant, moderately resistant, moderately susceptible and susceptible to the insect. On the basis of initial field evaluation of rice germplasm, the lines/varieties with higher leaf infestation were discarded. Rice lines/varieties with less than 20% leaf damage were selected and a total of 235 rice lines/varieties were selected for greenhouse/laboratory evaluation. The results revealed that none of the rice line/variety was found resistant to C. medinalis under laboratory/greenhouse evaluation. According to IRRI’s Standard Evaluation System for Rice, 4 lines were in score 3, 24 in score 5, 147 in score 7 and 60 were in score 9 respectively (Table 2). Of these tested lines, only 67 accessions were selected for further testing.

Sixty seven rice lines selected from 235 previously tested lines in the field were screened against C. medinalis under semi-controlled condition at NARC during years 2001-02. None of the lines was resistant to the insect (Table 3). According to the damage rating calculated using IRRI‘s Standard Evaluation System for Rice, 17 rice lines were placed under scale 3, 31 under scale 5, 17 under scale 7 and 2 under scale 9.

Forty six entries of National Uniform Rice Yield Trial (NURYT) were evaluated under semi-controlled condition at NARC during 2001-02 and 2002-03 crop years. None of the candidate rice varieties was found resistant to this pest (Table 4). However, 10 candidate varieties were moderately resistant and remaining lines were either moderately susceptible or susceptible to C. medinalis.

280

Table 2: Screening of rice germplasm against C. medinalis

Status Score Number of lines

Resistant 1 0

Moderately resistant 3 4

Moderately susceptible 5 24

Susceptible 7 147

Highly susceptible 9 60

_____________

Total 235

Source: Rehman and Salim, 2003

Table 3: Screening of rice germplasm against C. medinalis under natural condition

Status Score Number of lines

Resistant 1 0

Moderately resistant 3 17

Moderately susceptible 5 31

Susceptible 7 17

Highly susceptible 9 2

_____________

Total 67

Source: Rehman and Salim, 2003

281

Table 4: Screening of candidate rice varieties against C. medinalis under natural condition.

Number of lines

Status Score 2001-02 2002-03

Resistant 1 0 0

Moderately resistant 3 0 4

Moderately susceptible 5 10 12

Susceptible 7 6 7

Highly susceptible 9 1 0

________ _______

Total 23 23

Source: Rehman and Salim, 2003

Table 5: Evaluation of commercial varieties to C. medinalis under semi- natural condition

Variety Damage rating (%) Scale Level of Resistant

Basmati 370 30.1 3 MR

Basmati 385 50.5 7 S

Super Basmati 42.7 5 MS

Basmati 2000 49.8 5 MS

Shaheen Nafees 38.4 5 MS

Kashmir Basmati 30-8 5 MS

Basmati 6129 31.5 5 MR

Basmati 198 25.9 3 MS

Ks 282 38.2 5 MS

IR 6 34.9 5 S

DR 82 42.6 5 MS

DR 83 53.6 7 MS

Sada Hayat 38.5 5 MS

DR 50 35.1 5 S

DR 53 29.5 3 S

282

Jajjai 77 62.3 7 MS

DM 24 62.1 7 S

JP 5 33.8 5 MS

Swat II 51.7 7 MS

Pakhal 38.6 5 MS

Kunhar 34.1 5 MS

Source: National Coordinated Rice Research Programmeme, NARC, Islamabad.

Table 6. Resistance level of promising rice germplasm against major insect pests of rice in Pakistan

Promising line

S. incertulas C. madenalis S.furcifera

40265 - HS MR

49731 - MS MR

49737 - S MS

NIAB-2001 - S MS

BASMATI 2000 MS MS MS

SUPER BASMATI MS MS MS

PK 3355-5-1-4 - MS MS

PK 3300-12-2 - MS S

NIAB IR-9 MR MS MS

IR6-15 - S MS

DR-64 - MS MS

GOMAL 6 - MS MS

GOMAL 7 - MR MS

L-12 - MR MR

283

L-33 - MR S

L-142 - MR MS

L-143 - MR MR

IR 6 MS S S

KS-282 S S S

TAISEN YU 255 - MS S

KAOHSIUNG SEN 339

- S S

DILROSH 97 - MS S

JP- 5 - MR S

PK 5261-1-2-1 S MR MR

98801 MS MS R

98410 S MS MS

PB-95 S MS MS

SRI-8 S S MS

BASMATI-15-2 S S MS

NIAB 2000 S MS MS

KSK-201 MS S MS

KSK-202 S S S

PK 3699-43 S S MS

IR-25/A MS S S

P 51 MR MR S

P-52 R MR S

284

YRL/L-202-6S-5S-1S-1S

- S S

YRL/L-202-6S-5S-1S-2S

- S HS

CT674-9-21-4-7-M-1-M

- S HS

BG 1639 - S MS

DAK 83-99-27-3 - MS HS

GBR-1 HS MS MS

P52-9-2 - S S

P38-6-1 - MS MS

97502 S MS MR

98504 S S MR

98316 S S MS

KSK-301 S S MS

RD-25 MS S MS

99421 MS MS MR

T5 S HS MS

SRI-12 S MS MR

BASMATI-15-1 S MS MS

P 38-6-1 S MS MS

P 52-9-2 S S MS

98506 MS MS MR

99417 S HS MR

285

99512 S S MS

99513 S S S

BASMATI PAK MS MR MR

GBR-2 HS MS MS

GBR-3 HS MS MS

99421 S MS MS

99723 S S S

KSK- 401 MR S S

KSK-402 S S S

PK 7797-1-2-1-1 MS S MS

GPP 101 MS S S

00515 HS MS MS

00521 S MS MR

99404 HS S MR

DIK-1 MR S S

KSK-406 S MS S

00518-1 HS S MS

PK 8337-2-2-1 S MS MS

99316 HS S MS

PS 1 S MS MS

JAJAI 25/A S S MS

DM 1-30-34-99 HS S MR

286

EF 1-20-119-02 S MS MR

T15 HS S MS

KSK 418 S MS S

KSK 429 S S S

KSK 432 MS MS S

DR 57 MS MS HS

DR 58 S S HS

PK 7429-5-14-1-1 HS S MS

PS 2 MS MS MR

KSK 431 S MS S

KSK 434 S MS HS

KSK 436 S MS S

DR 65 S S S

PSP 1 HS S HS

PSP 2 MR MS S

ILLABONG - S HS

PR 26881-PJ16-4B-78-5-1

- S HS

YUNLEN-2 - MS MS

IR 1384 - S MS

GZ 5830-63-1-2 - S MS

Table 7. Level of resistance of rice hybrids against major insect pests

287

HYBRIDS Leaffolder WBPH Stem borer

-

MKH YBRID109 MS MS -

MK HYBRID 110 S MS -

MK HYBRID 111 MS MR -

MK HYBRID 117 S S -

PHB 71 HS MS -

27 P 72 MR MS -

H 8002 MS S -

RH 257 S MS -

GNY 401 MS MR -

GNY 402 S S -

GNY 403 HS S -

GNY 404 S S -

GNY 405 HS MS -

GNY 406 S S -

ARIZE 403 S MR -

KS 282 S MS -

HR 40 S MS -

HS 06-3 MS MR -

HSLY-287 MS S -

DAGHA-1 HS S -

288

R6606 MS S MR

R6301 S S MS

SY-470 MS S MR

SY 444 MS MS MR

INDAM-1 MS MS S

S S MS

INDAM- 11 HS S MS

INDAM- 12 MS MS S

IR-6 S HS MS

99-413 S HS -

SH-UL S S -

RA-203 HS HS -

RA-204 HS MS -

LP-1 S S -

LP-2 MS MS -

LP-3 MS MS -

DIANFAXMNG YOU-43

S MS -

DIANBAYOU-21 MS S -

YUNGUANG-14 S HS -

YUNGUANG-17 MS MS -

SY-531 MS S -

HSLY-9 MS S -

289

ARIZE H 701 S S -

ARIZE H 016 S HS -

HS-98 S S -

HS-99-5 MS S -

DAZH 1-1 S S -

RA 201 S HS -

CKD-775 S S -

CKD-776 S HS -

SVS-777 MS HS -

SVS-778 MS S -

CJV-5 S S -

HJ-19 S HS -

CK-12 HS S -

CK-8 S S -

YUANS BASMATI HYBRID-1

S HS -

Source: National Rice Research Programme , NARC , Islamabad.

---------------------------------------------------------------------------------------------------------- --

- Data not available S Susceptible

HS Highly susceptible MS Moderately susceptible

MR Moderately resistant WBPH Whitebacked planthopper

Twenty two commercially grown rice varieties were screened for resistance against C. medinalis under semi-controlled condition at NARC. According to IRRI’s Standard Evaluation System for Rice, the results revealed that none of existing rice varieties was resistant to the insect (Table 5). However, Basmati 370, Basmati 198 and DR 53 were moderately resistant to C. medinalis. Remaining varieties were either moderately susceptible or susceptible to the

290

insect. The overall level of resistance in commercial rice varieties against Scirpophaga incertulas, C. medinalis and Sogatella furcifera is given in table 6 and those of rice hybrids in table 7.

8.8.5. WHEAT

Sixteen varieties/advance lines of wheat (93032, 93105, 93108, 93111, 94234, 92R10, 93C066, 92B2535, 93B,T022, 6544-6, D-93620, D-93640, T-93705, Pasban-90 and Inqilab-91) were evaluated for resistance against Sitobion avenae and Schizaphis graminum. No significant difference in the level of resistances was found among the genotypes against the insect (Zia et al., 1999).

Tested 20 cultivars/genotypes of wheat for resistance against aphid, Rhopalosiphum padi and Sitobion avenae. The population of insect varied from 2.33-12.22 on different test varieties/ lines (Pervez and Ali, 1999).

Twenty wheat varieties/advance lines (Chakwal-186, D-84637,D-84658, Faisalabad-85, Pak-81, Shalimar-88, 82274-1, 83035-2, 83171, 84021, 84133-6, 85054, 85060-2, 85078, 85162, 85195, 85276-2, 86299, 86369 and 86371) were tested for resistance against aphid. A wheat line, D-84658 was found more susceptible to aphid infestation (15.12/tiller). D-84637 was resistant with lower aphid density (5.47/tiller) and higher grain yield (3147 kg/ha) (Aheer et al., 1993).

Ten wheat varieties (Pitic-62, Pak-81, Barani-83, Kohinoor-83, FSD-85, Punjab-85, Rawal-87, Chakwal-86, Pasban-90 and FDS-83) were evaluated for resistance against S. avenae. The most resistant wheat varieties to the insect were Pitic-62 and Punjab-85 (Nasir, 2001).

8.8.6. SUNFLOWER

Sixteen genotypes of sunflower were evaluated against green leafhopper, Empoasca Spp; grasshopper, Chrotogonus spp. and thrips, Thrips tabaci. These genotypes include: SH-3322, DK-3915, HYSUN-33, 76A-24, PARSUN-1, CRN-1435, SF-187, 6451, SMH-9796, 64-A-93, SF-177, SMH-9707, SW-278, 25, SS-1 and SS-2. The genotype 64-A-93 was resistant, whereas Super-25, and CRN-1435 and were moderately resistant and other genotypes were susceptible to Empoasca Spp. 64-A-93 was resistant, Super-25 was moderately resistant against Chrotogonus spp. Other genotypes were susceptible or highly susceptible (Aslam and Misbah-ul-Haq, 2003).

Tested 69 entries of sunflower including hybrids and open pollinated cultivars for resistance against sunflower stem weevil, Cylindrocopturus adsperrus. From the spring planting cultivar X-2578 was classified as resistant (10% infestation) and 13 entries as moderately resistant (Amjid et al., 1992).

291

8.8.7. MAIZE

Screening and evaluation of 20 maize cultivars was done for resistance against Atherigona spp. On the basis of initial screening, selected five cultivars (Composite-15, Swat-1, Gouhar, Zia and Sarhad white) for further studies (Kasana and Wahla, 1996a).

Studies on host plant resistance were carried out on six maize varieties (Agati 85, Akrar, golden, Ev 1098, Pak-afgoyee, EV 5089). There was no significant diffeence among the varieties for resistance against Atherigona spp. at different weeks after sowing (Akbar et al., 2002).

Composite 15, Swat 1, Gouhar, Zia and Sarhad white maize cultivars were tested for resistance against Atherigona spp. The resistance in maize cultivars to the insect was affected significantly by the changes in plant height, thickness of leaf midrib, length of leaf sheath wrapping, tenacity of the leaf sheath wrapping and its nature, diameter of stem, thickness of leaf sheath wrapping and the intensity of leaf colour. Of these traits, plant height, thickness of leaf midrib, length of leaf sheath wrapping, tenacity of leaf sheath wrapping and its nature were positively correlated to the Atherigona resistance. Lignified vascular bundle and leaf epidermal silica bodies were negatively correlated with oviposition by Atherigona spp.

Fig.3. Maintenance of maize plants resistant to maize stem borer

292

Fig. 4. Damage by maize stem borer on susceptible maize cultivar

Fig. 5. Damage by maize stem borer on resistant maize cultivar

Fig. 6. Comparison of resistant and susceptible maize genotypes against maize stem borer

293

Table 8.Level of resistance of maize varieties/genotypes against C. partellus in Pakistan

Varieties/genotype Scale Level of Resistance

Agaiti-2002 6 S

Agaiti-85 6 S

Sahiwal-2002 5 MS

Sadaf 5 MS

Sarhad Yellow 6 S

Kissan 7 S

Gauher 7 S

Soan-3 6 S

EV-1098 4 MR

EV-1097 6 S

EV-5098 6 S

EV-6098 5 MS

EV-3001 4 MR

EV-4001 6 S

EV-2097 7 S

Rakaposhi 5 MS

Margala 6 S

NC-25 2 R

C-9 2 R

C-40 3 R

C-16-1 3 R

C-43-3 3 R

BR-1 1 HR

BR-2 1 HR

BR-3 2 R

BR-4 2 R

C-6765-9 2 R

294

C-6765-23 2 R

C-6765-40 4 MR

C-6765-54 4 MR

C-1752-14-2 4 MR

C-1752-16-2 3 R

C-1752-17-3 2 R

C-1752-23-2 2 R

C-1752-43-3 4 MR

C-1752-44-2 3 R

C-1751-147-3 4 MR

Source: National Coordinated Maize Research Programmeme, NARC, Islamabad

0-1 Highly Resistant 2-3 Resistant

4 Moderately Resistant 5 Moderately Susceptible

6-7 Susceptible 8-9 Highly Susceptible

screening and evaluation of maize germplasm for resistance against maize borer, Chilo partellus was done under natural and artificial condition. Initially, 400 different exotic and indigenous maize germplasm accessions were screened for resistance to C. partellus under natural conditions during Spring 2000. Ninety five accessions were selected for further evaluation. During Kharif 2000, out of the ninety five accessions, 10 accessions under artificial and 50 accessions under natural conditions were comparatively resistant. For artificial infestation, 15 newly hatched larvae were released at 4-5 leaf stage of the crop by camel hair brush near the whorls. The progenies were reproduced form their survived plants by self-pollination. During Spring 2001, the ears maintained were used for further screening and testing. During Kharif 2001, stem borer larvae were released twice at 4-5 leaf stage of the crop and then 10 days after first release for final selection. The selected materials were combined through bulk pollination and made three resistant genotypes: BR-1, BR-2, and BR-3 (Javed 2005). The procedure regarding screening and evaluation of maize germplasm for resistance against maize borer is given in Figs. 3 to 6. Information on the level of resistance of maize approved varieties and promising germplasm against C. partellus is given in Table 8.

8.8.8. SOYBEAN

Ten soybean genotypes (PSC-62, NARC-VII, Ajmeri, VI, Soy-95-1, Davis, NARC-VI, S-69-94, PSC-56 and S-72-60) were tested for resistance against white fly, Bemisia tabaci, jassid, Amrasca bigutella and soybean looper,

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Preudoplusia includens. The variety VI and Ajmeri were comparatively resistant having population of B. tabaci 1.29, A. bigutolla 0.62 per leaf. PSC-56 suffered minimum infestation of P. includens. Davis was most susceptible to all three insect pests having infestation of B. tabaci, A. bigutella and P. includens respectively, 6.39, 2.09 and 33.33% (Ihsan-ul-Haq et al., 2003).

8.8.9. BRASSICA

Carried out studies on sarsoon, Brassica juncea cultivars/varieties for resistance against insect pests. There was no correlation between whitefly resistance and aphid resistance in sarsoon cultivars. Varieties RC-23 and ER-14 were most susceptible to whitefly but resistant against aphids, (Anonymous, 1996). The procedure regarding screening and evaluation of rapeseed musted germplasm for resistance against aphid is given in Figs. 7 to 11. Level of resistance of important oilseed crops to major insect pests in Pakistan are given in table 9.

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Fig.7. Leaf curling (source 4) of rapeseed

mustard by turnip aphid

Fig. 8. Screening of repeseed mustard

for resistance against aphid

Fig.9 .Antibiosis studies of cabbage aphid on rapeseed mustard

Fig.10. Population growth studies of

aphids on rapeseed mustard

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Fig. 11. Preference studies of rapeseed mustard against aphids

Table 9: Level of resistance in oilseed crops against major insect pests in Pakistan

Variety name Insect Level of resistance

Sarsun-II Whitefly (Bemisia tabaci) Moderately resistance

Planthopper (Amrasca devastans) Moderately resistance

Rapeseed Mustard

Westor Turnip aphid Susceptible

Shirlee Lipaphis Susceptible

Con-I erysimi Susceptible

Con-II Susceptible

Con-III Susceptible

BARD-I Susceptible

Khanpur Raya Moderately resistance

Chakwal Raya Moderately resistance

Source: National Coordinated Oilseed Research Programmeme, NARC, Islamabad

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8.8.10. MULBERRY

Five exotic mulberry varieties (Kanmasi, Morus latifolia, Garyansuban, Qumji, Husang) and one local variety (PFI-I) were screened for resistance against leafroller, Margaronia pyloalis. Local variety was relatively resistant with mean infestation of 25.9%. Infestation for other varieties ranged from 35.5 – 65.7%: Garyanosuban (65.7%, M. latifolia (59.1%), Husang (35.5%), Kanmasi (42.5%) (Bajwa and Ashiq, 1999).

8.8.11. BARLEY

Ten barley varieties/advanced lines (Arbiabiad, B-91-100, PRB-11, Harmal, B-90015, B-91037, Pedigree, B-91101, PRB-10 and JAU-87 were studied for resistance against aphid, Sitobion avenae. Significantly minimum number of aphids (2.85 – 3.47/tiller) were recorded on AI/C, B-91-100 and Pedigree. Advanced lines B-9110 and B-91101 proved better among all lines showing minimum aphid population with better yield performance (Zia et al., 1996).

8.8.12. CUCUMBER

All the tested cucumber varieties (Local, Royal silus and L green) proved susceptible against Bactocera spp. However on local variety infestation of the insect was minimum (36%) followed by on Roal Silus (52%) and Khira L. green (70%) (Qureshi et al., 2000).

8.8.13. OKRA

Studies on the varietal resistance of okra against spotted bollworms, Earias spp. were carried out. Okra varieties tested were: Jalandri, Green polo, Parbhani karanti, Pusa sawani, Faisalabad-M-I and Desi. Infestation of insect varied significantly in different varieties of okra. The okra variety Green polo was found the least susceptible and the Desi variety was the most susceptible (Memon et al., 2004).

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CHAPTER-9 GENETICS OF INSECT RESISTANCE IN CROP PLANTS

The information on the genetics of resistance is critical to breeder in deciding the methodology and strategy for the effective transfer and efficient selection of resistance genes (Gallun and Khush 1980). Understanding the genetics of plant resistance to insects is critical in planning breeding strategy through selection of suitable sources of resistance and breeding methodology and exercising the right option of gene deployment for the target region (Bentur, 2005). The polygenic nature of insect resistance widely occurs in nature. Many single genes have been incorporated into cultivars to provide resistance against various insects. Such as resistance in wheat against Hessian fly, ,Mayetiola destractor, and in rice against brown planthopper Nilaparvata lugens. Major gene exerts strong selection pressure on insect population and the resistance is often short lived due to the development of new biotypes.

9.1. INSECT RESISTANCE IN MAJOR CROPS

9.1.1. RICE

Several studies have been conducted on genetic characterization of resistance in rice against major insect pests at International Rice Research Institute, (IRRI), Philippines and in different other countries. Due to the prevalence of distinct geographic populations of insect pests in different countries, the genetic information obtained in one country may not always hold good in other countries. This is well documented, since several of the resistance sources reported in one country are found to be susceptible in other countries (Dhaliwal and Singh, 2005). A lot of work has been done on different insect pests of rice but only platnhoppers are discussed briefly as an example.

9.1.1.1 BROWN PLANTHOPPER (BPH), NILAPARVATA LUGENS

Genetic analysis of over 90 resistant rice varieties mainly at the IRRI, Philippines, Bangladesh and in Japan led to identification of ten major resistance genes of which five are recessive. These genes include Bph1, bph2, Bph3, bph4, bph5, Bph6, bph7, bph8, Bph9 and Bph10. In addition several minor genes also contribute to the expression of resistance in rice varieties against planthoppers (Bharati and Chelliah, 1991). Two resistant rice varieties (IR26 and IR64) which contain the same major gene (Bph1), later showed moderate resistance against N. lugens colony fully adapted to Bph1 gene. N. Lugens population in Central Luzon, Philippines, also adapted to this gene (Cohen et al., 1997). Using the mapping population of double haploid lines from IR64/Azucena cross, seven quantitative trait loci (QTLs) were found to be associated with N. lugens resistance (Alam and Cohen, 1998). Using

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another mapping population from the cross lemont/teqing, seven main effect QTLs and several epistatic QTLs were associated with N. lugens resistance in China (Xu et al., 2002). A third mapping population from the cross Nipponbare/Kasalath was used to detect three QTLs controlling the pest resistance in China (Su et al., 2002).

Four N. lugens, biotypes are known in different parts of the world. Biotype 1 and 2 are widely distributed in Southeast Asia, biotype 3 is a laboratory biotype and biotype 4 is used to represent undefined population spread across the South Asia (Table 1).

Table 1: Reaction of brown planthopper, N. lugens biotypes to different varieties

Reaction to brown planthopper biotype

Gene Germplasm 1 2 3 4

Bph1 Mudgo R S R S

bph2 ASD 7 R R S S

Bph3 Rathu Heenati R R R R

bph4 Babawee R R R R

bph5 ARC 10550 S S S R

Bph6 Swarnalatha S S S R

bph7 T12 R R R S

bph8 Shin Saba R R R S

Bph9 Balamawee R R R

Source: Khush and Brar (1991)

In N. lugens, resistance was qualitative in nature and controlled by a single gene (Krishna and Seshu, 1984). The involvement of two or more number of genes have also been recorded (Verma et al., 1999). Resistance to N. lugens has been observed in wild rice such as Oryza latifolia, O. officinalis, O. australiensis, O. punctata, O. longistaminata and O. ulta (Velusamy, 1987).

9.1.1.2. WHITEBACKED PLANTHOPPER, SOGATELLA FURCIFERA

Genetic studies covering over 75 rice varieties at IRRI, Philippines identified four major dominant genes and one major recessive gene in rice against S. furcifera. A single dominant gene was found to confer resistance against Indian population of the insect in Ptb33, ARC14636, ARC14767 (Krishna and Seshu, 1980), ptb19, while

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a recessive gene was responsible for resistance in ARC 11324 and various other varieties (Singh et al., 1990). A single dominant gene (wbph6) confer resistance in four local varieties Giu-Yi-Gu, Bian Gu, Da-Di-Gu and Da-hua-Gu in China (Shaokai et al., 1991). A single dominant gene was also found to confer resistance in Vietnam in their local varieties (Base, Rong Lem, Keo Cha A and La). Two dominant genes have been identified in IR2035-117-3 and Xoai Cat (Sen, 1994). Resistant gene in rice against S. furcifera include wbph1, wbph2, wbph3, wbph4 and wbph5.

Using the mapping population of IR64/Azucena cross a major QTL associated with plant dry weight loss per mg of S. furcifera dry weight has been identified in India. The QTL explained 79% of variation in the phenotype determining tolerance component of resistance (Kadirvel and Maheswarm, 1999). A total of 10 QTLs for ovicidal response based on analysis of population from Asominori/IR24 cross were identified. One of the QTLs was most significantly associated with the ovicidal response accounting for 46% of phenotypic variation in egg mortality (Yamasaki et al., 1999).

Single dominant or recessive gene in rice was responsible for S. furcifera, resistance (Shaokai et al., 2001). Polygenic resistance in rice against this insect has also been reported.

9.1.2. WHEAT

The Hessian fly, Mayetiola destructor was introduced into the USA during the later half of the eighteenth century, and quickly became key insect pest in the major wheat growing regions although, in its native Europe, it does not seem to a serious pest of wheat.

A search for resistance was initiated in Kansas in 1914, and several resistant varieties have been developed against this pest. Kawvale, a variety derived from a soft wheat variety Indiana Swamp, was released as a resistant variety in 1928. It was superseded by Pawnee, a higher quality variety with improved resistance, which was derived from a cross between Kawvale and a susceptible variety Tenmarq. Pawnee gave good yields of grain, even when heavily infested with M. destructor. A variety with a greater level of resistance, Ponca, was released in 1951; in 27 field tests in Kansas only two per cent of Ponca plants were infested, compared with 48 per cent for Pawnee and 75 per cent for a susceptible variety Tenmarq. The growing of resistant varieties in Kansas since 1965 has resulted in the virtual disappearance of the pest from the State, except in areas where susceptible varieties are still grown. In 1969, twenty four different resistant varieties were grown on 33.5 million hectares in the USA. By 1974, the area of resistant varieties in the USA had increased to more than six million hectares. New sources of resistance to the pest have been discovered in varieties from Spain and Portugal and these are being used extensively in breeding programmemes.

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Resistance in wheat against M. destructor is controlled by a series of dominant or partially dominant genes (designated H1 – H8) and several F1 hybrids between resistant plants carrying the H3 gene (which governs resistance to race C of M. destructor) and susceptible varieties, express a level of resistance intermediate to that of the parents. The resistance is expressed as reduced damage to the host plant (tolerance) and as antibiosis, the larvae being fewer and smaller on resistant plants.

There is a gene-for-gene relationship between the resistance genes of wheat and the virulence genes of the pest. Wheat varieties carrying the H1 and H2 genes are very resistant to M. destructor in California, less resistant in Kansas and susceptible elsewhere in the USA; this is because biotypes capable of attacking varieties with resistance controlled by the H1 and H2 genes are absent from California. Eight biotypes, designated Great Plains and A to G, have been identified in the USA. These biotypes vary in their ability to survive on resistant wheat. For example, the Great Plains biotype cannot survive on varieties carrying any of the eight resistance gene that have been identified so far. Biotypes E and F can attack wheat with resistant genes H3, H4, H5 and H6, respectively. Biotype D, on the other hand, is controlled only on wheat varieties

carrying the H5 genes.

Although, it is likely that more biotypes of M. destuctor will be discovered. However, resistance breaking biotypes will not necessarily be a problem in the use of resistant varieties. Some races of the fly are less competitive than others in the field and populations of some resistance-breaking biotypes may, therefore, not reach damaging proportions. Some previously immune wheat varieties have been badly attacked by biotypes that were not previously common in some parts of the USA, and the dangers of resistance breaking biotypes should, therefore not be underestimated. The situation concerning biotypes of M. destructor is further complicated by the differential reaction of resistant wheat genotypes to specific biotypes at different temperatures. Sources of resistance that will give an effective control of all known races over a wide range of different temperatures are now being sought.

The benefits that have already resulted from the growing of M. destructor resistant varieties have been very great. It has been estimated, for example, that the value of wheat crop in the USA has been increased by many millions of dollars annually through control by resistant varieties. For further details readers are suggested to consult Russel (1978).

So far 27 different M. destructor resistance genes (H1 to H27) have been identified. Genes H1 to H20 are dominant or partially dominant except h4 (Patterson et al., 1992). Two resistant genes (H1 and H25) from rye, Secale cereale have been introgressed into bread wheat, Triticum aestivum and durum wheat, T. turgidum via chromosomal translocation (Friebe et al., 1999). Gene H27 has been transferred from the wild grass, Aegilops ventriosa.

Twenty seven resistance genes have been incorporated into wheat cultivars and 14 biotypes of the M. destructor have been characterized (Dweikat

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et al., 2002). Virulence in the insect is inherited as recessive trait conferred by a single gene in respect of each of the dominant resistance gene in the plant. Three of the avirulence genes (vH6, vH9 and vH13) that confer avirulence against wheat resistance genes H6, H9, H13 respectively, have been tagged with molecular markers and all the three genes have been mapped to sex chromosomes X1 in close vicinity of each other (Schulte et al., 1999).

Studies were carried out on genotypic interaction between resistance genes in plant and virulence genes in the insect. These studies revealed that H3 gene either in homozygous or heterozygous state conferred high level of resistance against insects carrying H3 gene in homozygous or heterozygous state. While H7 and H8 genes were highly effective in the homozygous condition but displayed a reduced level of resistance in the heterozygous condition. The H6 and H9 genes showed different levels of resistance against the reciprocal heterozygous larvae. Avirulence in the insect to these resistance genes is incompletely dominant. Reciprocally, heterozygous plants showed greater survival of homozygous avirulent larvae suggesting incomplete dominance of the plant genes (Bohssini et al., 2001).

Seven major genes (Dn1 to Dn7) have been identified in wheat accessions against Russian wheat aphid, Diuraphis noxia. Six genes are dominant and one gene (dn3) is recessive (Marais et al., 1994). Two other genes (Dn8 and Dn9) have been identified and assigned chromosomal location for six of the genes using microsatellite markers. Another dominant gene has been described from a wheat accession P147545 (Linscott et al., 2001), Though its allelic status with reference to known genes is not known.

9.1.3. MAIZE

9.1.3.1 EUROPEAN CORN BORER, OSTRINIA NUBILALIS

In Europe, the pest has one generation in a year, while in USA it has two generations, the first attacks young plants and the second affects the developing grains. Same genes do not condition resistance to the first and second generations. Genetic studies prior to 1990 generally concluded that resistance in maize to O. nubilasis was polygenic. Seven QTLs associated with second generation tunnel length in populations derived from B73 (susceptible) and B52 (resistant) cross have been identified (Beavis et al., 1994). Three QTLs were found to be linked to second generation damage rating in another population from B73/B52. Nine QTLs for European corn borer tunneling which accounted for 59% of genetic variation have been identified (Cardinal et al., 2001). Analysis of another population involving a novel source of the pest resistance, M047, nine QTLs for leaf feeding damage by first generation and seven QTLs for leaf feeding damage by second generation and seven QTLs for stalk tunneling by second generation larvae have been identified (Jampatang et al., 2002). Studies in Europe involving F3 families from the cross D06 (resistant) D408 (susceptible) cross identified six QTLs for

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tunnel length and five QTLs for stalk damage rating which together accounted for 50% of the phenotypic variation (Bohn et al. 2000).

9.1.3.2 CORN EARWORMS, HELICOVERPA ZEA AND H. ARMIGERA

Earlier genetic studies did not identify major genes conferring corn earworm resistance in maize lines. Now studies have focused on genetics of flavonoid biosynthetic pathway that would explain quantitative variation in maysin concentration in the silks and thereby account for corn earworm resistance (Yencho et al., 2000). Using different mapping populations, 1 to 6 QTLs have been identified for silk maysin/apimaysin concentration or for larval weights recorded in diet based silk bioassays. A model assuming dominance at a single locus for low maysin content which is governed by the presence of a dominant modifier gene. It explains the segregation ratios obtained in the F2 and first backcrosses of two corn earworm resistant maize inbreds (GT114 x GT119) which had low levels of maysin but proved resistant to corn earworms (Wilson et al., 1995).

9.1.3.3. STEM BORER, CHILO PARTELLUS

It has been reported that resistance to primary damage (leaf feeding) is governed by additive and additive x additive type of gene action while additive and non-additive type gene action is important for secondary damage (stem tunneling). The inheritance for primary damage and secondary damage is independent. Resistance to borer for deadhearts is governed by both additive and non-additive type of gene actions, while resistance for stem tunneling is governed predominantly by additive gene action. The inheritance pattern of deadhearts and stem tunneling is different. Under natural infestation, resistance is controlled by additive and dominant major gene effects. Cytoplosmic influences play an important role in the inheritance of C. partellus resistance. Resistance in the double cross hybrid resulted from complementary type of non-allellic gene interaction. It has been reported that resistance to insect is mainly under the influence of additive genes (Bhanot et al., 2005).

9.1.3.4. SHOOT FLY, ATHERIGONA SOCCATA

Additive genetic variance was noticed for ‘deadhearts’ underlying ovipositional non-preference (antixenosis) and tillering in F1 consequent to ‘deadheart’ formation was controlled by non-additive genes. Qualitative analysis of oviposition and ‘deadheart’ formation in F2 progenies between susceptible and resistant crosses revealed monogenic recessive (npo npj) and two duplicate recessive genes (dhj dhj dh) governing non-preference (antixenosis) for oviposition and ‘deadhearts’, respectively (Prem Kishore, 2005).

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9.1.4 SORGHUM

Four greenbug biotypes (C,E,I,K) have been identified that damage resistant cultivars of sorghum. Biotype E was recognized as it was found to damage biotype C resistant SA7536-1 and KS30 sorghum lines. Biotype I was described in 1990 based on its ability to damage all known biotype E resistant sorghum sources. Biotype K was identified in 1996 as virulent on all except two sorghum resistant sources. Studies on gene complementarity in sorghum to greenbug biotype E concluded that these sources complemented each other to give increased level of resistance (Dixion et al., 1991). Genetic analysis of greenbug biotype I resistance in sorghum line KS97 suggested incomplete dominance with two dominant genes requiring complementary gene action controlling resistance (Tuinstra et al., 2001).

Molecular analysis of four greenbug resistance sources identified at least nine QTLs dispersed on eight linkage groups affecting the resistance. Most of these loci were additive or incompletely dominant. Using another set of recombinant inbred lines from a cross between sorghum lines GB1K and Redlan a set of nine QTLs affecting resistance to greenbug biotypes I and K were identified. Four of these were associated with biotype non-specific resistance and accounted for 5.6 to 38.4% of phenotypic variation (Agrama et al., 2002).

9.1.5. COTTON

Of the four principal upland cotton types cultivated world wide, the coker, Deltapine and Stoneville types have a common ancestor in the Bohemian variety. The fourth type Acala cottons have a different ancestory. Five categories of genetic variability are available for use in breeding programmeme for resistance to insect pests: (i) contemporary commercial varieties and strains, (ii) obsolete varieties and strains, (iii) exotic uplands, Iiv) primitive stock of Gossypium hirsutum and (v) wild species of Gossypium. Generally resistant sourcves are not available in cultivated cotton. These sources have been incorporated in cotton through biotechnological approaches from wild species and from distant sources (transgenic cotton).

Hairiness on the cotton leaves is an important trait of resistance to A. devastans. Thickness of palisade tissues also imparts resistance against this pest. Leaf hairness and thickness is responsible for imparting resistance in cotton against B. tabaci. Smooth cotton leaves are less preferred by aphid than hairy varieties. Hairy cotton varieties in general are preferred for oviposition by spotted bollworms (E. insulana, E. vittella). Okra leaf cultivars of cotton are comparatively resistant to these insects. Stiffness of shoot tip is an important resistant trait in cotton against spotted bollworms. Frego bract character has been exploited for reducing bollworm damage in cotton. Generally red cottons show less incidence of the insect. The leaf trichomes affect the mobility of young larvae of H. armigera. Glandless and nectriless cotton cultivars are comparatively resistant against E. insulana, E. vittella and H. armigera.

In general, the cotton varieties containing high gossypol content are less damaged by bollworms. High gossypol increase the post-embryonic development

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period of larvae. Gossypol in cotton leaves had significantly negative correlation with the number of eggs and population build up of cotton jassid.

Large number of plant traits have been evaluated for their effects on insect pests response in cotton as mentioned above. Few examples are given on the genes of such traits. Frego bract (fgfg) is an uplant mutant inherited as a monofactorial recessive and results in elongated, twisted bracts. Phenotypic expression of fgfg genotypes varies appreciably, depending on genetic background. The nectariless trait (n1n1n2n2) is controlled by two pairs of recessive genes. The glabrous or smooth leaf trait is governed by at least four alleles. The recessive sm3 allele has been widely used in development of commercial smooth-leaf varieties. Increases of plant hairiness above the degree normally found in the pubescent varieties are governed by two major genes and a complex of modifier genes. This key major gene is designated as H1. A second major gene, H2, controls the finely dense pubescence in an upland mutant designated as ‘Pilose’ and the same gene (or its allele) is responsible for densely pubescent characteristic of Gossypium tomentosum. In F1 populations, both H1 and H2 show incomplete dominance. Pigment glands of cotton, contain various biologically active constituents of which gossypol and heliocide H1 are of great significance. Generally, increasing gland density in the cotton plant results in increasing concentrations of these toxic compounds. Gland density is regulated by genes designated as gl1 through gl6. At each locus, the alleles that increase gland formation are identified Gl; those that reduce gland desity are designated gl. Gland density and terpenoid content in upland cotton can be increased by substitution of Gl alleles from other species or from wild forms of G. hirsutum.

Okra leaf is a monogenic trait governed by the Lo gene which is incompletely dominant to normal lo. Super-okra leaf shape is produced by the Ls allele at the Lo locus. Red plant colour is available from different sources. R1 is an incompletely dominant gene that results in red colouration of different plant parts e.g. R2 in the ‘AK Djura’ and ‘North Carolina margin’ stocks (Niles, 1979).

In transgenic cotton gene Cry 1 Ac has played an important role in suppressing the population/incidence of cotton bollworm complex: Heliothis virescens, H. armigera, H. zea Pectinophora gossypiella, Earias inculana and E. vittella etc. Generally this signle gene construct is being marketed by companies. Due to reliance on one gene the transgenic cotton crop will increase the chances of resistance breakdown because of high selection pressure due to (i) presence of toxin throughout the season, (ii) transgenic insecticide has single toxin requiring single gene to overcome it, (iii) cultivation on wider area where pest is serious, (iv) completion of various generations of pest on the crop.

Some successful efforts have been made to incorporate other genes for resistance. The high plant expression of Cry2 Ab has contributed to higher efficacy against important lepidopteran insects in cotton. Varieties with multiple gene constructs is better option.

With emphasis shifting to transgenic approach and successful commercial exploitation of Bt cotton conferring resistance against range of

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lepidopteran pests, studies on native host-plant resistance have slackened. Studies on inheritance of resistance indicated that inheritance of glanding trait that confers a significant level of resistance to bollworm and tobacco budworm in isoline of the high glanding breeding line x G15. It has been concluded that a special G13 allele confer high glanding phenotype in this line (Bentur, 2005).

9.2 STRATEGIES TO PROLONG THE UTILITY OF MAJOR GENES

The utility of major genes can be prolonged by (i) recycling and sequential release of resistance genes, (ii) pyramiding of resistance genes (iii) development of multiline cultivars and (iv) regional deployment of resistance genes.

The strategy of sequential release has been employed for resistance in rice to N. lugens at IRRI, Philippines (Khush, 1979). Prediction of biotypic changes and identification of effective resistance genes are needed for the success of this strategy. Based on such information the genes should be used in a sequential manner.

Pyramiding refers to the simultaneous introduction of diverse resistance genes into a single genotype. The potential value of gene pyramiding has been demonstrated in a number of cases. The major difficulty in developing gene pyramids is the selection of plants actually containing multiple resistance genes. Any single gene that is effective will mark the detection of other resistance gene. This problem can be overcome by (a) selection based on differential reaction of gene against different biotype and (b) selection based on linked molecular marker. The differential reactions can be effectively used for pyramiding of resistance genes. The technique has been successfully used is pyramiding of two gall midge resistance genes (Gm2 and Gm6) (Katiyar et al., 2001). The molecular markers linked to the individual resistance genes can be effectively used for the identification of lines carrying two genes for resistance and discriminate them from phenotypically resistant lines carrying only single gene for resistance.

Multiline cultivar with one or two resistance genes with a mixture of similar looking susceptible lines seems appropriate for the management of insect pests with limited mobility. This strategy is not so effective against those insect pests which are very mobile. The susceptible lines can act as refuge and thus reduce the selection pressure on insect and thus reduce the development of resistance/new biotypes and prolong the life of resistance genes.

Selection for insect resistance under field conditions is difficult and time consuming. Efficiency of selection can be increased with the help of molecular markers. Marker aided selection (MAS) allows the target genes to be identified and tracked in a segregation population at any plant growth stage based on linked DNA markers.

Different resistance genes can effectively be used at regional level based on the knowledge of insect population structure and biotypic distribution. Efforts are being made to characterize the insect population at the molecular level. This will help in understanding the population structure of various insects and thus help in deployment of resistance genes.

Molecular markers are not affected by environmental factors, therefore, precise estimates of genotype are obtained. These are tissue and stage non-specific, thus

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selection can be done at early growth stage, as well as number of selection cycles can be effectively increased to substantially reduce the breeding period. Generally co-dominant, allowing all possible genotypes to be distinguished in any segregating generations. Pyramiding of epistatic and hypostatic genes in a single genotype can be achieved (Singh and Yerulkar, 2005).

Plant resistance to insects is most often a quantitatively inherited trait. Strong environmental effects and genetic variability of insect often complicate the identification, transfer and selection of quantitative resistance. Conventional approaches for selection of horizontal resistance are time consuming and exhibit moderate correlation with resistance and not very effective. At the genetic level, quantitative variation in phenotype can be explained by the combined action of many genetic factors, each having rather a small effect on the overall phenotype and the influence of environment. Location and effect of the genes controlling a quantitative trait can be determined by marker based genetic analysis. Molecular genetic markers and QTL analysis offers a more efficient approach for working with quantitative traits (Yencho et al, 2000).

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Tanksley, S. D., N. D. Young, A. H. Patterson, and M. W. Boinerbale. 1989. RFLP mapping in plant breeding: new tools for an old science. Biotechnology. 7: 257-264.

Tayler, J. M. and J. H. Hatchett. 1983. Temperature influence on expression of resistance to Hessian fly (Diptera: Cecidomyiidae) in wheat derived from Triticum tanschii, J. Econ. Ent. 76: 323-326.

Tuinstra, M. R., G. E. Wilde, and T. Kriegshauser, 2001. Genetic analysis of biotype I greenbug resistance in sorghum .Euphytica. 121: 87-91.

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Xu, X. F., W. H. Mei, L. J. Luo, X. N. Cheng, and Z. K. Li. 2002. RFLP facilitated investigation of the quantitative resistance of rice to brown planthopper (Nilvarpata lugens). Theor. Appl. Genet. 104: 248-253.

Yamasaki, M., A. T. H. Yoshimura. N. Iwata, and H. Yasui. 1999. Quantitative trait locus mapping of ovicidal response in rice against whitebacked planthopper, Sagotella furcifera (Hovarth). Crop. Sci. 39: 1178-1183.

Yencho, G. C., M. B. Cohen, and P. F. Byrne. 2000. Application of mapping and tagging insect resistance loci in plants. A. Rev. Ent. 45: 393-422.

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CHAPTER-10 ROLE OF HOST PLANT RESISTANCE IN INTEGRATED PEST MANAGEMENT

10.1. SIGNIFICANCE OF HOST PLANT RESISTANCE

The primary objective of programmes on insect resistance in crop plants is to develop cultivars that are resistant to insect pests while maintaining or improving their basic agronomic characteristics. The approach for the development of insect resistant crop varieties may vary with each crop and with each insect species. Resistant cultivars need less frequent treatment with a pesticide or require lower rates of insecticide application of other control measures. Insect resistance has been the principal pest control method in a number of crops. Resistance to insects can serve as a means of insect control in unique niches, where other controls are not feasible or are difficult to use. Plant resistance may have significant advantages in situations where (i) a critical timing regime in which an insect is exposed for only a brief period of its life cycle; (ii) the crop is of low economic value; (iii) the pest is continuously present and is the single most limiting factor in successful cultivation of a crop in wide area; or (iv) other controls are not available (Ortman and Peters, 1979).

Insect resistance has been introduced into large number of crop varieties. A resistant variety can provide a foundation on which to build an integrated control system and in fact, may be most productive when used in adjunct with cultural, chemical and biological control methods. With some crops, particularly those having low cash value per hectare, the use of resistant varieties may offer the best (and perhaps only) economical method of control of certain pests.

Plant resistance as a method of insect control offers many advantages. In some cases it is the only method that is effective, practical or economical. Resistance developed in plants for one pest species may provide resistance to several others insect pests as well. Resistant varieties are not a panacea for all pest problems. Use of resistant varieties in pest management has been categorized as (i) the principal control method, (ii) an adjunct to other measures, and (iii) a safeguard against the release of more susceptible varieties than existing varieties. Resistant varieties usually have to be integrated with other methods of pest control to achieve stable pest suppression. Resistant varieties have been used as principal control method in case of grape phylloxera, Phylloxera vitifolia; cotton jassid, Amrasca facialis; wooly apple aphid, Eriosoma lanigerum; Hessian fly, Mayetiola destructor (Maxwell, 1972).

Plant resistance is usually specific to a pest or a complex of pests. Plant resistance has cumulative effect in supperessing the population development of insect pests. Even a limited degree of resistance is sufficient not only to check but even nearly exterminate the insect pest population in successive generations. Most of the resistant varieties usually maintain high levels of resistance for a longer period. It has harmony

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with the environment. Resistant varieties can easily be incorporated into normal farm practices at little or no extra cost. It is compatible with other components of IPM. The most important advantage of plant resistance is that its effect on the pest population is specific, cumulative and persistent. A resistant plant variety that reduces the insect population by 50% in each generation is sufficient to eliminate an insect of economic importance within a few generations. Resistance in host plant is manifested by the disruption of the rate of growth and reproduction, laying fewer eggs than on susceptible varieties, damage of insect’s eggs when laid, lower fecundity of females, lower weights of pest individuals, shortening of adult life or lengthening of the larval or nymphal life. Varieties with low or moderate levels of resistance can be used successfully for the suppression of pest populations through integration with other components of IPM. There is need to make adjustments to suppress the population of pests and conserve the population of biocontrol agents.

10.2. COMPATIBILITY WITH OTHER COMPONENTS OF IPM.

Host plant resistance is compatible with other components of IPM such as chemical control, biological control, cultural control etc. These components are briefly described.

10.2.1. CHEMICAL CONTROL

Prophylatic use of pesticides has posed multifarious and multifacet complicated problems such as resurgence of target pests, outbreak of secondary pests, destruction of biocontrol agents and other non-target organisms, environmental pollution, health hazards, pesticide residues in the produce etc. It is, therefore, imperative to eliminate or at least minimize the use of pesticides. Crop varieties with high level of resistance against insect pests generally do not require the use of any insecticide. Only few insecticide treatments are required on crop varieties with moderate level of resistance and need-based use of insecticides seems to be a viable option to minimize losses caused by insect pests and minimize insecticide-induced problems.

The most common form of integrated pest management involving resistant varieties is the use of carefully supervised treatments of insecticides to control outbreak of pests on varieties having low or moderate level of resistance. The major advantage of using the resistant variety is to induce a constant level of suppression on each pest generation. If resistant varieties are planted on large scale the reduction in pest number will be cumulative over time. Insecticides may be used with much greater efficiency on resistant than on susceptible varieties. Fewer insect pests develop on the resistant variety than on the susceptible one. The reduced rate of pest increase may greatly prolong the time required by the pests to reach the economic threshold (Knipling, 1964).

Resistant varieties generally reduce the viability of insects and render them more vulnerable to insecticides. Resistant varieties require less insecticidal treatments than susceptible varieties with the same level of insect infestation (IRRI, 1969). Likewise, on resistant varieties lower insecticidal rates are required (Brett and Sullivan, 1970). Reduction in insect pest number by resistant varieties

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makes control by insecticides easier. Toxic substances or nutritional inadequacies in the resistant plants make the pest more susceptible to various insecticides (Potter and Gillham, 1957). Resistant varieties affect the viability of the insects and render them more vulnerable to insecticides. Chalfant and Brett (1967) reported that mortality of Trichoplusia ni and Pieris rapae on resistant cabbage varieties wAS higher than on susceptible varieties. Fewer insecticide treatments were needed to control rice borers on moderately resistant varieties than on susceptible varieties (Pathak and Dyck, 1973).

The incorporation of a resistant variety into an integrated pest management system minimizes the production cost, conserves natural enemies, improves environmental conditions and reduces the chances of development of resistance to insecticides. However, there is no variety highly resistant to all insect pests, so in general complete elimination of insecticides is not feasible in the production systems. Even the varieties with multiple resistance based on the environmental conditions sometimes require the use of insecticides for the suppression of the population of insect pests.

10.2.2. CULTURAL CONTROL

The integration of insect resistant cultivars with cultural management can be a powerful tool in managing insect pests. The insect pest control involves two basic approaches; one is to make the environment least favourable to insect pests and the other is to favour the natural enemies of insect pests. Early maturing cultivars and planting dates are the usual cultural practices employed to manage insect pests of different crops. For example, in field studies in Philippines, brown planthopper, Nilaparvata lugens predator ratio on resistant varieties was better than on susceptible ones.

Resistant varieties are highly useful in cultural control systems designed to keep the population of key insect pests below the economic threshold while preserving insect natural enemies. In the irrigated deserts of the Western United States, the pink bollworm, Pectinophora gossypiella is the key insect pest of cotton and insecticides must be applied for its control. The insecticides kill biocontrol agents, increasing the population of bollworm and tobacco budworm. These insects are difficult and costly to control with the use of insecticides. P. gossypiella can be controlled by cultural practices designed to reduce its overwintering populations. This involves early uniform planting, early maturity, defoliation and stalk destruction of the cotton in late August and September before the larvae go into diapause due to short days and cool nights. Certain nectariless varieties may reduce the numbers of P. gossypiella to develop in a cotton field by more than 50% when compared with susceptible varieties. The greatest impact on the insect pest could be realized by combining the nectarilles character with varietal earliness. In this case only three generations of the insect would develop in a field (Adkisson and Dyck, 1979). With reduction in the number of generations the population of the insect pest can be reduced drastically. Insect infestation can be controlled in relation to sowing dates. Early planting supported high larval population and per cent fruit injury caused by Helicoverpa armigera (Mustafa et al, 2004).

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10.2.3. BIOLOGICAL CONTROL

The integration of host plant resistance with biological control tactics may be additive, or even synergistic, in their effects on decreasing populations of rice insect pests (Smith, 1994). Generally resistant varieties have positive effect on natural enemies by minimizing the use of toxic insecticides.

Resistant varieties are highly compatible with biological control. Host plant resistance even at low level may be beneficial in increasing the effectiveness of natural enemies of insect pests. The populations of insect pests at sub-economic levels can serve as food for the natural enemies. Natural enemies help control target as well as other insect pests to which the variety is not resistant. Plant resistance is usually specific to a pest or complex of pests and has no direct detrimental effect on beneficial insects and no danger of contaminating the environment.

Insects feeding on resistant hosts often lack virility and are slow in developmental growth, which greatly affect their ability to combat environmental stresses. Their restless behaviour on resistant plants expose pests to predators longer and small sized insect pests on resistant plants are likely to be reduced in number comparatively more than on susceptible plants (Pathak, 1975).

Predators inefficient on susceptible varieties may be more effective on resistant varieties. Resistant varieties may be more attractive to predators. The morphology of resistant plants may make it easier for predators to find the prey. A study by Teetes (1980) compared the ratio of predators to green bugs in sorghum; the ratio was about equal or greater in the resistant than in the susceptible sorghum plants. The predator/prey ratio indicated that the green bug resistant sorghum may compliment biological control.

In California, development of alfalfa resistant varieties to spotted alfalfa aphid in the early 1960s became a significant feature for the control of this insect pest. Resistance together with actions of introduced and exotic natural enemies made it unnecessary to use insecticides (Stern et al, 1959).

A braconid parasitoid, Lysiphlebus testaceips was able to keep the population of green bug nearly static on both resistant and susceptible barley varieties when the initial population of green bug was 12 per plant, the parasitoid prevented the build up of insect on the resistant barley variety but not on susceptible varieties (Stark et al, 1972).

Van Emden and Wearing (1966) calculated that a predator whose voracity failed to exert control on the population of its host when the host multiplied daily by a factor of 1.2 could adequately regulate it, if its host increased by a factor of 1.15. It is clear that even a mild resistance factor which reduced pest population increased by 0.05 is adequate to turn an inefficient predator into effective one.

Feeding efficiency of four predator species: Lycosa pseudoannulata, Callitrichia formisana, Cyrtorhinus lividipennis and Microvelia atrolineata was high when combined with the effect of varietal resistance. This was due to the

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cumulative effect of varietal resistance and predation. In the field, the prey/predator ratio was low on resistant varieties, intermediate and high on moderately resistant and susceptible varieties respectively. Combination of green leafhopper resistant cultivars and predation by Cyrtorhinus lividipennis have a cumulative effect on reducing the population of green leafhopper. C. lividipennis predation rate increased when the prey Nilaparvata lugens fed on the resistant rice cultivar IR36. Increased movement of N.lugens nymphs on resistant plants facilitated detection of prey for C.lividipennis (Kartohardjono and Heinrichas, 1984).

Salim and Heinrichs (1986) carried out studies on the effect of different levels of host plant resistance on the mortality of whitebacked planthopper, Sogatella furcifera due to predators and the level of resistance. These studies were carried out in the green house. Four species of promising predators: (i) L. pseudonnulata;(Fig.1) (ii) C. lividipennis; (Fig. 2) (iii) ladybird beetle, Harmonia octomaculata (Fig.3) and (iv) the rove beetle, Paederus fuscipes (Fig. 4) were used in thses studies..

Treatments of each experiment consisted of one predator and six rice cultivars: these were IR2035, highly resistant with two genes Wbph 1 and Wbph 2; WC 1240, resistant with Wbph 1 + 1 recessive gene; Colombo, resistant with Wbph 2 + 1 recessive gene; ARC 10239, resistant with the W2 gene; N22, resistant with the Wbph 1 gene and TNI, susceptible.

Seeds of selected rice cultivars were sown in soil in seedboxes. One week after sowing, seedlings were transplanted into soil in clay pots (14 cm diameter) at the rate of four plants per pot. At 25 days after transplanting rice plants were covered with Mylar film cages (11 cm x 108 cm). The predator and prey (S. furcifera) were released in the cages. S. furcifera mortality was determined by counting the number of live insects.

Total mortality of S. furcifera was based on a combination of that caused by the predator and varietal resistance. Mortality due to varietal resistance was based on mortality in cages without the predator.

Last instar spiderlings of wolf spider, Lycosa pseudoannulata were kept singly in glass tubes, covered with caps having small holes for aeration. Moisture was supplied on small swabs of cotton. After the last moult, spiders of uniform age were used for the feeding studies. In first test sixteen freshly emerged S. furcifera (brachypterous females) adults and one L. pseudoannulata were released per cage. Observations on the mortality of S. furcifera with and without L. pseudoannulata were recorded daily and continued for 10 consecutive days. In the second test fifty S. furcifera adult (brachypterous females) and L. pseudoannulata adults were released per cage. Observation on the mortality of S. furcifera were recorded at 4 days interval after infestation.

Pupae of ladybird beetle, Harmonia octomaculata were collected from the rice field and kept in a cage in the laboratory. Moisture was supplied with wet cotton swabs. Adults of uniform age were used for predation studies. In first test, four H. octomaculata adults and ten S. furcifera adults (brachypterous females)

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were released per cage. Observations of mortality of S. furcifera were recorded daily. All prey were replaced after each observation. In second test, forty S. furcifera adults (female) and two H. octomaculata adults were placed in each cage. Observations on the mortality of S. furcifera were recorded at four days interval after infestation.

The cultures of mirid bug, Cyrtorhinus lividipennis and S. furcifera were maintained in the greenhouse. Both prey and predators were collected from the cages at 10 days after infestation and transferred into labeled glass tubes. S. furcifera were counted in the laboratory and mortality was calculated.

In first test, where all S. furcifera were replaced daily to maintain a population of 16 adults, the effect of varietal resistance did not have sufficient time to be an important factor in S. furcifera mortality. Average mortality over the 10 day period on the highly resistant cultivar IR2035 was 21% whereas in susceptible TNI it was 17%. To calculate the extent of the effect of predation on mortality without L. pseudoannulata was subtracted from the mortality with L. pseudoannulata. The degree of S. furcifera mortality attributable to predation ranged from 3% to 27% on a daily basis and was significantly higher than the mortality without L. pseudoannulata in most cases (Table 1).

In second test, where S. furcifera mortality was recorded four days after infestation,There was sufficient time for the adverse effect of the resistant cultivars to be observed. Results of the three trials were similar (Table 2). Mortality of S. furcifera when caged in the absence…………

Fig. 1. Wolf spider, Lycosa pseudoannulata

Fig. 2. Mirid bug, Cyrtorhinus lividipennis

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Fig. 3. Ladybird beetle, Harmonia octomaculta

Fig. 4. Rove beetle, Paederus fuscipes

of the predator L. pseudoannulata was highest (74-78%) on the highly resistant cultivar IR2035 and lowest (8-10%) on susceptible TNI. Mortality of prey in the resistant cultivars WC 1240, ARC 10239 and Colombo was intermediate, ranging from 53% to 62%.

On the basis of the results of the three trials, it is evident that when one predator L. preudoannulata was added to the cage of 50 S. furcifera adults, its mortality in a 4 day period increased from 10% to 33% on various cultivars (Table 2). The increase was simply an additive effect and there was no evidence of a higher predation rate on the highly resistant cultivar compared with the resistant or susceptible cultivar. In fact, the mortality increased 33% when the predator was added to the susceptible TNI cages, whereas the increase was 19% on the highly resistant cultivar IR2035.

Prey mortality by H. octomaculata in test 1 was little affected by the level of resistance of the various cultivars because all prey were replaced daily and there were no sufficient resistance factors to act (Table 3). The combined mortality due to predation and varietal resistance was the same on all cultivars. Mortality with H. octomaculata was 37-39%. The increase in mortality with the predator was significant for all cultivars, being 31-35%.

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Table 1: Predation efficiency of L. pseudoannulata against S. furcifera on

ice cultivars with different levels of resistance.

S. furcifera mortality (%)

DAIo IR2035 WC 1240 Colombo ARC 10239 N22 TNI

Varietal resistance + predation

1 31.2a (a) 25.0 ab(ab) 14.1 b(b) 21.9 abc(ab) 20.3 abc(ab)

17.2 abc(b)

2 2-8 c (b) 23.4 abc(a) 12.5 b(ab) 17.2 abc(ab) 14.1 bc(ab) 9.4 c(b)

3 28.1 a(a) 7.8 d(b) 15.6 ab(b) 10.9 c(b) 10.9 c(b) 17.2 abc(ab)

4 14.1 bc(b) 32.8 a(a) 23.4 ab(ab) 21.9 ab(ab) 23.4 ab(ab) 14.1 bc(b)

5 23.4 ab(a) 12.5 cd(ab) 12.5 b(ab) 15.6 abc(ab) 10.9 c(b) 15.6 bc(ab)

6 25.0 ab(ab) 32.2 a(a) 15.6 ab(bc) 23.4 ab(ab) 9.4 c(c) 23.4 ab(ab)

7 9.4 c(b) 14.1 bcd(b) 25.0 bc(a) 12.5 bc(b) 29.7 a(a) 23.4 ab(a)

8 29.7 (a) 14.1 bcd(b) 20.3 ab(ab) 28.1 a(a) 15.6 bc(b) 14.1 bc(b)

9 15.6 bc(c) 25.0 ab(ab) 12.5 b(c) 20.3 bc(abc) 14.1 bc(bc) 29.7 a(a)

10 26.6 a(a) 14.1 bcd(b) 28.1 a(a) 18.8 abc(ab) 31.2 a(a) 9.4 c(b)

Predation (with L. pseudoannulata – without L. psecudoannulata)*

1 26.5** 20.3** 11.0** 17.2** 17.2** 15.6**

2 3.1 NS 17.2** 6.3 NS 12.5** 8.2 NS 4.7 NS

3 18.7** 1.6 NS 10.9** 6.2 NS 6.2** 17.2**

4 7.9* 26.6** 15.6** 18.8** 20.3** 11.0**

5 18.7** 6.3* 7.8** 12.5** 7.8** 14.0**

6 20.3** 23.4** 10.9** 18.7** 6.3* 17.2**

7 3.2 NS 9.4* 23.4** 6.3 NS 25.0** 20.3**

8 20.3** 11.0** 14.1** 25.0** 10.9** 9.4**

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9 7.8 NS 18.8** 7.8* 15.6** 9.4** 26.6**

10 18.8** 6.3 NS 23.4** 14.1** 28.1** 4.7NS

o DAI Days after infestation

In a column and in a row (in parenthesis), means followed by a common letter are not significantly different at 5% level by Duncan’s multiple range test (DMRT).

ns = non-significant * Significant at 5% level ** Significant at 1% level

Prey mortality in the three trials of test 2 was recorded 4 days after infestation and thus there was a significant effect of varietal resistance on the S. furcifera population (Table 4). S. furcifera mortality without the predator on IR2035 ranged from 69% to 79% in the three trials, 49-60% on the resistant cultivars and 10-12% on TNI.

Table 2: S. furcifera mortality due to varietal resistance and L. pseudoannulata predation.

S. furcifera mortality (%)

Trial I Trial II Trial III

With Without With Without With Without

Cultivar predator predator predator predator predator predator

IR2035 96 a 78 b 96 a 74 bc 96 a 78 b

WC 1240 82 b 60 c 84 b 61 d 82 b 60 c

Colombo 80 b 56 cd 82 b 62 cd 80 b 56 c

ARC 10239 77 b 53 cd 80 b 56 d 77 b 53 cd

N22 78 b 54 cd 80 b 57 d 78 b 54 cd

TNI 43 d 10 e 42 e 8 f 43 d 10 e

All means in two columns within each trial (with predator and without predator) followed by a common letter are not significantly different at the 5% level by DMRT. Mortality after 10 days of release.

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Table 3: S furcifera population as affected bv levels of varietal resistance and H. octomaculata predation.

Cultivar S.furcifera mortality (%)

With predator Without predator Difference

IR2035 39.0 a 6.8 a 32.2**

WC 1240 39.5 a 5.8 ab 33.7**

Colombo 38.0 a 6.0 a 32.0**

ARC 10239 36.5 a 5.2 ab 31.3**

N22 37.5 a 5.0 ab 32.5**

TNI 39.0 a 3.8 b 35.2**

All means in two columns within each trial (with predator and without predator) followed by a common letter are not significantly different at the 5% level by Duncan’s multiple range test. (DMRT). Average mortality after 10 days

Table 4: S. furcifera mortality due to varietal resistance and H. octomaculata predation.

Cultivar S. furcifera mortality (%)

Trial I Trial II Trial III

With Without With Without With Without

predator predator predator predator predator predatot

IR2035 98 a 72 e 98 a 69 c 99 a 79 b

WC 1240 81 b 50 ef 81 b 52 de 81 b 49 c

Colombo 81 b 54 def 80 b 54 de 80 b 53 c

ARC 10239 84 b 59 d 84 b 60 cd 84 b 56 c

N22 81 b 55 de 81 b 55 de 80 b 56 c

TNI 46 f 10 g 46 e 46 f 48 c 11 d

All means in two columns within each trial (with predator and without predator) followed by a common letter are not significantly different at the 5% level by DMRT.

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Table 5: Effect of levels of varietal resistance and C. lividpennis predation on

S. furcifera nymphs.

S. furcifera mortality (%)

Cultivar With predator Without predator Differenc

IR2035 58.0 a 39.0 a 19.0**

WC 1240 56.5 a 37.5 a 19.0**

Colombo 48.5 b 25.0 b 23.5**

ARC 10239 41.0 bc 21.0 b 20.0**

N22 42.5 bc 19.5 b 23.0**

TNI 36.5 c 12.5 c 24.0**

In a column, means followed by a common letter are not significantly different at the 5% level by Dncan’s multiple range test (DMRT).

** Significnt at 1% level.

Table 6: S. furcifera population as affected by levels of varietal resistance and

P. fuscipes predation.

S. furcifera mortality (%)

Cultivar With predator Without predator Difference

IR2035 65.0 a 37.5 a 27.5**

WC 1240 62.5 a 34.0 a 28.5**

Colombo 49.5 b 22.5 b 27.0**

ARC 10239 51.0 b 19.5 bc 31.5**

N22 41.0 c 16. 5c 24.5**

TNI 42.0 c 8.5 d 33.5**

In a column,means followed by a common letter are not significantly different at the 5% level bny by DMRT. ** Significant at 1% level.

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S furcifera mortality in the presence of the predator H. octomaculata increased by about 30% on all cultivars. Mortality on IR2035 was 98-99%, about 80% on WC 1240, Colombo, ARC 10239 N22 and 46-48% on susceptible TNI. The combined prey mortality due to varietal resistance and predation on IR2035 was about twice that on TNI.

The mortality of S. furcifera nymphs at 10 days after infestation in the C. lividipennis test increased with increase in the level of resistance of rice cultivars. However, there was an effect of varietal resistance on mortality, as indicated (Table 5). Mortality being significantly higher on IR2035 and WC 1240 than on the other cultivars. Mortality on Colombo, ARC 10239 and N22 was intermediate. In the presence of the predator C. lividipennis, there was a simple additive effect, the increase in mortality due to predation was 19-24% on the various cultivars.

Without P. fuscipes, mortality of S. furcifera nymphs at 10 days after infestation was highest on IR2035 and WC 1240 at 38% and 34% respectively, intermediate on Colombo, ARC 10239 and N22 and lowest (9%) on TNI (Table 6). There was about 30% increase in mortality when the predator P. fuscipes was added to the caged plants and the combined effect of predation and varietal resistance resulted on 65% mortality on IR2035 and 42% on TNI.

10.3. CONCLUSION

The highly resistant cultivar IR2035 and the resistant cultivars WC 1240, Colombo, ARC 10239 and N22 all had a significant impact on S. furcifera populations. When predation was combined with the effect of varietal resistance, S. furcifera mortality increased for the four predators tested. Increase in mortality due to predators was generally 30% for all predator species.

The additive effect of varietal resistance and predation in the integrated control of S. furcifera is expected to be an effective means of managing S. furcifera populations. It is important that these predators, which are abundant in rice fields should be conserved through the judicious use of insecticides. Although S. furcifera resistant cultivars have not been released, breeding lines are currently being tested. When resistant cultivars are released for commercial cultivation, the integrated effect of varietal resistance and predation is expected to provide effective control of S. furcifera populations.

It is evident from large number of studies that host plant resistance is compatible with different components of IPM. The potential of this compatibility can further be exploited with more studies under local conditions on promising predator species in different crop production systems.

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10.4. REFERENCES

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Kartohardjono, A., and E. A. Heinrichs. 1984. Population of the brown planthopper, Nilaparvata lugens (Stal), (Homoptera: Delpahacidae), and its predators on rice varieties with differing levels of resistance. Envir. Ent. 13: 359-365.

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Maxwell, F. G. 1991. Use of insect resistant plants in integrated pest management programmemes. FAO. Pl. Prot. Bull. 39: 139-146.

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Pathak, M. D. 1975. Utilization of insect plant interactions in pest control. pp. 121-148. In: Insects, Science and Society. New York, Acad. Press, Inc.

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GLOSSARY ACCESSION.

A variety or strain or bulk population registered at a natural centre and worth conservation.

ACTIVE VEGETATIVE STAGE.

The growth stage of plants, when there is rapid development of leaves, tillers, branches, or stems.

ADAPTATION.

The process of becoming suited to new or different environmental conditions or for particular functions.

ADDITIVE RESISTANCE.

Resistance governed by more than one gene, each of which can be expressed independently, but which is reinforced by the expression of each of the additional genes.

ADULT OR MATURE STAGE.

It is that stage which comes after the nymphal stage in insects that have incomplete or no metamorphosis and after the pupal stage in insects that have complete metamorphosis.

ADULT PLANT RESISTANCE.

It is generally horizontal resistance of adult plants, and also known as old age resistance or mature plant resistance.

AGROCLIMATIC.

Relating to the relationship between crop adaptation and climate.

AGROECOSYSTEM.

Cultivated plants and the environmental factors (including human intervention) that act upon or influence them under field conditions.

ALLELOCHEMIC.

A non-nutritional chemical, produced by an individual of one species that affects the growth, health, behaviour, or population biology of another species.

ALLELOPATHY.

The phenomenon of suppressing the growth of one plant species by another through the release of toxic substance(s).

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ALLOMONE.

An allelochemic that induces a response in an individual of another species (e.g. an insect) that is beneficial to the emitting organism. Many allomones are essentially defensive, that is, toxic or repugnant to potential attackers. However, a scent that attracts bees and, therefore, facilitates pollination is also an allomone.

ANAESTHETIZE.

To immobilize an insect by exposing it to carbon dioxide.

ANEMOTAXIS.

The tendency of certain insects to orient themselves in relation to wind direction.

ANTHESIS.

The action or period of opening a flower. The period of pollination, especially the time when the stigma is ready to receive the dispersed pollen and fertilization takes place.

ANTIBIOSIS.

It is a component of host plant resistance; insects do not grow, survive, or reproduce properly because of toxic or other detrimental effects of the host plant.

ANTIFEEDANT.

A substance that deters feeding by an insect but does not kill the insect directly.

ANTIXENOSIS.

It is a term proposed by Kogan and Ortman to replace the term non-preference.

ARMYWORM.

The larva of the famility noctuidae which often travels in large population from field to field.

ARTIFICIAL INSECT INOCULATION.

Infesting plants with insects by placing them on or near the plant with an instrument.

AVIRULENT GENE.

A gene in an insect which is unable to break down the gene for insect resistance in a plant.

BACKCROSS.

It is a breeding method in which a desired character such as insect resistance is transferred into an improved variety by the repeated use of the variety carrying it as a recurrent parent to reinforce or increase the gene frequency of the character.

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BIOASSAY.

Measurement of the effect on an organism of a given stress.

BIOTECHNOLOGY.

Any technique that manipulates living organisms (or part of organisms) to make or modify products, to improve plants or animals, or to develop microorganisms for specific uses.

BIOTYPE.

An individual insect or its population that is distinguished from the rest of its species by criteria other than general morphology, for example a difference in parasite ability.

BIOTYPE SPECIFIC RESISTANCE.

Resistance that is effective only for a given biotype of the pest species. Such resistance is often, but not necessarily, vertical.

BLACKHEAD STAGE.

The stage in the development of insect eggs wherein the head can be observed as a black spot through the chorion.

BORE.

To make a hole or tunnel by the feeding action of an insect.

BORER.

An insect larva making tunnels or burrows inside the plant stem or branches.

BREAKDOWN OF RESISTANCE.

The inability to maintain resistance when attacked by a newly insect biotype that has a gene for virulence at every locus corresponding to a gene for resistance in host.

BROOD.

A generation of insects from the egg to the adult stage and back to the egg stage.

CARYOPSIS.

A small one-seeded dry indehiscent fruit with a thin membranous pericarp adhering so closely to the seed that fruit and seed are incorporated in a body forming a single grain.

CHEMICAL CHEMISTRY.

The chemistry and biochemistry of ecological situations involving interactions among organisms and between organisms and their environment.

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COEFFICIENT OF SELECTION.

A measure of the relative change in gene frequency between generations as a result of differential selection.

COMPLEMENTARY RESISTANCE.

Resistance governed jointly by two or more genes, one of them alone being ineffective.

COUMARIN.

A substance that deters feeding by certain insect pests. Derived from glucosides in the plant, and is formed when tissues are disrupted by the feeding insect.

CROP DAMAGE.

Any reduction in quantity or quality of yield that results from injury caused by environmental factors, chemicals or insect pests.

CROP DEVELOPMENT.

The sequence of processes and events involved in producing new tissues and organs throughout the crop cycle. Change in growth stage and morphogenesis.

CROP GROWTH RATE.

The crop’s rate of dry matter accumulation.

CROP.

Plants on a farm that are managed for economic purposes, producing a physical product for farm use or sale.

CROSS CONTAMINATION.

In insect rearing, a situation where a culture of one insect species is infested with insects of another species.

CULTIVAR.

A cultivated variety.

CULTIVATED VARIETY.

A named group of plants within a cultivated species that is distinguishable by a character or group of characters and that maintains its identity when propagated.

CULTURAL CONTROL.

The use of agronomic practices to reduce pest population or pest infestation or pest damage on field crops.

CULTURAL PRACTICES.

Activities or operations that are usually carried out in raising field crops.

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CULTURE MEDIUM.

In insect rearing, a food source for the insects.

CULTURE.

The rearing of organisms such as insects to serve as a source for varietal resistance or other studies.

CULUM.

The stem of a grass or sedge. The round smooth-surfaced, ascending axis of the shoot, consisting of hollow internodes joined by solid nodes.

DEADHEART.

Dead tiller (rice) or shoot (sugarcane, maize etc.) caused by the attack of the insect (borer).

DEFICIENCY.

Any inadequacy or shortage of substances essential for growth and development of plants.

DEFOLIATOR. (NUTRIENT)

Any chewing insect that feeds on the leaves of plants or chemical that removes foliage of plants.

DIAPAUSE.

A period of dormancy.

DIALLEL CROSS.

The intercrossing of parents in all combinations of two; all possible crosses among individuals in a group.

DIMBOA.

Acronym for 2,4-dihydroxy-7-methosyl-1,4-benzoxazin-3-one, a naturally occurring compound that confers resistance to certain pests in corn.

DIMERIC.

Consistaing of two parts.

DIRECTIONAL SELECTION.

The adjustment, through natural selection, of a pest population to a change in host resistance or of a host population to a change in the parasitic ability of its pests. It is also called as Stabilizing Selection.

DOMESTICATION.

The process of breeding for a given desirable characteristic found in the wild relatives so as to increase, enhance, and stabilize its occurrence in cultivated plants.

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DONOR.

In plant breeding, a variety that serves as a source of a characteristic such as insect resistance.

ECONOMIC INJURY LEVEL.

The degree of crop damage at which economic losses become significant.

ENVIRONMENTAL BIOTYPES.

Biotypes distinguished by their degree of sensitivity to environmental factors (e.g. moisture, temperature) rather than to host resistance factors.

EPISTASIS.

The suppression of the effect of a gene by a non-allelic gene.

ESCAPE.

Individual susceptible plants that show no infestation or damage because pests did not happen to attack or feed on them.

ESSENTIAL CROP NUTRIENTS.

These are the elements or simple inorganic compounds, indispensable for the growth of crops and are not synthesized by the plants during the normal metabolic processes.

FIELD RESISTANCE.

Resistance observed under field conditions as distinguished from resistance observed in the laboratory or greenhouse.

FIELD SCREENING.

Evaluating varieties for resistance in the field in contrast to greenhouse and screen-house evaluation.

FLAVONOID.

Any of a number of compounds, including plant pigments, related to or similar in chemical structure to flavone (C15H10O2).

GENETIC POTENTIAL.

The capability of a genetic material to produce many new genotypic combinations.

GENETIC PURITY.

Trueness to type, seeds or plant genetic potential.

GENOME.

A complete single set of the genetic material of a cell or organism. The complete set of genes in a gamete, the single DNA/RNA molecule of bacteria, phages, and most viruses. One haploid set of chromosomes with their constituent genes.

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GERMPLASM.

The material basis of heredity; the potential hereditary materials within a species, taken collectively.

GLABROUS.

Hairless the opposite of pubescent.

GOSSYPOL.

A pigment C30H30O that occurs naturally in cotton plants and is toxic to some insect pests.

GROWTH.

The change in the size of an organism, resulting in increased volume, increased biomass (dry) and protein content.

HOMOZYGOSITY.

The condition or state of having pairs of genes that are identical for one or more allelomorphic characters.

HONEYDEW.

A sugary liquid excretion of plant-sucking hoppers which consists of a mixture of undigested plant material and excretory products.

HORIZONTAL RESISTANCE.

A resistance trait that does not involve a gene-for-gene relationship, that is, any resistance other than Vertical Resistance.

HOST EVASION.

A type of pseudoresistance where the plant evades insect injury by passing through the susceptible stage quickly or when insect numbers are low.

HOST PLANT RESISTANCE.

The relative genetic ability of a cultivar to produce a larger or higher quality crop compared with other cultivars exposed to the same infestation level.

HOT SPOT.

Site where the natural field infestation of a particular insect is high, proving sufficient pressure for reliable results in the studies on host plant resistance.

HYBRID VIGOUR.

The increase in vigour of hybrids over their parental inbred types; also known as heterosis.

IDEOTYPE.

An ideal plant type used as a model for selective breeding to develop specific plant characteristics desired or required in particular ecosystems.

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IN-SITU.

It is Latin word meaning, in the natural or original location.

IN-VIRTO.

It means (Latin), outside the living cell. In a test tube or other artificial medium denotes the growth of explants.

IN-VIVO.

In Latin, meaning, in real life; in the living cell.

INBRED LINE.

A nearly homozygous breeding line produced by continued self-fertilization.

INBRED.

An individual resulting from the mating of closely related parents or selfing.

INBREEDING DEPRESSION.

Loss of vigour and increased mortality in successive generations due to inbreeding. The result of inbreeding as in the continued rearing of insects without the occasional introduction of field- collected insects where the continuous breeding of genetically related individuals affects the biology i.e. decrease in reproductive capacity.

INDIRECT SELECTION.

Selection for a trait other than the one desired to improve, based upon the existence of a genetic correlation between two traits.

INDUCED RESISTANCE.

A type of pseudo-resistance in which a plant temporarily acquires increased resistance from some conditions of plant or environment.

INTER ALLELIC.

Expression governed by alleles of more than one pair.

INTERFERENCE.

Distortion of test results by an extraneous factor or factors, specifically, increased levels of infestation on resistant test plants due to their proximity to susceptible plants.

INTRA-ALLELIC.

Expression governed by one of a single pair of alleles.

INTROGRESSION.

The entry or introduction of a gene from one gene complex into another.

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ISOPRENOID.

Of or related to the hydrocarbon isoprene C5H8.

JUGLONE.

A reddish yellow crystalline compound C10H5O2(OH).

JUVENILE RESISTANCE.

The characteristically vertical resistance of plants in the seedling stage; also known as seedling resistance.

JUVENILE STAGE.

The immature or young growth period.

KAIROMONE.

A chemical or mixture of chemicals emitted by an organism (e.g. a plant) that induces a response in an individual of another species (e.g. an insect) that is beneficial to the receiving organism. An example of a kairomone is a plant scent that makes the plant more easily indentifiable to an insect pest.

KEY PESTS.

Serious, perennially occurring persistent species that dominate control practices and that in the absence of human intervention commonly attain population densities that exceed economic injury levels.

LAND VARIETY OR LANDRACE.

A variety or race developed locally by indigenous people, presumably without benefit of scientific knowledge of plant breeding.

LEAF AREA INDEX.

The sum of the leaf area of all green leaves divided by the ground area above which the leaves are growing.

LEAF SCRAPPING.

The removal of the epidermal portion of a leaf by the feeding of an insect.

LEAF SHEATH.

The lower part of the leaf originating from a node and enclosing the culum above the node.

LEAFHOPPER.

Insects of order Homoptera, family Cicadellidae, which feed on the leaf portion of the plant by sucking plant sap.

LIGNIN.

A complex amorphous substance that in association with cellulose, causes the thickening of plant cell walls and thereby forms wood or woody parts.

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LINE.

In plant breeding, plants originating from common parents or a common cross that is undergoing selection or improvement and which may become an identified variety.

LODGING.

The condition of a plant that has been beaten to the ground or otherwise damaged so that it cannot stand upright.

MAJOR GENE RESISTANCE.

Genes that show clear-cut and discrete segregation in the F2 or later generations of crossed between resistant and susceptible parents. Effects are thus qualitative.

MASS REARING.

Rearing large number of insects, which serve as test organism in varietal resistance studies.

MASS SCREENING.

Screening a large number of varieties in preliminary studies. Varieties selected as resistant in the mass screening trial are retested to confirm their resistance.

MECHANISM OF RESISTANCE.

Process involved in the resistance of a plant to an insect, including non- preference, antibiosis and tolerance.

MEIOSIS.

The stage of the reproductive process in which the pairs of chromosomes within the nucleus of a cell separate so that only one chromosome from each pair enters each daughter cell.

MODERATE RESISTANCE.

Intermediate level of resistance, between highly resistant and susceptible.

MONOGENIC.

Relating to or controlling by a single gene.

MONOPHAGOUS.

Feeding on or utilizing a single kind of food or single species of plants.

MULTIGENIC.

Relating to or controlled by more than one gene.

MULTILINE.

A crop or field grown plants from a mixture of seed of several resistant lines, thereby confronting insect pests with a mixture of host genotypes.

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MULTIPLE

Resistance to several stresses such as insects, nematodes, diseases, drought etc.

NOCTURNAL.

The animal which sleeps or remains inactive during the day and becomes active at night.

NON-PREFERENCE.

The behaviour of insects when they avoid or exhibit negative reactions to a plant of a host species; also used to describe resistance trait that induces such behaviour. The term antixenosis can also be used instead of non-preference.

NUTRIENT STRESS.

A condition of plant growth when inadequate nutrient supply restricts growth or excessive supply of nutrient restricts plant growth primarily by causing toxicity in plants.

NUTRIENT.

A chemical element essential for the growth and development of an organism.

NUTRITIONAL DISORDER.

Any abnormality in organism (plant) caused by a deficiency of any essential element; or toxicity caused by a high level of any substance or ion etc.

OCCASIONAL PEST.

A pest generally under natural control, that exceeds the economic injury level only sporadically or in localized areas.

OLFACTOMETER.

An instrument for measuring the sensitivity of the sense of smell.

OLIGOGENIC.

Governed by a few genes.

OLIGOPHAGOUS.

Feeding on or utilizing a few kinds of food.

PALEA.

The upper bract that with the lemma encloses the flower in grasses.

PARENCHYMA.

A tissue of higher plants consisting of thin-walled living cells that remain capable of cell division even when mature.

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PHOTO PERIOD.

The duration of the light period during a given day, which fluctuates during the year and according to latitude.

PHOTOTROPIC.

Moving toward (positively) or away (negatively) from a light source.

PILOSITY.

Being covered with short, usually soft hair; pubescence.

PLANTHOPPER.

Insects of the order Homoptera, family Delphacidae which feed primarily on the stem at the lower portion of the plant.

PLEIOTROPIC.

Having multiple phenotypic expressions.

POLYGENIC.

Governed by several or many genes.

POLYPHAGOUS.

Feeding on or utilizing many kinds of food or many species of plants.

PERSISTENT.

With reference to a disease agent, one that remains in a virulent state in the vectors system for an extended period, often for the lifetime of the vector.

PROPRIOCEPTOR.

A receptor that responds to stimuli arising within the organism.

PSEUDO-RESISTANCE.

Apparent resistance in potentially susceptible host plants resulting from chance, from transitory (non-heritable) traits, or from environmental conditions.

RATING.

Classifying test entries based on degree of plant damage, number of insects, etc. by expressing on a numerical scale, usually 0-9.

RECEME.

An influorescence, usually conical in form, consisting of an elongated axis bearing flowers on short stems.

RESISTANT.

Possessing the ability to resist or counteract.

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RESURGENCE.

A significant increase in an insect population of insecticide treated plants as compared to untreated plants.

RETEST.

In screening for resistance, re-evaluating in subsequent tests an entry selected in the mass screening test.

ROGUE.

To weed out or cull diseased or off type or defective individuals from a crop field.

ROW GRADING.

Evaluating for insect damage where the evaluator, while standing at one spot, rapidly observes all the plants in a row and mentally estimates the average damage rating for the row.

SALINE SOIL.

A soil that contains sufficient salt in the root zone to impair plant growth.

SALINITY.

The state of consisting of or containing salt.

SCALE.

The range of damage ratings, based on a numerical system, usually 0-9 where 0 = no plant damage and 9= severe plant damage.

SCLERENCHYMA.

A protective or supporting plant tissue made up of cells with thickened, lignified walls.

SCREENING.

Evaluation of varieties or breeding lines for resistance where the resistant ones are selected for further studies and possible use as donors in the breeding programme and the susceptible are eliminated.

SECONDARY PESTS.

Pests that do not normally occur in economically important numbers but which can exceed the economic injury level as a result of changes in cultural practices or crop varieties, or the injudicious use of insecticides applied for a key pest.

SEEDLING RESISTANCE.

The characteristically vertical resistance of plants in the seedling stage.

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SEGREGATING POPULATION.

A population, generally the progeny of a cross, in which genetic differences are detectable, thus permitting identification of individuals having a desired trait and their selection for further breeding.

SELECTED ENTRIES.

Varieties or breeding lines that are resistant in initial screening tests and are selected for further screening in replicated test to confirm the resistance.

SELECTION PRESSURE.

Exposure to stresses, such as pests, that tend to induce natural selection in the exposed population. Negative selection pressure is used by Robinson to denote the absence of or interference with selection pressure.

SELF.

To induce pollination with pollen from the same plant.

SHOOT.

The vegetative parts of a plant above the ground level.

SOURCE OF RESISTANCE.

Varieties or breeding lines that have genes for insect resistance.

STABILIZING SELECTION.

Natural selection tending to maintain an established equilibrium between host and pest populations.

STOCK CULTURE.

An insect culture which serves as a source of insects for beginning a rearing programme.

STRESS.

Biologists have adopted the term stress for any environmental factor potentially unfavourable for living organisms. The living organisms may show a physical strain or chemical strain due to stress. Stress reduces the size of energy and nutrient budget of plants.

STRESS RESISTANCE.

It is the ability of a plant to survive the stress or even to grow in the presence of stress. The plant may be able to repair the strain due to stress by an active expenditure of metabolic energy.

SYNTHETIC.

A line or variety produced by combining genes unlikely to occur in nature; lines having a high general combining ability.

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TERPENE.

A class of hydrocarbons characterized by (C5H8) and occurring in plant oils and resins.

Test entries.

Varieties or breeding lines being evaluated.

Test Insect.

The insect species and biotype against which a test entry is being evaluated.

TILLER.

A sprout or stalk, especially one from the base of a plant or from the axils of its lower leaves.

TOLERANCE.

Ability of a plant to withstand infestation and to support insect populations that could severely damage susceptible plants.

TUNNELLING.

Making a passage through plant material by the feeding of larvae.

TURGOR PRESSURE.

The pressure excerted against the wall of a plant cell by the fluid contents of the cell.

VARIETY.

A variety is a group of similar plants which, by structural features and performance fall in the same species. It differs from a breeding line in that it has been named and made commercially available to farmers.

VEGETATIVE PHASE/STAGE.

From seed germination to the panicle initiation stage.

VERNALIZATION.

To treats seeds, bulbs or seedling of a plant to shorten its vegetative period and thereby induce earlier flowering and fruiting.

VERTICAL RESISTANCE.

In theory, resistance governed by one or more genes in the host plant, each of which corresponds to a matching gene to parasitic ability in the pest species; sometimes called gene-for-gene resistance.

WHITEHEAD.

White, empty spikelets resulting from the attack of a stem borer which cuts the lower portion of the culum stopping the flow of nutrients to the panicles.

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353

INDEX

Acyrthosiphon pisum 25, 70, 87, 117, 154

Adult plant resistance30, 31, 314

African pink stem borer 257 Agrobacterium mediated DNA transformation

231

Alfalfa 88, 93, 97, 99, 154, 205, 302 Alfalfa aphid 99, 154, 219

Alfalfa seed chalcid 88. Alkaloids 2, 69, 120, 121, 122, 1206

Allelochemicals/Allelo chemics 29, 33, 35, 60, 67, 71, 72, 94, 98, 115, 116, 128, 130, 165, 171, 186,193, 194, 196, 198

Allelochemicals/Allelo chemics 29, 33, 35, 60, 67, 71, 72, 94, 98, 115, 116, 128, 130, 165, 171, 186, 193, 194, 196, 198

Allopatric resistance 28, 29 Aluminium 130, 174, 175, 176, 177

Amino acids 58, 59, 60, 68, 70, 76, 115, 117, 119, 120, 127, 128,129, 130, 178, 179, 180, 189, 190, 194, 207, 213, 214

Amrasca devastans 5, 13, 89, 93, 124, 200, 213, 251, 274

Amrasca biguttula 89,91,213 Anthocyanin 58, 122

Antagonists 60, 78, 79, 125 Antibiosis 3, 8, 9, 10, 33, 36, 37, 38, 39, 41, 68, 71, 190, 204, 212, 220, 236, 257, 273, 286, 315, 323

Anthonomus grandis 6, 69, 87, 93, 99, 122

Antixenosis 8, 9, 33, 34, 35, 36, 38, 39, 41, 154, 288, 315, 324

Antifeedants 2, 74, 117, 123, 124, 315 Aphis craccivora 99

Aphid 2, 4, 27, 28, 38, 39,41, 56, 57, 59, 71, 80, 87, 89, 90, 91, 92, 93, 94, 95, 96, 98, 99, 119, 120, 123, 124, 125, 128, 154, 180, 190, 204, 205, 253, 254, 268, 272, 273, 274, 275, 287, 289, 299, 302

Aphis gossypii 95, 97, 98

Aphis fabae 71, 125 Armyworm 43, 122, 123, 124, 125, 315

354

Apple 4, 57, 97 Asiatic rice borer 28, 200

Ascorbic acid 60, 178, 213 Asparagine 129,180

Atherigona soccata 35, 41, 87, 88, 90, 119, 127, 187, 200, 214, 243, 288

Avoidance 1, 8, 38, 72, 74, 89, 116

Bacillus thuringiensis (Bt) 7, 249, 254, 268, 290, 330

Basis of resistance 87, 115, 117, 125, 221

Backcross 248, 288, 315 Bean 26, 35, 88, 94, 96, 97, 99, 118, 123, 204, 213

Bean fly 88 Beetles 3, 4, 15, 26, 56, 57, 70, 73, 74, 75, 88, 91, 94, 95, 96, 97, 98, 99, 117, 118, 119, 122, 123, 124, 200, 205, 206, 303, 305

Bemisia tabaci 27, 88, 89, 94, 187, 213, 251, 272, 274

Benzylisoquinolines 121

Biolistic 250 Biological control 9, 75, 299, 300, 302

Biotechnology 316 Biotype specific resistance 316

Biotypes 10, 11, 12, 13, 25, 30, 31, 41, 235, 284, 286, 288, 289, 291, 316

Bitter gourd 125

Blissus leucopterus 37 Boll worm complex 27, 36, 39, 41, 89, 93, 95, 99, 123, 126, 127, 212, 251, 252, 253, 254, 275, 289, 290, 301

Boll weevil 6, 69, 87, 99, 122, 123 Borers 7, 10, 25, 27, 28, 29, 30, 33, 35, 36, 37, 38, 39, 40, 41, 55, 88, 89, 90, 92, 93, 96, 97, 107, 124, 127, 128, 129, 200, 201, 204, 206, 207, 212, 239, 254, 255, 256, 257, 269, 270, 271, 272, 287, 288, 295, 301, 316, 318, 328

Brassica compestris 38, 39, 55, 56, 57, 58, 68, 70, 73, 88, 98, 120, 125, 128, 214, 272

Brassica alba 212

Brassica juncea 98, 120, 272 Brassica oleracea 91, 125

355

Breeding for resisance 13, 230, 237 Brevicoryne brassicae 87, 91, 214

Brinjal 27, 88, 89, 96, 97 Brown planthopper 30, 92, 125, 129, 156, 212, 239, 244, 283, 284, 301

Bruchophagus raddi 88 Brussels sprout 88, 120

Bt transgenic crop 249 Bug 37, 90, 92, 93, 124, 139, 154, 187, 289, 302, 304

Bulk method 245 Butterfly 56, 57, 58, 77, 122, 123, 145, 150

Cabbage 55, 56, 57, 58, 63, 87, 91, 92, 123, 125, 301

Cabbage aphid 91, 125

Cabbage butterfly 56, 58, 123 Cabbage root fly 55, 57

Callosobruchus chinensis 255, 256 Canavanine 117

Carbohydrates 59, 70, 118, 127, 130, 178, 190, 194, 254

Carotenoid 127

Carrot 122 Catepillars 73, 120, 123

Cephus cinctus 6, 89, 90, 93, 206 Ceratitis capitata 24

Cereal 5, 94, 96, 97, 99, 118, 286 Chemical 1, 2, 3, 8, 9, 14, 27, 28, 29, 54, 56, 57, 58, 60, 67, 68, 69, 70, 71, 72, 73, 75, 76, 77, 78, 87, 91, 92, 94, 97, 98, 115, 116, 117, 119, 121, 123, 125, 126, 128, 130, 134, 153, 155, 163, 165, 167, 171, 174, 178, 179, 182, 186, 187, 189, 192, 194, 196, 198, 201, 202, 206, 229, 230, 235, 236, 248, 250, 252, 300, 314, 316, 317, 318, 319, 322, 324, 327

Chemical composition 69, 92, 125, 159, 163, 165, 167, 171, 174, 175, 179, 182, 186, 187, 189, 192, 196, 198, 201, 202, 252

Chemical defences 1, 75, 77, 116, 119

Chemoreceptors 58, 73, 80, 119 Chickpea 36, 39, 40, 89, 214, 255, 256

356

Chilo infuscatellus 93, 214, 254 Chilotraea infuscatellus 200

Chilo partellus 29, 36, 119, 127, 200, 213, 214, 271, 288

Chilo suppressalis 88, 90, 200

Chromosomal translocation 286 Citrus 90

Cnaphalocrosis medinalis (C. medinalis ) 34, 37, 44, 257, 258, 259, 267, 268

Colorado potato beetle 56, 70, 73, 94, 98, 123, 205, 206

Constitutive defence 1 Conventional breeding 6,7

Corn 5, 25, 28, 30, 60, 92, 96, 122, 123, 124, 125, 126, 204, 206, 287, 288, 318

Corn borer 25, 30

Corn earworm 92,122,123,288 Corn rootworm 60, 124

Cotton 5, 6, 7, 13, 27, 36, 39, 41, 88, 89, 91, 92, 93, 94, 95, 96, 97, 98, 99, 123, 124, 126, 127, 164, 187, 200, 207, 212, 213, 229, 251, 252, 253, 254, 289, 290, 299, 301, 303, 320

Cotton aphids 95, 98, 253, 254

Cotton bollworm 123, 127, 290 Cotton bollweevil 6, 123

Cotton jassid 5, 13, 27, 41, 89, 126, 127, 200, 225, 289, 299

Cotton leafworm 98

Cotton pink bollworm 123 Cotton varieties 5, 6,13, 27, 41, 91, 126, 127, 200, 251, 252, 253, 289

Cowpea 35, 39, 88, 89, 90, 97 Cultural control 300, 301, 317

Cucurbit 3, 89, 90 Cucurbitacin 2, 3, 4, 70, 205, 206

Cynogenic 116, 119, 120 Cyrtorhinus lividipennis 302, 303, 304

Dacus cucurbitae 91, 200 Defence 1, 2, 3, 29, 67, 68, 76, 79, 118

Deterrents 58, 60, 69, 70, 73, 74, 91, 94, 121, 122, 123, 124, 125

Diabrotica virgifera 60

Diamond backmoth 91 Diatraea saccharalis 92

357

Diuraphis noxia 287 Dual discrimination theory 68, 70

Earhead bug 90 Earias insulana 253

Earias vittella 89, 127 Ecological resistance 25

Embryo 118, 124, 128 Empoasca spp 268

Empoasca fabae 89, 93 Enzyme inhibitors 117

Epilachna tredecimnotata 2, 206 Eriosoma lanigerm 4

Eruca sativa 38, 39, 190 Escape mechanism 27

Ethylene 31, 245 European corn borer 25, 30, 55, 124, 206, 287, 292

Extrinsic resistance 29 Evolution 1, 3, 10, 25, 28, 29, 67, 75, 76, 77, 78, 79, 80, 81, 116, 121

Fall armyworm 122 Fatty acid 56, 59, 60

Feeding deterrents 91, 119, 121, 122, 123, 125

Field resistance 30, 31, 319

Flavonoids 69, 122, 123 Folic acid 60

Frego bract 6,93,289,290 French bean 99

Fruit fly 57

Gall midge 11, 14, 130, 204, 207 Genes 3, 4, 6, 7, 10, 11, 12, 13, 15, 28, 29, 30, 31, 33, 81, 119, 230, 231, 236, 247, 248, 249, 250, 251, 276, 283, 284, 285, 286, 287, 288, 289, 290, 291, 303, 315, 316, 317, 319, 321, 323, 328

Gene complementarity 89 Gene pyramiding 291

Genetic engineering 6, 120 Genetics of resistance 13, 18, 283, 292

Genetic resources 12, 13 Genetic variability 14, 32, 245, 289, 292

358

Geographic mosaic theory 81 Genotypes 13, 14, 31, 36, 37, 39, 40, 45, 88, 89, 92, 94, 98, 99, 125, 127, 151, 187, 212, 245, 246, 252, 253, 254, 256, 257, 268, 286, 290, 291, 323

Geotaxis 56, 68 Germplasm 6, 7, 12, 13, 30, 36, 37, 230, 231, 232, 234, 235, 236, 238, 240, 245, 248, 257, 258, 260, 271, 272, 284, 292, 320

Glucose 58, 59, 76, 98, 119 Glucosinolates 116, 119, 120

Glycoalkaloids 2, 206 Gossypium arboreum 41, 92

Gossypol 3, 39, 89, 92, 93, 97, 99, 125, 127, 253, 289, 290, 320

Gossypium hirsutum 92, 289

Gram 99 Graminae 69

Grape phyloxera 5 Grasshoppers 57, 124

Green bug 154, 302 Green leafhopper 13, 129, 130, 268, 303

Greenhouse resistance 30 Gurdaspur borer 255

Gypsy Moth 2, 205 Heliconiine butterfly 77

Heliconius melpomene 77 Helicoverpa armigera 39, 88

Heliothis armigera 35 Heliothis virescens 3, 107, 290

Herbivores 1, 2, 28, 29, 75, 76, 77, 81, 85, 92, 116, 122, 126, 205, 206

Hessian fly 2, 4, 5, 11, 154, 229, 283, 285, 299

Hexose 59 Highly resistant 5, 41, 95, 96, 207, 237, 271, 272, 301, 303, 304, 305, 310, 323

Highly susceptible 163, 253, 258, 259, 267, 268, 271

Hispa 43

Hooked trichomes 77, 94, 96, 99 Hopperburn 239

Horizontal resistance 30, 31, 32, 33, 314, 320

Host acceptance 55, 59, 68

359

Host evasion 25, 26 Host finding 55, 56, 72, 74

Host finding 55, 56, 72, 74 Host plant resistance 4, 6, 7, 8, 9, 10, 12, 15, 25, 46, 115, 153, 179, 186, 210, 215, 229, 230, 231, 235, 236, 245, 249, 251, 269, 299, 300, 302, 303, 310, 320

Host plant selection 55, 57, 68, 69, 120, 125

Host recognition 57

Host suitability 55, 56, 68 Host-insect interactions 1, 33, 98, 116, 153, 154, 163, 171, 174, 179, 186, 189, 190, 202, 204, 223

Hylemya brassicae 55 Immunity 41

Inbreeding depression 1, 321 Incitant 58, 68, 72

Induced defence 1, 2 Induced resistance 30, 290

Inheritance of resistance 30, 290 Inositol 58

Insect development 96, 123, 154, 155, 190

Insect - plant interactions 67, 79, 101

Integrated pest management 299, 300 Interactions 1, 3, 7, 10, 11, 12, 33, 55, 67, 68, 78, 79, 80, 81, 89, 92, 94, 98, 115, 116, 120, 122, 153, 154, 163, 171, 174, 179, 186, 187, 189, 190, 202, 204, 206, 251

Internode borer 39, 40, 92 Intrinsic resistance 29

Iron 27, 128, 186, 188, 194, 195, 196, 203, 212

Isogenic lines 13

Isoleucine 59, 178 Jassid 5, 13, 27, 36, 41, 89, 93, 95, 96, 97, 98, 126, 127, 212, 253, 272, 289, 299

Juglone 322 Juvenile hormone 124

Juvenile resistance 30, 322 Kairomones 2, 3, 60, 115, 116

Key pests 322 Laodelphax striatellus 156

360

Larval growth inhibitors 128 Larval weight 38, 39, 95, 118, 201, 288

Leaf corn aphid 28 Leaf folder 34, 35, 38, 201, 241, 257

Leafhoppers 94, 129, 179, 180, 239 Lectins 118, 254

Legume pod borer 97, 107 Legumes 97, 117, 118

Lepidoptera 28, 60, 67, 124, 125, 249, 290

Leucine 59, 178

Leucinodes orbonalis 27, 204 Level of resistance 6, 7, 11, 25, 26, 30, 32, 87, 92, 99, 119, 153, 154, 155, 163, 171, 179, 200, 201, 203, 208, 212, 245, 251, 252, 254, 264, 268, 270, 272, 274, 285, 286, 287, 289, 290, 300, 303, 305, 313, 323

Light intensity 206 Lignification 1, 88, 93, 186

Lignin 39, 95, 122, 129, 322 Linolenic acid 59

Lipaphis erysimi 38, 39, 88, 98, 120, 128, 190

Lipids 59,60,117,125,127,252

Locomation of insect 98 Low resistance 41

Lycosa preudoannulata 302, 303 Lygus Hesperus 97

Lymantria dispar 2, 205 Lysine 59, 178

Macrosiphum euphorbiae 89 Maize 7, 30, 36, 37, 55, 93, 96, 127, 200, 206, 212, 213, 215, 222, 224, 250, 269, 270, 271, 272, 287, 288

Maize stem borer 29, 36, 269, 270 Major gene resistance 33, 323

Manduca sexta 98, 117, 205 Marker aided selection 291

Mass rearing 237 Maysin 122, 288

Mayetiola destructor 4, 5, 11, 96, 154, 200, 229, 285, 286, 299

Mealworm 117

361

Mechanism of resistance 25, 34, 36, 37, 45, 98, 323

Mendelian 6

Methionine 59, 178 Mexican bean beetle 97, 123

Microsiphum euphorbiae 2 Minor gene resistance 33

Moderately resistant 41, 256, 257, 258, 259, 267, 268, 271, 272, 301, 303

Molecular biology 122, 231

Monophagous 67, 72, 73, 323 Monosaccharides 59

Monoterpenoids 133, 124 Morphological characters 33, 35, 36, 79, 80, 87

Mulberry 68, 274 Multiline resistance 33

Mustard aphid 38, 39, 98, 128, 190 Mustard beetle 88, 123, 200

Mustard oil 58, 70, 75 Mutation 4, 11, 13, 30, 276, 248, 249

Neem 83, 125 Nicotinic acid 60

Nilaparvata Iugens 11, 30, 92, 125, 129, 190, 201, 204, 212, 216, 239, 244, 283, 292, 301, 303

Nitrogen 2, 27, 119, 120, 121, 127, 165, 179, 180, 186, 187, 242, 244

Non-perference 6, 9, 33, 35, 39, 68, 71, 72, 74, 91, 99, 257, 288

Oak 26, 60, 76, 205

Odd plant substances 69, 70 Oilgonichus indicus 87

Oligogenic resistance 30 Oligophagous 67

Operophtera brumata 61 Ophiomyia centrosematris 88

Organic acids 69 Orientation 56, 57, 67, 68, 71, 99, 115, 133,134, 135, 155, 164, 175, 177, 181, 183, 201, 208, 209, 210

Orseolia oryzae 11 Ostrinia nubilalis 25, 55, 206, 287

Passiflora adenopoda 77 Pea aphi 57, 87, 204, 205

Pectinophora gossypiella 27, 92, 253, Phagostimulants 59, 60, 70, 73, 91,

362

290, 301 119, 120, 126, 139

Phenolic compounds 21, 98, 129 Phenology 79

Phenylalanine 59 Phototaxis 56, 68

Phyllotreta crucifera 75, 91 Phytochemicals 1, 67

Phytophagous insects 1, 3, 55, 56, 60, 67, 69, 71, 72, 73, 75, 78, 79, 80, 87, 116, 119, 153, 154, 163, 171, 178

Pieris brassicae 56, 58, 68, 70, 73, 87, 88, 119

Pink bollworm 27, 93, 99, 123, 126, 253, 254, 301

Plant resistance 4, 5, 6, 7, 8, 9, 10, 12, 25, 29, 30, 31, 41, 55, 72, 115, 125, 153, 155, 179, 186, 201, 210, 215, 229, 231, 235, 236, 245, 249, 251, 283, 290, 292, 299, 300, 302, 303, 310, 314, 315, 320

Plutella xylostella 91, 125 Pod borer 40, 97, 256

Pod fly Polygenic resistance 13, 30, 285

Polyphagous 72, 178, 325 Polyphenols 69, 122, 126, 127

Polysaccharides 59 Potato 2, 7, 55, 56, 70, 73, 89, 94, 96, 98, 99, 118, 123, 205, 206

Potato aphid 2, 89 Progeny 14, 238, 244, 245, 246,247, 248, 327

Proline 59, 60, 126 Pseudosarcophaga affinis

Pyridoxine 60 Races 5, 6, 31, 230, 286

Red mite 87 Repellent 9, 36, 58, 72, 73, 77, 90, 94, 97, 98, 123, 124, 129

Resistant 1, 2, 3, 4, 5, 6,7 ,8, 9, 10, 11, 12, 13, 14, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 55, 59, 71, 72, 76, 78, 79, 87, 88, 89, 90, 91, 92, 93, 94, 95, 98, 99, 115, 116, 117, 118, 119, 122, 124, 125, 126, 127, 128, 129, 130, 133, 153, 154, 155, 163, 164, 171, 179, 186, 187, 189, 190, 194, 200, 201,

Rice 5, 7, 10, 12, 13, 14, 26, 27, 28, 30, 32, 34, 35, 37, 38, 40, 41, 43, 45, 88, 90, 91,92, 93, 95, 125, 128, 129, 130, 131, 134, 135, 36, 137, 154, 155, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 170, 171, 172, 73, 174, 175, 176, 177, 178, 179, 180, 181, 82, 186, 187, 188, 189, 190, 191, 192, 93, 194,

363

202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 229, 230, 231, 235, 236, 237, 238, 240, 243, 245, 246, 247, 249, 251, 252, 253, 254, 255, 256, 257, 260, 264, 267, 268, 269, 270, 271, 272, 273, 274, 275, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 299, 300, 301, 302, 303, 304, 305, 307, 308, 309, 310, 314, 315, 316, 317, 318, 320, 321,322, 323, 324, 325, 326, 327, 328

195, 196, 197, 199, 200, 201208, 210, 211, 212, 213, 231, 238, 240, 242, 244, 250, 57, 258, 260, 264, 267, 283, 284, 285, 291, 01, 303, 310, 318, 202, 203, 204, 206, 207,

Rhagoletis fly 87

Riboflavin 60

Saponin 69, 123 Schizaphis graminum 41, 154, 187, 204, 268

Scirophaga incertulas 35, 38, 41, 90, 129, 200, 206, 214, 239, 257, 268

Scirpophaga nivella 88, 127, 254

Scripophaga excerptalis 35, 36, 39, 49

Self pollinated 245 Shoot borer 27, 40, 89, 200, 204

Shoot fly 26, 35, 41, 54, 88, 90, 97, 98, 127, 187, 200, 212, 243, 244, 288

Sinigrin 58,68,120

Sogatella furcifera 26, 27, 41, 130, 163, 189, 195, 201, 207

Sorbitol 58

Sorghum 26, 37, 41, 47, 87, 88, 90, 91, 92, 98, 187, 200, 214, 243, 244, 250, 288, 289, 302

Soybean 26, 88, 89, 90, 97, 99, 118, 272

Squash vine borer 89, 90 Sterols 59, 60, 115, 149, 156, 178

Stripe stem borer 10, 88, 90, 128 Subterranean termite

Sugar 40, 58, 59, 70, 126, 127, 128, 129, 130, 160, 168, 175, 180, 182, 186, 189, 190, 193, 194, 198, 200, 213, 214, 252, 256

Sugarcane 36, 39, 40, 88, 89, 92, 93, 96, 200, 254, 255

Sugarcane internode borer 39 Sugarcane top borer 26, 89

364

Suppressant 72 Susceptible 5, 8, 9, 10, 11, 25, 26, 27, 31, 32, 34, 36, 37, 38, 39, 40, 41, 42, 44, 60, 71, 87, 89, 90, 92, 93, 99, 125, 127, 128, 129, 130, 133, 134, 154, 155, 160, 161, 163, 164, 166, 169, 171, 186, 187, 189, 199, 200, 201, 207, 208, 212, 230, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 246, 247, 251, 252, 253, 254, 256, 257, 258, 259, 267, 268, 269, 270, 271, 272, 274, 275, 283, 285, 286, 287, 288, 291, 299, 300, 301, 302, 303, 304, 305, 310, 319, 320, 321, 323, 325, 326, 28

Sympatric resistance 28 Tannins 60, 122, 126

Temperature 27,56130,131, 134, 137, 153,154, 155, 159, 160, 161, 162, 174, 186, 201, 207, 208, 214, 286, 319

Tenebrio molitor 117

Termite 60 Terpenoids 117, 123, 124, 134

Tetranychus spp 99 Tetranychus telarius 253

Thiamin 60 Threonine 59, 178

Thrips 89, 91, 94, 95, 127, 253, 268 Tobacco 7, 56, 98, 117, 123,125, 205, 290, 301

Tobacco budworm 123, 125, 290, 301 Tobacco hornworm 56, 65, 98, 117, 205

Tolerance 1, 7, 8, 9, 33, 40, 41, 71, 98, 116, 153, 163, 253, 285, 286

Toxins 3, 7, 29, 117, 290

Trichomes 29, 77, 87, 94, 96, 97, 98, 99, 289

Trichoplusia ni 99, 123, 301

Tryptophan 59, 140, 178 Turnip 88, 273, 274

Umbelliferae 69 Valine 59, 178

Vertical resistance 31, 32, 33, 247, 320, 322, 326, 328

Vicia sativa 117

365

Virulence 11, 12, 286, 287, 292, 316 Virulent biotypes 10

Volatile chemicals 56, 57, 58, 97 Waxes 87, 91, 92, 125, 134

Wbeworm 72 Weevil 6, 36, 57, 69, 87, 90, 95, 99, 122, 123, 127

Western corn rootworm 60, 124 Wheat 4, 5, 6, 11, 14, 32, 89, 90, 92, 93, 94, 95, 96, 97, 99, 154, 187, 200, 206, 229, 268, 283, 284, 285, 286, 287, 292

Wheat stem saw fly 6 Whitebacked planthopper 26, 27, 130, 147, 155, 163, 189, 201, 207, 238, 239, 244, 267, 284

Whitefly 27, 89, 90, 91, 93, 94, 95, 96, 97, 126, 187, 212, 272

Winter moth 26, 61

Yellow stem borer 35, 38, 41, 49, 90, 93, 129, 200, 201, 204, 239, 257

Yield losses 229

Zea mays 35, 38