impact of natural products on the physiology of

Proc. Indian Acad. Sci. (Anim. Sci.), Vol. 99, No.3, May 1990, pp. 185--198.© Printed in India.

Impact of natural products on the physiology of phytophagous insects

B P SAXENA and K TIKKUDivision of Insect Physiology, Regional Research Laboratory, Jammu Tawi 180001,India

AbstracL The influence of naturally occurring toxic substances of plants such asalkaloids, rotenoids, antifeedants, growth inhibitors. hormone analogues, sterilants andantigonadial agents, on the physiology of some phytophagous insects is discussed.

Keywords. Natural products; insects; physiology.

1. Introduction

Insects have developed a variety of strategies in the guise of toxic molecules alongwith an appropriate delivery mechanism to face varied encounters in nature. Thesuccess of these natural weapons depend on the ways in which inbuilt mechanismshave been devised for synthesizing all sorts of molecules that are effective either ona specific target or act as general shoot out signals. Majority of these compoundsact by interfering in the normal signalling mechanism of the nerve cells i.e.basically in the manner similar to the modem insecticides like the DDT andpyrethroids that interrupt the membrane permeability of cells due to an effect onbound proteins. Leads of similar nature have resulted in the industrial synthesis ofmany compounds that have their base in naturally occurring toxins like eserine,nereistoxin and pyrethrum etc. but unfortunately even after about 49 years of thediscovery of DDT, out of about 200 synthetic insecticides discovered, the structureof only a few is based upon the clues provided by nature. The generallyprojected view is to discover the mode of action of natural products on pestsincluding the insects so that synthesis is directed towards environmentally safe andcommercially viable compounds that will lower down the populations of pests ofagriculture and of vectors of diseases.

2. Toxic substances

2.1 Alkaloids and glycosides

These compounds have been found to exhibit some physiological action on theanimals and are toxic to the man as well as to the insects. Though instances ofalkaloids/glycosides being utilized or stored by some specific feeders like theMonarch butterfly (that stores cardiac glycosides to deter the predatory birds)(Roeske et al 1976) and Creatonotos moth (that utilizes the pyrrolizidine alkaloidsof plants for synthesizing its pheromone) (Schneider et al 1982) are available butthese compounds in general are very toxic to the insects and one of the firstpesticides used by the man was the alkaloid nicotine from the tobacco plantNicotiana tabacum L. A comprehensive review of this category of compounds hasbeen published by Levinson (1976).

185

186 B P Saxena and K Tikku

2.2 Pyrethrum

This insecticide belongs to the first generation of insecticides and was isolated fromthe plant Chrysanthemum cineariaefolium (Casida 1973). Because of its property of ahigh toxicity against insects and negligible toxicity against the man and other warmblooded animals, it gained much popularity. However, due to its high degradationrate in the atmosphere especially in the presence of light, the use of pyrethrum waslimited to the control of mainly the household pests. With the synthesis ofpyrethrins and permethrin, more stability has been achieved in this group ofcompounds (Elliot 1977).

2.3 Rotenoids

These active components from Derris and Tephrosia are toxic to the insects and thefish. The active ingredients rotenone and deguelin are powerful insecticides but allspecies do not contain rotenoids (Marini Bettolo and Delle Monache 1975).

2.4 Ryanodine

This active principle from the woody portion of the south American shrub Ryania(family; Flacourtiaceae) is a complex substance (Wiesner et al 1967) used for thecontrol of insects in the fields.

3. Andfeedants

These chemicals preventing insect attack belong to a large family of compoundsmost of which act specifically against specific insects and only a few have universalapplication. Many uncommon amino acids, found generally in seeds and preventingthe attack of beetles and other insects (Bell 1977, 1980), non-hydrolysable tannins(flavanotannins) and cyanogenetic glycosides belong to the latter category. Suchamino acids may also be present in the leaves and rhizomes of some plants.

Much work has been done on canavanine (2-amino-4-guanidinoxybutyric acid),the amino acid present in the seeds of several species of plants e.g. Gymonocladusdioicusand D. megacarpa. The compound mimics arginine and is toxic to the insectsand mammals on hydrolysis to L-canaline (figure 1) which acts as a neurotoxin.However, the bruchid beetle Caryedes brasiliensis can attack Canaoalia seeds as its

~H ~H2H2N-C-NH-O-CH2-CH2-CHC02H

L-Canavanine

L-Canaline

Figure I.

Natural products impact on phytophagous insects 187

enzyme system is capable of breaking canavaline (Rosenthal et al 1978).Experimentally it has been shown that uncommon amino acids act as generalantifeedants for Locusta, Schistocerca gregaria and Chortoicetes terminifera (MariniBettolo 1983). Certain other feeding inhibitors from plants like Parabenzointribolium, Clerodendron tricotomum, Orixa japonica and Angelica japonica areknown (Wada and Munakata 1971) while antifeedant activity has been seen in theoils of lsocoma wriqhtii and Mentha pulegium (Zalkow et al 1979). Some otherplants having antifeedant properties worth mentioning are Warburgia ugandensisand W. stuhlmannii (Kubo and Ganjian 1981), Artemisia capillaris (Yano 1983),Catharanthus roseus (Meisner et al 1981), Physalis; Withania and Nicandra sps.(Ascher et al 1981), Encelia (Isman and Proksch 1985) and Ajuga sps. (Belles et al1985). The steroidal alkaloid conessine from Holarrhena antidysenterica acts as anantifeedant for Spodoptera litura and Pieris brassicae (Saxena and Tikku 1988) whilethe alkaloids of Adhatoda vasica viz. vasicine, vasicinol, vasicinone, deoxyvasicineand deoxyvasicinone, deter feeding of Aulacophora foveicollis and Epilachnaoiqintioctopunctata (Saxena et al 1986). Azadirachtin· from Indian neem treeAzadirachta indica, is a most versatile antifeedant and also exhibits many otherphysiological activities. .

4. Growth inhibitors

Some terpenoids of sunflower e.g. kaurenoic and trachylobanoic acids inhibitlarval growth (Elliger et al 1976) while others e.g. carvacrol, eugenol, famesol andgeraniol inhibit the embryonic development (Mehta 1979) of insects. Some of theother phytochemicals acting against the development of insects are chavicol(Ohigashi and Koshimizu 1976), two compounds from Coleopsis lanceolata(Nakajima and Kawazu 1980), C-glycosylflavones from Zea mays (Elliger et al1980), certain norditerpene dilactones (Singh et a11973) and sendanin from Trichiliaroka (Kubo and Klocke 1982) etc. Conessine, from H. antidysenterica has larvalgrowth inhibitory activity against Aedes aegypti (Saxena and Tikku 1988). The nonprotein amino acid L-canavanine derails the growth and development of Manducasexta (Dahlman 1977), Musca domestica, M. autumnalis, Haematobia irritans andStomoxys calcitrans (Dahlman et al 1979) in addition of reducing the fecundity andfertility of M. sexta and effecting its nervous system (Kammer et al 1978; Palumboand Dahlman 1978).

5. Hormone analogues

Till .the discovery of the 'paper factor' (Slama and Williams 1966; Williams andSlama 1966) insect endocrinology was confined only to the academic interest. As thefactor could produce permanently juvenile adults incapable of reproduction it wasthought to make use of such compounds for insect control and Williams (1967)termed them as the 'third generation pesticides'. Because only the papersmanufactured from the coniferous trees showed juvenile hormone (JH) activity,therefore the general belief at the time was that only gymnosperms possessed theJH mimicking substance [or the JH analogues (JHA)]. The very first reportfollowed by a second one about such a type of activity being present in the


flowering plants was that of Saxena and Srivastava (1972, 1973) and this led tosearch for a trail of JHA existing in the flowering plants.

5.1 Evaluation and biological assay of JHjJHA

In order to term a compound as JHA it is very essential to know the true test for itsevaluation. Primarily a JH exerts its effect on metamorphosis and a test compoundmust produce morphogenetic changes in the test insect while all other effects are ofa secondary importance (Saxena et al 1978). Slama et al (1974) have described indetail the methodology that should be followed for a proper evaluation of suchtypes of compounds and have calculated the ID/EDso values. For determining theeffect on metamorphosis last instar nymphs or pupae should be subjected to thetests. The lesser important criteria for JHA tests are: (i) an inhibition of theembryonic development, (ii) stimulation of ovarian development and (iii) population assays.

The five natural JH identified so far are illustrated in figure 2. Numerousanalogues of JH cave also been synthesised by the private companies especially bythe Zoecon Corporation of USA whose highly active patented products are themethoprene and hydroprene but on account of the development of resistance by theinsects towards these JHA, the compounds could not find a successful place in themarket. Moreover the slow action of such types of compounds could not standbefore the rapid overnight control of the insect pests brought about by thepesticides. Such drawbacks have made the developed world lose interest in theseanalogues and now their use is recommended only in specific places like thehospitals etc for controlling pests like the pharaon ants since poisonous pesticideapplications are avoided at such places.

12

JH r R1 =R2 =C2 HS, R=CH3

JHIl Rl=C2HS,R2=R=CH3

JHIII Rl=R2=R= C H3

JH 0 Rl = R2 =R =C2 HS

•iso JH 0 = 4-meth y l - J H I

*Relative & absolute configurationat C-4 unknown

Figure 2.

OMe

Natural products impact on phytophagous insects

6. Moulting hormones

189

These steroidal hormonal secretions of the prothoracic glands of insect larvae andof spongy cells of adult testes (Saxena and Tikku 1989) govern the insect moulting.A variety of ecdysterols are also present in the plants (Hikino and Hikino 1975) asthese phytosterols can be easily synthesised by the plants by metabolism from theother steroidal sources (Levinson 1972). Reports are available on the presence ofecdysones in the pteridophytes (6 families, 44 sps.), the gymnosperms (6 families,26 sps.), and the angiosperms (22 families, 46 sps.) (Jacobson and Crosby 1971).

7. Antijuvenile hormones

This new category of compounds has been placed under the class of the fourthgeneration pesticides. Bowers et al (1976) found two naturally occurring chromenederivatives in Ageratum houstonianum that could produce precocious adults (i.e.adults moulted directly from the fourth instar nymphs by deleting the formation ofthe ruth instars during the development process) and also effected the ovaries ofsuch adults. These derivatives were termed as Precocene I and Precocene II (figure 3)and in the insect body these compounds are broken down resulting into the

/'(y0'f(

"o~PRECOCENE I

CO Reus ALlATUM

Figure 3.

PRECOCENE II

Metabolised


production of another compound (3,4-epoxide1 that reacts with the proteins in thecorpus allatum leading to their destruction (Bowers and Martinez-Pardo 1977) andthereby to an impaired JH synthesis. The low titres of JH in the nymphs or thelarvae result into their transformation into adults.

8. Sterilants

The fecundity and fertility of the insects can be disturbed in multiple ways e.g.either by a direct effect on the germ cells or by an indirect hormonal disruption,whether of neurosecretion br of the juvenile or the moulting hormones. Theclassical chemosterilants are alkylating agents bringing about sterility by theirproperty of effecting the DNA and thereby the germ cells while the sterilants differfrom such alkylating agents in lowering down the rates of fecundity and fertility butnot causing any harm to the germ cells. Such sterilants are exemplified bycompounds like conessine, several alkaloids of A. vasica, aristolochic acid, andplumbagin (Saxena et a11979, 1986; Saxena and Tikku 1988; Thappa et al 1988).

8.1 Conessine

this steroidal alkaloid from H. antidysenterica sterilizes Dysdercus koenigii (Thappaet al 1988) when given in drinking water (doses used: 5 and 10 ppm) for 72 handuntreated mates provided to the treated individuals. In 5 ppm the most remarkableaction of the compound was being more effective in inducing sterility in theuntreated females crossed with treated males rather than showing a more strongaction against the females that were given the alkaloid in feed but with the increaseof dose to 10 ppm the fecundity of treated females was also reduced and the laideggs were sterile for treated individuals of both the sexes (table 1). The effect onboth fecundity and fertility of normal females crossed with treated males indicatesthe transfer of some factor from the affected males into the female genital systemthat reduces the number of eggs and makes such eggs sterile while conversely thenormal sperm of the untreated male provided to the alkaloid-fed female perhaps isnot able to fertilize its eggs due to a changed physiological environment of thefemale tract that does not provide an environment conducive for normalfertilization and fertile eggs are not formed in this way.

8.2 Alkaloids of A. vasica

The fecundity and fertility of D. koenigii was adversely affected by 5 tested alkaloids

Table 1. The percentage reduction in laying and hatching of eggs of adult D. koenigiiaffected by conessine.

Dose Ist batch 2nd batch 3rd batch(ppm) Laying Hatching Laying Hatching Laying Hatching

Untreated ~ treated <1 5 18'69 00-0 23·95 2·2 31'79 27·810 20-96 ooo 50-38 1·5 66·87 0-5

Treated ~ untreated <1 5 oooo 45·6 oooo 14·8 33-94 31·910 25-68 roo 19·86 roo 66'35 0-8

Control 00-00 100 00-00 100 07·60 100


of A. vasica (Saxena et al 1986) given for 72 h in drinking water (dose: 0·1 and Q-3%).The alkaloids were vasicine, vasicinol, vasicinone, deoxyvasicine and deoxyvasicinone (table 2). Vasicinol displayed the maximum and deoxyvasicinone the minimumactivity. The female D. koenigii adult lays its normal number of eggs whether matedor not (unpublished results) but a collapse of eggs (within 2-3 days of oviposition)of untreated females paired with vasicinol treated males having sperms of normalappearance indicates some disturbance in the chorion formation of such eggs whichleads to a slight blackening in their colour followed by crumpling. In the case ofvasicinol treated females the lowering in the number of laid eggs is apparentlyrelated to a blocking of the oviduct by oocytes which inhibits the descent of thevitellar oocytes and their resorption causes the death of such females. All otherallelochemics of Adhatoda bring about a reduction in the oocyte numbers. TheAdhatoda alkaloids are also effective in reduction of the progeny of Triboliumcastaneum in doses of 0·1 and 0·5%.

8.3 Aristolochic acid-I

This product from Aristolochia bracteata in addition of acting as a sterilant forD. koenig ii, Ae. aegypti and T. castaneum, also interferes in the ecdysis (Saxena et al1979). The high dosage of 0·002% totally prevents egg laying in the adults moultedfrom treated 5th iustar nymphs of D. koeniqii, the next dose i.e. 0·001 % is againtotally inhibitory for females while comparatively the male treatments are not aseffective though hatching percentage becomes highly reduced in the eggs, the lowestdose of 0·0005% does not cause any reduction in the first batch of eggs but in the

Table 2- Cumulative fecundity and fertility of 3 batches of eggs in D. koenigii adultstreated with Adhatoda alkaloids at Q-l and 0·3% level (source: Saxena et al 1986).

Average fecundity Average fertility(no. ± SE) (%) Sterility index

Sex crossTreatment <S x ~ Q-l% Q-3% Q-l% (}3'110 (}1% (}3%

Vasicine-HCl T x T 74·9±6·9 64·3±6·4 60-3 67·9 64-0 65-0T x U 93'1 ±6'4 . 90-7± 10-0 80-0 67-0 40-4 51·2U x T 101·5.±7-o 72-8± 5-0 77-3 72-0 37-0 58-0

Vasicinol T x T 56·3±6·6 46·2±3-o 49-1 3(}5 77·8 88·7T x U 4(}Q±5·8 42-3±H 37'7 2(}4 87-8 96'7U x T 53·5±7·5 53·8±4-o 49·8 45-0 78·6 8Q-7

Deoxyvasicine T x T 102·2±6-3 91·4±7-o 65·3 63-3 46'4 53'6T x U 96·7±9·9 93·7±6·5 54'1 63-1 58-0 52'5U x T 94·7 ± 7·0 85·1 ±6·3 61-3 53-0 53-3 63'8

Vasicinone-HQ T x T 94·7±6·4 36·8±4·3 58'9 40-7 55·2 88·0T x U 91-o±7-4 104'3±8-o 64'1 56·7 53'2 52·5U x T 88·3±7·0 84·5±5·7 51-2 43-0 63'7 7Q-9

Deoxyvasicinone-HCl T x T lOO5±6-2 104·3±8·8 90-8 1000 26-7 18·6T x U 99·2±7-3 111-o±6-8 90-5 98-0 27-9 15·2U x T 12(}3±9-2 IOS-o±8-0 89'8 92'5 13·2 22·1

Control U x U 127·5±6-2 13Q-9 ± 9-0 97·7 98'0

T, Treated; U, untreated.


subsequent second and third batch the fecundity and fertility goes on declining (70and 30 eggs respectively in 2nd and 3rd batch in comparison with 110 eggs of thecontrols). In the case of adult treatments the effect was not as marked as in thetreatments to 5th instars though the hatching capabilities get significantly reduced.While lower doses are not effectively able to control the vitellogenesis, though themanner in which yolk is deposited seems very haphazard and the number ofoocytes is reduced, the higher doses prevent maturation of ovaries of both the adulttreatments and of the adults moulted from 5th instar nymphs. However all thedoses affected acutely the ovaries of adults formed from treated 4th instars as suchovaries remained very small consisting of almost only the tropharium. The vitellarportion was undeveloped and ovaries retained their larval yellow covering. Figures4-6 show a large number of trophocytes and their nutritive cords but only a fewoocytes in the tropharium. Sometimes the undifferentiated vitellar portioncontained 2-3 oocytes in addition to the corpus luteum and the oviduct, indicatingan imbalance in the hormonal system governing the maturation of the oocytes or atthe cystocyte stage of the ovarian development. Such a type of effect is totallydifferent from that of the classical chemosterilants that directly influence the DNAof the cells. Impairment of vitellogenesis was also observed in the ovaries of Ae.aegypti (doses used: 0'001-0'00025%) and that of T. castaneum (doses used:1-10 ppm).

8.4 Plumbagin

This nephthoquinone reduces the fecundity of the red cotton bugs (Joshi et at 1989),affects spermatogenesis (Saxena and Tikku 1988) and causes sterility in thehouseflies (unpublished results). Low doses of 1-2 pg of plumbagin brought about a

Figures~. Affected ovary of D. koeniqii. 4. Tropharium with trophocytes (T) andnutritive cords (arrow). The vitellarium (V) is empty. 5. Juvenile ovary with emptyvitellarium (V) and corpus luteum (arrow). Arrow-head points to the oocytes, 6. Magnifiedview of lower portion of tropharium showing a single oocyte (arrow).


delay of the ovarian development of adult bugs but higher doses disruptedvitellogenesis (Joshi et al 1989). The experiments on male houseflies (unpublishedresults) have revealed that treatments at adult stages were very effective (table 3) ascompared to those given to the wandering larvae.

According to Joshi and Sehnal (1989), plumbagin inhibits or delays the imaginalecdysis and oviduct metamorphosis in Dysdercus cinqulatus and an injection ofmakisterone A reverses the effect of the compound. They found that plumbaginaffects the prothoracic gland and the pericardial cells which in turn suppresses thebiosynthesis of ecdysteroids. In in vitro experiments it was shown by them that thepresence of plumbagin in culture media could restrain the ecdysteroid biosynthesisand it was concluded that plumbagin exerts a direct action on the prothoracicgland. According to other reports plumbagin inhibits chitin synthetase activity(Kubo et a11983) and interferes in ecdysone biosynthesis due to its inhibitory actionon the enzyme ecdysone 20-monooxygenase (Mitchell and Smith 1988). Ourexperiments have shown that the compound affects the spongy tissue of the testis ofD. koenigii (Saxena and Tikku 1988) whose secretion is steroidal in nature (Saxenaand Tikku 1989). The disintegration of the cells of the spongy tissue indicates thatthe testis is not provided with the requisite quantities of steroids required for .anuninterrupted spermiogenesis. Also, the appearance of vacuoles in spermatocytesand a reduction in the number of axonemal microtubules of the sperms ofD. koenigii (Saxena and Tikku 1988) points out to the fact that plumbagin does notspecifically act on the prothoracic gland and inhibits biosynthesis of ecdysteroidsbut such an effect is a generalised one on cells including the glandular ones.

9. Antigonadal agent

The term 'antigonadal' is applied for those compounds that act particularly againstthe gonads i.e. their action is neither mediated by the endocrine system of the insectnor do they behave as alkylating agents. fJ-asarone is one such compound (Saxenaet al 1977), isolated from the oil of Acorus calamus L. that brings about a regressionof the ovaries of D. koenigii by its antigonadal action. The oil in vapour form alsosterilizes Thermobia domestica, M. domestica, the females and males of D. koeniqi:and a number of stored product insects (Saxena and Rohdendorf 1974; Mathur andSaxena 1975; Saxena and Mathur 1976; Saxena et al 1976; Koul et al 1977).

Products of similar nature that are detrimental for the survival of innumerablepests of the insect kingdom are stored in the existing flora in unlimited quantitiesand tapping of such sources is the urgent need of the hour. The essential role played

Table 3. Effect of plumbagin treatment on the egg laying andhatching of adult males of M. domestica crossed with untreatedfemales.

Reduction inDose (2IJI/fly)I egg laying (%) Hatching (%)

0·05% 76 0(H)l% 71 00·005% 64 0Control 0

.- U7


by the plant is to supplement the insect thriving on it with the nutrients required forits growth and development. In addition, while providing almost all the precursorsfor the constituents of some very important hormones and the pheromones, theplants concomitantly ensure their own existence against the voracious feeders bysynthesizing certain chemicals like the tannins to reduce the intensity of the attacksbecause tannins are known to be indigestible. Some other compounds likeazadirachtin, gossypol and canavanine are also utilized by the plants forstrengthening their defences. The saponin gossypol is a growth retardant, decreaseslarval survival, causes pupal deaths and also increases the time taken for pupation(Dongre and Rahalkar 1980). This important pest controlling agent is found in theleaf glands of cotton plants and in the enthusiasm for increasing the yields, thecultivated varieties became glandless and prone to infestation. Azadirachtins'antifeedant action is just one of the multifaceted aspects of this phytochemical thatis most effective in disrupting growth, acting as a sterilant and inhibiting the insectmoulting. In locusts the compound deters feeding even if the azadirachtin-sensitivechemoreceptors on the mouthparts are bypassed by directly injecting it into thehaemolymph which indicates its effect on the passage of food through the gut andthe speculations are that the azadirachtin-poisoned gut is unable to expandsufficiently for a successful moulting (Mordue (Luntz) et al 1985). Azadirachtin alsolowers down the ecdysteroid titres and according to the findings of Pener et al (1988)the compound affects ecdysteroid metabolism and/or prothoracicotropic hoamonerelease. Canavanine's toxic and sterilant action also acts as a safeguard for the hostplants.

The role played by the moulting hormone in an insect's life is well knownbecause the insect is totally dependent upon phytosterols for the synthesis of thehormone. The physiology and reproduction is governed by ecdysteroids but themajor drawback that limits their desirable exploitation is the difficulty ofpenetration of sterols through the insect cuticle. Therefore till the stage is reachedwhen absorption of these vital plant products through the gut or the body ispossible, the ecdysteroids have a limited scope for use as insect controlling agents.

The isoprene units of terpenoids are available in plants but so far it is not knownwhether the phytophagous insect gets any precursor from the host plant for theproduction .ofJH, the most important physiological secretion of the corpus allatum.The synthesized analogues are also losing ground due to the preferences for fastacting compounds and, because of the development of resistance by someeconomically important insects (Brown and Hooper 1979), a potent compound likethe methoprene has also become of a limited use. Precocenes which appeared to bephysiologicallyvery important compounds in acting as antijuvenile hormone agentshave receded in the background on account of their cytotoxicity.

The other hormones including the neurosecretion are indigenous products of theinsect body and till now no link has been established with their relation to theinsects' feeding activity.

Pheromones, having a direct bearing on the modulation of the insect behaviour,are comprised of widely unrelated groups like the aldehydes, ketones, alcohols etc.and these groups are the constituents of a number of phytochemicals that theinsect gets from its host plants but whether the compounds have a decisiveparticipation in the synthesis of the volatile product is not yet known. However thework of Schneider et al (1982) has shown that totally unrelated compounds like the


pyrrolizidine alkaloids are used in pheromone production and more work needs tobe done along this direction to explore the co-relation between natural productsand synthesis of pheromones by the insects as this area has an immense use infuture.

The sterilant chemicals from plants truly appear to be the answer for thephysiological agents that are in demand due to their property of causing adisharmony in the hormonal balance of the pest. Compounds like conessine,Adhatoda alkaloids, aristolochic acid, and plumbagin are the products of plantorigin and sterilize male as well as female insects. These phytochemicals areapplied in very low dosages and their detailed mode of action at cellular orhormonal level needs to be probed further. The antigonadal property of theactivecis-form of f3-asarone has given a clue for the synthesis of similar compounds withthe appropriate positions of Osmethyl-groups so that only compounds exhibitingactivity of similar nature are obtained rather than toxic or antifeedant compounds(Saxena et al 1977).

Now-a-days work on genetic engineering and DNA recombination techniques arebelieved to be the answer for future control measures in the light of the toxicitiescaused by Bacillus thurinqiensis in the insect pest but recent work has revealed thatmosquito larvae have already overcome the toxic effects of B. sphaericus (Aly et al1989) and if one pest has developed detoxification mechanism the possibility ofother such organisms following suit is always there. JHA protagonists also at onetime were putting forward the argument that as these compounds were structurallysimilar to those produced by the insect body, so chances of development ofresistance were the least but what happened was contrary to the expectations.Similarly if mosquito larvae are able to digest the Bacillus poison and the trendgoes on increasing, genetically engineered plants will lose their efficacy. The otheraspect of the problem that needs a thoughtful approach is that, presently the B.thuringiensis incorporated tobacco plants are considered a safe bet against thetobacco hornworm but tomorrow if similar methodologies are adopted to protectother crops, will not the toxins have a disastrous accumulative effect on thoseliving beings (including mammals) who would regularly consume the edible parts.Therefore toxicological studies need to be conducted in advance beforecommercialization of the process.

Our stronghold against the pests can only be maintained by a proper screening ofthe plants and the evaluation of their products on the physiology of the insect. Ifthe activity of the phytochemical is of a desirable nature, constraints should befollowed in its overexploitation so that the problems of resistance that are beingencountered for the synthetic analogues of pyrethrins (Scott and Georghiou 1985)are not faced and at the same time no harm should come to the predators and theparasites. An ideal approach would be to put into use the aggregated effect ofvaried properties of the natural products e.g. the toxic, antifeedant, growthretardant, and sterilant etc. and advocate their proper application in integratedcontrol schemes so that the pest species are effectively brought below the level ofnuisance.

References

Aly C, Mulla M S and Federici B A 1989 Ingestion, dissolution and proteolysis of the B. sphaericus toxinby mosquito larvae; J. lllvertebr. Pathol. 53 12-20


Ascher K R S, Schmutterer H, Glotter E and Kirson I 1981 Withanolides and related ergostane-typesteroids as antifeedants for larvae of Epilachna varivestis (Coleoptera: Chrysomelidae); Phytoparasitica 9 197-205

Bell E A 1977 The possible significance of uncommon amino acids in plant-vertebrate, plant-insect andplant-plant relationship; in Natural products and the protection of plants (ed.) G B Marini Bettolo(Amsterdam: Elsevier) pp 571-593

Bell E S 1980 Encyclopedia of plant physiology B. 11 (Berlin, Heidelberg: Springer-Verlag)Belles X, Camps F, Coll I and Piulachs M D 1985 Insect antifeedant activity of c1erodane diterpenoids

against larvae of Spodoptera /ittoralis (Boisd.) (Lepidoptera); J. Chem. Ecol. 11 1439-1445Bowers W S, Ohta T, Cleere I S and Marsella P A 1976 Discovery of insect antijuvenile hormones in

plants; Science 193 542-547Bowers W S and Martinez-Pardo R 1977 Antiallatotropins: Inhibition of Corpus alia tum development;

Science 197 1369-1371Brown T M and Hooper G H S 1979 Metabolic detoxification as a mechanism of methoprene resistance

in Culex' pipiens pipiens; Pestic, Biochem. Physiol. 12 79-86Casida IE 1973 Pyrethrum, the natural insecticide (New York: Academic Press)Dahlman D L 1977 Effect of L-canavanine on the consumption and utilization of artificial diet by the

tobacco homworm. Manduca sexta; Entomol. Exp. Appl. 22 123-131Dahlman D L, Herald F and Knapp F W 1979 L-canavanine effects on growth and development of four

species of Muscidae; J. Econ. Entomol. 72 67~79Dongre T K and Rahalkar G W 1980 Growth and development of spotted bollworm E. vittella on

glanded and glandless cotton and diet containing gossypol; Entomol. Exp. Appl. 27 6-10Elliger C A. Zinkel D F. Chan B G and Waiss A C Ir 1976 Diterpene acids as larval growth inhibitors;

Experientia 32 1364-1365Elliger C A. Chan B G. Waiss A C Jr, Lundin R E and Haddon W F 1980 C-glycosyInavones from Zea

mays that inhibit.insect development; Phytochemistry 19 293-297Elliot M 1977 Synthetic insecticides designed from the natural pyrethrins; in Natural products and the

protection of plants (ed.) G B Marini Bettolo (Amsterdam: Elsevier) pp 157-176Hikino Hand Hikino Y 1975 Phytoecdysones; Fortschin. Naturstaffe 28 294-302Isman M Band Proksch P 1985 Deterrent and insecticidal chromones and benzofurans from Encelia

(Asteraceae); Phytochemistry 24 1949-1951Iacobson M and Crosby D G 1971 Naturally occurring insecticides (New York: M Dekker)Ioshi N K. Lathika K M. Rarnakrishnan V, Banerji A and Chadha M S 1989 Plumbagin-induced

inhibition of ovary development in Dysdercuscinqulatus; in Regulation of insect reproduction IV (eds)M Tonner, T Soldan and B Bennettova (Praha: Czech. Acad. Sci.) pp 287-292

Joshi N K and Sehnal F 1989 Inhibition of ecdysteroid production by plumbagin in Dysdercuscingulatus; J. Insect Physiol. 3S 737-741

Kammer A E, Dahlman D L and Rosenthal I G A 1978 Effects of the non-protein amino acids'.-canavanine and Lcanaline on the nervous system of moth Manduca sexta (L); J. Exp. BioI. 75123-132

Koul 0, Tikku K and Saxena B·P 1977 Mode of action of Acarus calamus L. oil vapours on adult malesterility in the red cotton bug; Experientia 33 29-31

Kubo I and Ganjian I 1981 Insect antifeedant terpenes, hot-tasting to humans; Experientia 37 1063-1064Kubo I and Klocke I A 1982 An insect growth inhibitor from Trichilia roka (Meliaceae); Expenentia 38

639-640' .

Kubo I. Uchida M and Klocke J A 1983 An insect ecdysis inhibitor from the African medicinal plant,Plumbago capensis (Plumbaginaceae); a naturally occurring chitin synthetase inhibitor; Agric. Biol.Chem. 47 911-913 .

Levinson H Z 1972 Zur evolution und Biosynthese der terpenoiden Pheromone un Hormone; Naturwiss59 477-484

Levinson HZ 1976 The defensive role of alkaloids in insects and plants; Experientia 32 408Marini Bettolo G B and Delle Monache F 1975 Flavonoids of uncommon structures; Proc. 5th

HungariaN Bioflaoonoids Symposium Akademiai Kiado, Budapest pp 1-13Marini Bettolo G B 1983 The role of natural products in plant insects and plant-fungi interaction; in

Natut:al products for innovative pest management (eds) D L Whitehead and W S Bowers (Oxford:Pergamon Press) pp 187-220

Mathur A C and Saxena B P 1975 Induction of sterility in male houseflies by vapours of Acarus calamusL. oil. die; Naturwissenschaften 62 576


Mehta R C 1979 Inhibition of embryonic development in Earias vittella by terpenoids; Naturwissenschaften 66 56

Meisner J, Weissenberg M, Palevitch D and Aharonson N 1981 Phagodeterrency induced by leaves andleaf extracts of Catharanthus roseus in the larva of Spodoptera /ittoralis; J. Econ. Entomol. 74 131-135

Mitchell M J and Smith S L 1988 Effect of the chitin synthetase inhibitor plumbagin and its 2-delmethylderivative juglone on insect ecdysone 20-monooxygenase activity; Experientia 44 990-991

Mordue (Luntz) A J, Cottee P K and Evans K A 1985 Azadirachtin: its effect on gut motility, growthand moulting in Locusta; Physiol. Entomol. 10431-437

Nakajima Sand Kawazu K 1980 Insect development inhibitors from Coleopsis lanceolata L.; Agric. Bio/.Chem. 44 1529-1533

Ohigashi Hand Koshimizu K 1976 Chavicol, as a larva-growth inhibitor, from Viburnum japonicumSpreng.; Aqric. Bioi. Chem. 40 2283-2287

Palumbo R E and Dahlman D L 1978 Reduction of Manduca sexta fecundity and fertility byL-canavanine; J; Econ. Entomol. 71 674-676

Pener M P, Rountree D B, BishoffSTand Gilbert L J 1988 Azadirachtin maintains prothoracic glandfunction but reduces ecdysteroid titres in Manduca sexrc pupae. In vivo and in vitro studies; inEndocrinological frontiers in physiological insect ecology (eds) F Sehnal, A Zabza and D L Denlinger(Wroc1aw: Wroc1aw Technical Univ. Press) pp 41-54

Roeske C N, Seiber J N, Brower L P and Moffitt C M 1976 Milkweed cardenolides and theircomparative processing by Monarch butterfly (Danaus plexippus L.); in Biochemical interactionbetween plants and insects (eds) J W Wallace and R. L Mansell (New York: Plenum Press) pp 93-167

Rosenthal G A, Dahlman D L and Jansen D H 1978 L-canaline detoxification: a seed precursor'sbiochemical mechanisms; Science 202 528-529

Saxena B P, Koul 0 and Tikku K 1976 Non-toxic protectant against the stored grain insect pests; Bull.Grain Techno/. 14 190-193

Saxena B P, Koul 0 and Tikku K 1978Citral-Not a juvenile hormone mimic against insects; Indian J.Exp. Bioi. 16703-705

Saxena B P, Koul 0, Tikku K and Atal C K 1977 A new insect chemosterilant isolated from Acoruscalamus L.; Nature (London) 270 512-513

Saxena B P, Koul 0, Tikku K, Atal C K, Suri 0 P and Suri K A 1979 Aristolochic acid-An insectchemosterilant from Aristolochia bracteata Ret:z.; Indian J. Exp. Bioi. 17 354-360

Saxena B P and Srivastava J B 1972 Studies on plant extracts with juvenile hormone activity. Effects ofIris ensata Thumb. (Iridaceae); Experientia 28 112-113

Saxena B P and Srivastava J B 1973 Tagetes minuta L. oil-a new source of juvenile hormone mimickingsubstance; Indian J. Exp. Bioi. 11 56--58

Saxena B P and Rohdendorf E B 1974 Morphological changes in Thermobia domestica under theinfluence of Acorus calamus oil vapours; Experientia 30 1298-1300

Saxena B P and Mathur A C 1976Loss of fecundity in Dysdercus koenigii F. due to vapours of Acoruscalamus L. oil; Experientia 32 315-316

Saxena B P and Tikku K 1988 Exploitation of lacunae by some a11elochemics in insect-plantinteractions; in Dynamics of insect-plant interaction: Recent advances and future trends (eds) T NAnanthakrishnan and A Raman (New Delhi: Oxford and IBH) pp 105-122

Saxena B P and Tikku K 1989 Ultrastructural studies of two tissues in Dysdercus koenigii testis and theirrole in spermiogenesis: in Regulation of insect reproduction IV (eds) M Tonner, T Soldan and BBennettova (Praha: Czech. Acad. Sci.) pp 141-151

Saxena B P, Tikku K, Atal C K and Koul 0 1986 Insect anti-fertility and antifeedant allelochemics inAdhatoda vasica; Insect Sci. Appl. 7 489-493

Schneider D, Boppre M, Zweig J, Horsley S B, Bell T W, Meinwald J, Hansen K and Diehl E W 1982Scent organ development in Creatonotos moths: Regulation by pyrro1izidine alkaloids; Science 2151264-1265

Scott J G and Georghiou G P 1985 Rapid development of high-level permethrin resistance in a fieldcollected strain of the housefly (Diptera: Muscidae) under laboratory selection; J. Ecen. Entomol. 78316--319

Singh P, Fenemore P G and Russell G B 1973 Insect-control chemicals from plants II. Effects of fivenatural norditerpene dilactones on the development of the housefly; Aust. J. Bioi. Sci. 26 911-915

Slama K, Romanuk M and Sorm F 1974 Insect hormones and Bioanalogues (Wien: Springer-Verlag)Slama K and Williams C M 1966 'Paper factor' as an inhibitor of the embryonic development of the

European bug, Pyrrhocoris apterus; Nature 210 329-330


Thappa R K, Tikku K, Saxena B P, Vaid R M and Bhutani K K 1989 Conessine as a larval growthinhibitor, sterilant, and antifeedant from Holarrhena antidysenterica Wall.; Insect Sci. Appl. 10149-155

Wada K and Munalcata K 1971 Insect feeding inhibitors in plants Part III. Feeding inhibitory activity ofterpenoids in plants; Agric. Bioi. Chern. 3S 11>-118

Wiesner K, Valenta Z and Findlay J 1967 The structure of Ryanodine; Tetrahedron Lett. 221-225Williams C M 1967 Third generation pesticides; Sci. Am. 117 13-17Williams C M and Slama K 1966 The juvenile hormone. VI. Effects of the 'paper factor' on the growth

and metamorphosis of the bug, Pyrrhocoris apterus; Bioi. Bull. 130 247-253Yano K 1983 Insect antifeeding phenylactylenes from growing buds of Artemisia capillaris; J. Agrlc

Food Chern. 31 667--668Zallcow L H, Gordon M M and Lanir N 1979 Antifeedants from rayless goldenrod and oil of

pennyroyal: Toxic effects for the fall army worm; J. Econ. Entomol. 72 812--815

impact of natural products on the physiology of

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