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Crop Pests and Control
Chapter 13
Factors that can limit crop yield: 1. Insect Pests 2. Disease
3. Weeds
4. Non-insect invertebrate or vertebrate pests
Problems That Have Occurred Due To The Use of Pesticides:
Integrated Pest Management (IPM): Makes use of many pest control methods
in order to preserve the environment and prevent pest resistance.
Goals of IPM
Basic Principles of IPM: Exclusion –
Suppression –
Eradication –
Plant resistance –
Steps of IPM: Identify pests Take preventative measures Crop scouts Use economic thresholds
IPM Involves The Use Of: 1. Natural controls
Biological control – Cultural control Mechanical control - Sanitation Chemical control
Successful IPM makes use of the following practices
Beneficial Insects: Insects that benefit crops by pollinating or
feeding on crop pests.
Keys to successful IPM Programs: Scouting Fields –
weeds should be scouted within 2 weeks of crop emergence
avoid boarder rows scout at least once a week. Samples should be representative of field
Economic Thresholds (ET)-
Damage Thresholds –
Used on pathogens – bacteria, fungi etc. Economic Injury Level (EIL) –
Insect Pests Crop Losses: Crop losses due to insects and mites range
from 5-15% annually = $4 billion in losses. Annual losses for:
Corn 12% Sorghum 9% Wheat 6% Soybean 3%
Damage Caused by Insects: 1. Defoliation – 2. Necrotic spots – 3. Boring or tunneling- 4. Lower quality – leave frass (excrement
and exoskeletons) 5. Higher harvest losses 6. More susceptible to virus’ – from open
wounds from sucking insects. 7. Increase erosion due to less dry matter
Insect Orders: Coleoptera - Diptera – Hymenoptera – Orthoptera – Lepidoptera – Acarina –
Insect Life Cycles: Metamorphosis – change in form from
egg to adult insect. 2 Types of Metamorphosis: Gradual - egg nymph adult Complete - egg larva pupa adult
Ametamorphic – without metamorphosis Hatches as a miniature adult
Two Major Types of Mouthparts: Chewing - jaws used to tear and grind
food i.e. Grasshoppers, crickets, honeybees
Sucking Mouthparts: used to obtain liquid foods.
Damage Caused By Chewing Insects:
Defoliators –
Borers –
Leaf miners -
Root Feeders -
Orders With Chewing Mouthparts: Orthoptera – Odanata – Neuroptera – Coleoptera – Hymenoptera –
Damage Caused by Sucking Insects: Distorted Plant growth:
Stippling Effect on Leaves:
Burning on leaves:
Pest Control Program: Made up of 3 basic principles:
1. 2. 3.
Methods of Insect Control: Genetic – using resistant or tolerant varieties Cultural - crop rotation, tillage methods,
resistant or tolerant varieties Mechanical – screens, traps, light and sound Biological – parasites, predators, diseases, male
sterility, temperature, moisture, pheromone traps
Chemicals – kill, repel, attract, sterilize Integrated control – using both artificial or
natural bio-control and chemicals.
Genetic Control: Non-preference control
Plant is not palatable to the pest – taste, smell, color, or texture is altered
Antibiosis Crop plant has harmful effect on the growth or
reproduction of the pest Tolerance
Plant doesn’t suffer economic damage even though it is infested
Chemical Control: About 75% of all pesticides are applied to
agricultural lands. Should only be used as last resort Classified by mode of action:
Contact – used for sucking insects Stomach – used for chewing insects Systemic – absorbed by plant and then
ingested during feeding.
3 Broad Classes of Insecticides: 1. Chlorinated hydrocarbons – DDT,
chlordane, lindnae Do not degrade easily Build up in fat tissues of animals and are passed
along the food chain Has led to a ban or restriction of most chemicals in
this category.
2. Organic phosphates – Ethyl and Methyl parathion and malathion Can be extremely toxic to mammals
3. Carbamate – Lower effect on mammals
Crop Losses From Diseases: 10-20% of production annually Diseases reduce yields:
Wheat & Soybean 14% annually Corn 11% Sorghum 9% Cotton 15% Potato 20-25% annually
Definitions: Symptoms - outward appearance of a plant that
are identifiable with a particular disease. Pathogen – causal organism, infectious agent Host – the plant infected with the pathogen Parasite – feeds off the host organism it invades.
Obligate – can survive only on the living tissue of the host Non-obligate - can survive on dead organic matter or
inorganic materials as saprophytes
Disease affects yield by: 1. Increasing respiration rate 2. Rob plant of: water, nutrients,
photosynthate. 3. Block translocation of nutrients 4. Replace the grain – ergot, smut 5. Increase plant respiration rate 6. Block photosynthesis 7. Affect crop quality too.
Classification of Diseases: 1. Infectious – caused by a living organism
that invades the host disrupting normal growth
2. Noninfectious – crop injury that involves no pathogen
Weather Nutrient deficiency Chemical injury
Pathogens Causing Disease: 1. Fungi-
Live in plant tissue, soil, weeds etc. Parasitic or saprophytic
2. Bacteria Parasitic or saprophytic Enter plants through openings or wounds Symptoms are: galls, blights, leaf spots, rots
3. Virus – Submicroscopic pathogenic particles Can not grow and multiply without a host Parasites Cause stunting and death; dwarf mosaic virus or dwarf potato virus
4. Nematode Microscopic worms that attach to roots, stems, leaves and flowers of
plants Live in soil or water
Fungi: about 100,000 fungal species have been
identified, about 8,000 of them are parasitic to plants. Have been responsible for a number of crop disasters that have lead to famines.
Classified mainly by what they feed on:
Obligate saprophytes – feed on dead plant and animal tissue
Obligate parasites – live only on living plants
Facultative saprophytes – can live on dead or living tissue
Facultative parasites – normally saprophytic but occasionallyparasitic.
How they work: Spread by – spores. Spores spread by: wind, water, insects, and other
agents. Enter plants through wounds, natural openings,
or by direct penetration of the epidermis. Upon germination spores produce hyphae that
grow and branch to produce other structures called mycelia or fungal bodies.
They attack – flowers, seeds, leaves, stems, and roots
Symptoms of Fungal Diseases: a. Necrosis – leaf spots, blight, die back,
canker, anthracnose, damping off, scab. b. Symptoms involving growth – galls leaf
curl hypertrophy or hyperplasia. Other symptoms – wilt, rust and mildew.
Control of Fungi:1. Chemicals – fungicides2. Treat seed3. Resistant cultivars4. Prevention – clean equipment
Bacteria and Mycoplasmas
1. Bacteria are classified by: *Form (or shape) spherical (cocci), rod-shaped
(bacilli), spiral (spirilli), and filament (filamentous)
*Reaction to Gram’s stain – either Gram+ (stain violet) or Gram – (stain pink-red)
2. Mycoplasmas are wall-less microbes that occur in phloem tissue.
-Have DNA and RNA but cannot reproduce themselves.
-Carried by vectors – such as grasshoppers, and aphids
-Treatment – spray insects
Bacteria and Mycoplasmas Occur where it is moist and warm Spread by: wind, splashing from
irrigation or rain, infected seed Enter host through – stomata or
wounds Symptoms – soft rot, bacterial cankers,
wilts, overgrowth, scabs and rots.
Control of Bacterial Diseases: *High temperature *Prevention *Bactericide *Resistant varieties * There are many beneficial
bacteria that are not pathogenic (Bradyrhizobium (fixes Nitrogen for legumes), Agrobacterium (used in biotechnology) etc.
Viruses – Need a host tissue to stay alive (obligate
parasite). Not true animals – cannot digest or respire. Consist of a core nucleic acid that may be RNA
or DNA encased in a protein or lipoprotein coat (capsid).
How viruses work Enter plants through a mechanical wound
or a vector (carrier). Viruses get into the phloem of the plant and move towards the apical meristem.
What are the vectors ? -Sucking and chewing insects (aphids and
thrips)- Seed transmitted virus might be in the
ovule of infected plant- Nematodes - Mites- Fungi
Symptoms of Viral Diseases:
Mosaics (light-green, yellow or white patches mingled with normal green)
Stunted growth Some are asymptomatic (show no
symptoms)
Classified mainly according to host, because they are host specific
Tobacco mosaic virus (TMV) Maize dwarf mosaic virus
(MDMV)
Control of plant viruses Prevention Resistance cultivars Heat therapy – high heat
for 2 to 4 weeks (100+ temps).
Requirements for Disease to Occur: 1. Pathogen –
Level of infestation
2. Host- Must be susceptible to invasion, establishment and
development
3. Favorable environment
Interact make the Disease Complex Severity of disease is decided by the level
of interaction of all three.
Epidemic Or Endemic? Epicemic Disease:
One that occurs frequently over a wide crop area but only periodically.
1971 Race T of Southern Corn Leaf Blight causing 14% yield loss.
Endemic Disease: Usually present in a crop area from year to
year causing moderate to severe damage. Corn stalk and root rots are examples
Stages of Disease Development: Page 177 – Disease Cycle: Stage:
1. Inoculation – pathogen enters host Insects, Wounds
2. Incubation – Pathogen becomes established in a plant Begins upon inoculation, ends when the plant reacts
to the disease 3. Infection -
Disease symptoms appear, producer notices Economic crop losses occur in this stage
Economic Impact Depends on: Monocyclic: disease goes through one life
cycle
Polycyclic: disease goes through several life cycles.
Disease Control: 1. Genetic
Avoidance Resistance Tolerance
2. Cultural Change in field operations that will alter life cycle of the
disease Crop rotation Planting time
3. Chemical- Seed treatment
Noninfectious Disease:1. Weather Damage:
FrostDroughtSun scaldHail damage
2. Soil Problems:Low or high pHNutrient deficiencyAccumulation of salts.
3. Chemical Damage:Over application of pesticidesCarry over from previous crop
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