All parts of the plants are subject to pest damage and the pest complex changes during the season as the plants grow.

Harvest are reduced in many ways: Loss of roots (root worms) Stems break (stem borers) Leaf area (many species) Reproductive structures Seed weevils Phloem sap (aphids)

Pupa of a stem borer

Damage by corn rootworm

Adults and eggs of bruchids(Larvae do the damage)

Leaf miner damage

Insect control: Bt crops are an example of the (so far) successful

application of biotechnology.

Some insects are generalists others are specialists

Whether insects are generalists (e.g.locusts) that eat everything or specialiststhat thrive on only a few species depends on their ability to overcome the plant’s defenses.Plants have evolved physical barriers (thick Cuticles), chemical barriers (specific defensechemicals) and inducible defenses (e.g.inhibitors of digestive proteases). The proboscisof an aphid species may not be able to penetratethe cuticle and cell wall, or the plant may have toxic chemicals or induce chemicals that preventgrowth of the aphid.

Control options for the farmer: avoiding the build-up by cultural control

Phenological asynchrony:Safe planting date for winter wheatto escape damage from Hessian fly

Crop rotation: when there is no host, the pest does not thrive

The other crops may harbor enemies of the pest

Control options for the farmerPlanting insect-resistant crop varieties

Antixenosis. A physical or chemical property of a plant can make it so unpalatable that it is largely protected from herbivore attack. This type of resistance is often known as nonpreference. It may involve the presence of feeding repellents (or the absence of feeding attractants), or it may involve physical traits such as hairs, waxes, or a thick, tough epidermis that do not provide the pest with a desirable feeding substrate. Alfalfa, for example, has been bred with hairy leaves to deter feeding by the spotted alfalfa aphid.

Control options for the farmer: Biological control agents

1. Narrow host range. Generalized predators may be good natural enemies but they don't kill enough pests when other types of prey are also available. 2. Climatic adaptability. Natural enemies must be able to survive the extremes of temperature and humidity that they will encounter in the new habitat. 3. Synchrony with host (prey) life cycle. The predator or parasite should be present when the pest first emerges or appears. 4. High reproductive potential. Good bio-control agents produce large numbers of offspring. Ideally, a parasite completes more than one generation during each generation of the pest. 5. Efficient search ability. In order to survive, effective natural enemies must be able to locate their host or prey even when it is scarce. In general, better search ability results in lower pest population densities.

To improve biological control one can import a new agent, conserveexisting agents by changing cultural practices or pesticides or augment agents by periodic release. Through success and failure weHave determined the characteristics of effective agents

Control options for the farmer: insecticides

Insecticides have evolved from general poisons to specific poisons!As a result, the total amount of pesticide used has leveled off and insecticide use has actually declined (not acreage, but pounds)

Many insecticides are toxic to humans (they affect nerve function),and need to be handled with care. This is not always the case, especially in developing countries.

Bioaccumulation of DDT in the food chain

Eggshell thinning was found to bethe main reason for the failure of the reproductive success of birds at the topof the food chainMany other chemicals bio-accumulate

The emergence of insecticide resistant insects is the result of continued pesticide use and creates the need

for new pesticides.

The goal of insecticides is to kill insects, not to create an ecosystem in which there is an acceptable level of the pest. Pesticide treatment is always followed by resurgence of the pest population.

Evolution of pesticide use 1940 - 2010

Molecular basis of insecticide resistance

A. Mutation that makes the target protein insensitive to the pesticide. Pesticide does not bind to the protein.

B. Mutation in the promoter of a detoxification enzyme to enhance the expression of the gene.

C. Amplification (increase in copy number) of the detoxification enzyme.

R S

Corn rootworm

With so many options, what is the farmer to do?Use integrated pest management (IPM), which requires monitoring of pests and taking action when required.

1. Cultural practices2. Genetics 3. Biological control4. Chemical control

Sticky traps are widely used but you need to be able to distinguish pests from non-pests! Farmers contract with special companies for these services. Traps with mating pheromones attract males of flying insects in a species-specific way. Monitoring has to be done throughout the season. Results can vary enor-mously from one year to the next.

Example: IPM of the sweet potato whitefly (Bemisia tabaci) in Cuba (infects many plants and spreads tomato yellow leaf curl virus)

Steps in the IPM program:1. Plant only early tolerant, or resistant varieties2. Healthy or disinfected cuttings3. Sex pheromones to disrupt mating4. Beauveria bassiana (entomophagous fungus)5. Colonization by predatory ants6. Irrigation management (no soil cracking)7. Early harvest8. Crop rotation9. Destroy crop residues and volunteer plants10. Nationwide monitoring program (traps)

Beauveria bassiana

Program developed by the Centro Internacionalde la Papa in Peru.

Whiteflies on the leaves

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

sporulation induces synthesis of crystals

pore-forming domain

receptor binding domains

crystals are made up of pore-forming toxins

Bacillus thuringiensis is a ubiquitous soil bacterium that produces proteins that kill

insects and nematodes.

The proteins are called “Cry”, because they occur in crystals

Physiological mechanism of Cry toxin action

Lepidoptera

Coleoptera

Diptera

Cry Toxin Specificity

Crickmore et al. 1998

Nematoda

Bacillus thuringiensis (Bt) spores can be formulated as dusts or sprays and have been used for years as

“natural” insecticides. About 100 different Cry proteins have been identified and all have some specificity.

Spores of Bt are dusted on vegetables by home gardeners and organic producers or sprayed (1000 spores per sq meter) to kill larvae of lepidoptera (e.g. corn ear worm)

Genetically engineered Bt cotton and Bt corn have been a

commercial success

Advantages of transgenic approach:

Reduction in insecticide sprays (labor and chemical costs). Increased activity of natural enemies. Biological control can be used on secondary pests.

Limitations of transgenic approach

You still need to control the “secondary” pests Cost of transgenics

Development of resistance because of persistent exposure.

A side benefit of Bt corn: reduction in mycotoxins

Infection of corn (and other seeds) by Fusarium is more common when corn is damaged by insects. Fusarium produces fumonisin, a potent mycotoxin. The FDA “Guidance for Industry” for fumonisin levels of 2 to 4 µg/g in human food and animal feeds, but higher levels are normally found in corn produced in some parts of the country or some years (5 to 10 g/g). The histogram below compares fumonisin levels in control (green) and Bt (blue) corn. Bt 176 does not make Bt protein and is a control.

Example of the economic benefit of Bt cotton to small farmers in South Africa.

The total worldwide cost of insect control is $ 8 B. About $ 2.6 B could be substituted by Bt crops.

Resistance to Bt sprays has already emerged for some insects. So, management of Bt crops is needed to avoid

the emergence of Bt resistant pests.

Two farmers stand among rows of non-Bt corn, which has suffered insect damage. To either side are rows of Bt corn, for which insect damage is greatly reduced. Carrie Daniel, Novartis Seeds

“Management” means that a certainacreage must be set aside for the non-GMcrop so that the insects will thrive there.This will reduce the selection pressure andthe occasional mutant that evolves will find a non-mutant mate. This greatly delaysthe emergence of resistance.

Does Bt corn endanger the monarch butterfly as alleged by many “Green” organizations?

Pollen shed by the Bt corn falls on milkweed that grows in and around corn fields. Monarch larvae feed exclusively on milkweed leaves. Cornell university

researchers showed that high levels of pollen on the leaves can kill the larvae. But are such high levels a frequent occurrence in nature?

A Risk Assessment...

Acute toxic effects of pollen

Probability of larvae being exposed to toxic levels in and around corn fields

Hazard =

Exposure =

Risk = “This two year study suggests that the impact of Bt corn pollen from current commercial hybrids on Monarch Butterfly populations is negligible.”

From Sears et al., 2001

X

Pests have short life cycles and their populations can build up during the season and over the years.

Populations fluctuate depending on conditions (food, enemies, weather)

Pest control options: (1) prevent the buildup, (2) decrease the levelor (3) delay the buildup beyond the point where it damages the crop.

Typical buildup and decline of a pest

population during the growing season

When is a herbivore a pest? Economic injury levelPest populations change over time and not every population will reach the economic injury level (EIL). The farmer has to balance the cost of control with the loss of revenue. When cosmetic appearance is important (fruit, sweet corn, canned/frozen vegetables) even a little damage (blemishes,

presence of insect larvae) may cause economic loss. Pest control has to be initiated before that level is reached at the economic threshold (ET).

A different approach uses inhibitors of digestive enzymes

Peas on the left have been transformed with the gene from bean that encodes aninhibitor of digestive -amylase. This inhibitory protein is normally present in bean seeds and prevents the seeds from being eaten by the larvae of certain species of bruchids, when the gene is transferred to peas and expressed in the pea seeds, the seeds are now resistant to those species of bruchids whose digestive amylase is inhibited by this inhibitor (amylases are needed to digest starch).

Pyrethrum is a non-synthetic

insecticide (botanical)

Pyrethrum is extracted from the flowers of the chrysanthemum grown in Kenya and Ecuador. It is one of the oldest and safest insecticides available. The ground, dried flowers were used in the early 19th century as the original louse powder to control body lice in the Napoleonic Wars. Pyrethrum acts on insects with phenomenal speed causing immediate paralysis, thus its popularity in fast knockdown household aerosols. However, unless it is formulated with one of the synergists, most of the paralyzed insects recover to once again become pests. Pyrethrum is a mixture of four compounds: pyrethrins I and II and cinerins I and II.

What happens to all those insecticides?Only a minor proportion falls on the plant

A study from Cornell University showed that pollen from Bt corn, when dusted on milkweed leaves killed the

monarch larvae. But how realistic are the conditions?

Plants, herbivores and their enemies all evolve together (co-evolution). Plants evolve defenses, but herbivores evolve to

overcome them. Predators evolve to live off the herbivores, but the herbivores evolve defenses.

Spined soldier bug attackinglarva of Mexican bean beetle.All the interactions between herbivores and their predatorsand diseases are still poorlyunderstood

Azadirachtin, an allelochemical from the Neem tree that is an anti-feedant and can be used as a spray. Plants contain tens of thousands of chemicals, most of which have not been identified let alone studied. Their role is in plant-plant or plant-herbivore interaction is poorly understood.

Many insecticides affect nerve functionThree major classes of insecticides:

DDT and other organochlorines) are now banned because of bio-accumulation and effects on mammals. Mode of action of DDT was not clearly established but it interferes with nerve function (not used in the US).

Organophosphates discovered by research on nerve gasses. Inhibit acetyl-choline esterase an enzymeessential for nerve function. Inhibition causes accu-mulation of acetyl-choline at the nerve synapses resulting in muscle twitching.

Pyrethrum is a naturally occurring insecticide, but it is unstable after isolation. Chemists have made a series of synthetic pyrethrins. They also block nerve function (keep Na+ channels in the open position).

ipmworld.umn.edu/chapters/ware.htm

Insects and insect control

Pests attack all parts of the plants; some are generalists, others specialists.Plants defend themselves in several ways, but co-evolution means that someone always gets eaten.The goal of pest control should be to have a stable manageable pest population that causes no economic hardship. The farmer has many pest control options: breeding, cultural methods, chemicals (pesticides, natural or synthetic), biological control.Selection pressure results in the emergence of pesticide-resistant pestsIntegrated pest management (IPM) is the best pest control methodBacillus thuringiensis (Bt) produces a protein toxin used by farmers to kill lepidopteraBt-crops are genetically engineered with the Bt gene encoding the Cry toxin protein. Bt crops are highly successful biotech crops.