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 specialists

that 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 defense

chemicals) and inducible defenses (e.g.

inhibitors of digestive proteases). The proboscis

of an aphid species may not be able to penetrate

the cuticle and cell wall, or the plant may have

toxic chemicals or induce chemicals that prevent

growth of the aphid.

Control options for the farmer: avoiding the build-

up by cultural control

Phenological asynchrony:

Safe planting date for winter wheat

to 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 farmer

Planting 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, conserve

existing agents by changing cultural practices or pesticides or

augment agents by periodic release. Through success and failure we

Have 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 be

the main reason for the failure of the

reproductive success of birds at the top

of the food chain Many 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 practices

2. Genetics

3. Biological control

4. 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 varieties

2. Healthy or disinfected cuttings

3. Sex pheromones to disrupt mating

4. Beauveria bassiana (entomophagous fungus)

5. Colonization by predatory ants

6. Irrigation management (no soil cracking)

7. Early harvest

8. Crop rotation

9. Destroy crop residues and volunteer plants

10. Nationwide monitoring program (traps)

Beauveria bassiana

Program developed by the Centro Internacional

de la Papa in Peru.

Whiteflies on the leaves

QuickTime™ and a TIFF (Uncompressed) decompressor are 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 µg/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 certain

acreage must be set aside for the non-GM

crop so that the insects will thrive there.

This will reduce the selection pressure and

the occasional mutant that evolves will

find a non-mutant mate. This greatly

delays

the 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 level

or (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 level Pest 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 an

inhibitor of digestive α-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 attacking

larva of Mexican bean beetle.

All the interactions between

herbivores and their predators

and diseases are still poorly

understood

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 function

Three 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 enzyme

essential 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 pests

Integrated pest management (IPM) is the best pest control method

Bacillus thuringiensis (Bt) produces a protein toxin used by farmers

to kill lepidoptera

Bt-crops are genetically engineered with the Bt gene encoding the

Cry toxin protein. Bt crops are highly successful biotech crops.