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

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All parts of the plants are subject to pest
damage and the pest complex changes during the
season as the plants grow.
Insect control Bt crops are an example of the
(so far) successful application of biotechnology.
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
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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 plants 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.
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Control options for the farmer avoiding the
build-up by cultural control
Crop rotation when there is no host, the pest
does not thrive
Phenological asynchrony Safe planting date for
winter wheat to escape damage from Hessian fly
The other crops may harbor enemies of the pest
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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.
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Control options for the farmer Biological
control agents
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
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.
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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.
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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
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The emergence of insecticide resistant insects is
the result of continued pesticide use and creates
the need for new pesticides.
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Evolution of pesticide use 1940 - 2010
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.
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Molecular basis of insecticide resistance
R S

• 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.

Corn rootworm
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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.
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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)
Whiteflies on the leaves
Program developed by the Centro Internacional de
la Papa in Peru.
Beauveria bassiana
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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
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Physiological mechanism of Cry toxin action
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Cry Toxin Specificity
Crickmore et al. 1998
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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)
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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.
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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 mg/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.
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Example of the economic benefit of Bt cotton to
small farmers in South Africa.
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The total worldwide cost of insect control is 8
B. About 2.6 B could be substituted by Bt crops.
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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.
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.
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
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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?
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A Risk Assessment...

• Acute toxic effects of pollen
• Probability of larvae being exposed to toxic
  levels in and around corn fields

Hazard Exposure Risk
X
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
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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)
Typical buildup and decline of a pest population
during the growing season
Pest control options (1) prevent the buildup,
(2) decrease the level or (3) delay the buildup
beyond the point where it damages the crop.
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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).
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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 ?-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).
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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.
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What happens to all those insecticides?Only a
minor proportion falls on the plant
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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?
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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.
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.
Spined soldier bug attacking larva of Mexican
bean beetle. All the interactions between
herbivores and their predators and diseases are
still poorly understood
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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
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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.