Pesticides

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• Pesticides

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Pests and pesticides

• Pest ANY UNWANTED organism that directly or
  indirectly interfere human activity is called a
  pest.
• Pesticides, or biocides, are substances that can
  kill organisms that we consider to be
  undesirable.
• The most widely used types of pesticides
• Insecticides
• Herbicides
• Fungicides
• Rodenticides

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Reasons for our use of pesticides

• Insects compete with us for food It is estimated
  that 30 of agricultural crops are consumed by
  insects worldwide.

Swarms of locusts like this one in Morocco can
completely destroy crops.
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Reasons for our use of pesticides Some insects
are diseases carriers
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The ideal pesticides

• Kill only the target pest
• Have no short- or long- term health effects on
  non-target organisms, including people.
• Be broken down into harmless chemicals in a
  fairly short time.
• Prevent the development of genetic resistance in
  target organisms.
• Saving money compared with making no effort to
  control pest species.

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The case for DDT
Para-dichlorodiphenyltrichloroethane
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DDT an almost ideal pesticide

• Chemically stable and degrades slowly
• Low volatility
• Low solubility in water
• Readily penetrates the waxy coating of insects
• Low toxicity to animals, including human beings.

Each application is effective for a long time
DDT binds to the nerve cells of insects ? hold
open the molecule channels for sodium ions ?
uncontrolled firing of the nerves ? kill the
insects.
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The minus side of DDT Bioaccumulation
Example Accumulation of DDT in the aquatic food
chain
Chemical basis for bioaccumulation DDT is more
soluble in fats than in water.
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DDT and mosquito-lizard-cat connection
Example Accumulation of DDT in the food chain on
land
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The minus side of DDT use
The accumulation of DDT inside birds tissue
disrupts calcium control mechanism during egg
formation. The egg shells are so fragile that
they crack and do not survive until hatching.
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Eagle eggs and DDT
The DDT ban (in 1972) was successful in improving
the survival rate of bald eagles. DDT was
outlawed in Hong Kong and China in 1983.
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Insect resistance to DDT
DDE has a distinctly different shape than DDT and
no longer binds strongly to insert nerve cells.
Most pest species can develop genetic resistance
to a chemical poison through natural selection in
a short time. (Thanks to the rapid reproduction
rate.)
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Example
Suppose an insecticide is applied to a crop to
eradicate fruit flies. Assume that one out one
million fruit flies possesses an enzyme that
breaks down the insecticide into nontoxic
metabolic products. Assume further that the
normal fruit flies die off quickly, the
population of the resistant flies increases
geometrically (that is, 1,2,4,8,). If a new
generation occurs every 23.5 days, in how many
days will the fruit fly population be restored?
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The nature of resistance means that no pesticides
can remain effective for long, and it spurs the
drive to develop new insecticides.
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DDT analogs
Make the new molecule more water-soluble, thus
less persistent in the environment.
Effective component retained
The DDT analogs are not as rapidly attacked by
resistant insects DDT-ase
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Nonpersistent insecticides

• Break down rapidly into harmless and water
  soluble products, once released into the
  environment.
• Two classes of widely used nonpersistent
  insecticides
• Organophosphates
• Carbamates
• Both are neurotoxins

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Organophosphates
General structure
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Neurotransmitter Acetylcholine

• Neurotransmitters are chemicals that transmit
  signals from one nerve fiber to another. Once the
  nerve impulse has been transmitted, the
  neurotransmitter is destroyed.

Acetylcholine
Acetylcholine is a neurotransmitter molecule
responsible for firing motor nerve cells in
higher live forms.
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Normal destruction of actylcholine
Enzyme regenerated. Neuro signal transmission
completed
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Inhibition of acetylchlorine destruction enzyme
by organophosphates
Organophosphate mimics acetylcholine in binding
with the enzyme
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Toxicity difference of parathion and malathion
Parathion and marathion possess similar
insecticidal activity, but malathion has much
lower mammalian toxicity than parathion
(Malathion has a LD50 for adult male rats about
100 times that of parathion.)
The enzyme that accomplish malathion hydrolysis
are possessed by mammals, but not by insects.
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Carbamates
General structure
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Inhibition of acetylchlorine destruction enzyme
by carbamate insecticides
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Broad-spectrum and narrow-spectrum insecticides

• Broad-spectrum insecticides kill a wide range of
  insects, including many that are beneficial.
• Examples organochlorinated hydrocarbons, most
  organophosphates.
• Narrow-spectrum insecticides are toxic to only a
  few types of insects.
• Examples most carbamates.

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The case FOR pesticides

• Pesticides save lives.
• In WWII, at least 7 millions of lives were saved
  because use of DDT and other pesticides kept
  insect-transmitted diseases under control.
• Pesticides increase food supplies and lower food
  cost.
• Pesticides increase profits for farmers
• For every 1 spent on pesticides, 3-5 would
  result from an increase in crop yield.
• They work faster and better than other
  alternatives.
• Pesticides control most pests quickly and at a
  reasonable price.
• Easily shipped and applied.
• Have a relatively long shelf.
• When genetic resistance occurs, pests can still
  be controlled by applying stronger doses and
  switching to other pesticides.

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The case FOR pesticides (Continued)

• The health risks of pesticides are insignificant
  compared with their health and other benefits.
  (see next slide)
• When handled probably, pesticides are safe to
  use.
• Safer and more effective products are continually
  being developed.

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The case AGAINST pesticides

• Development of genetic resistance
• Killing of natural pest enemies
• Bioaccumulation
• Short-term threats to human health from pesticide
  use and manufacture.

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Alternative methods of insect control

• Crop rotation
• Planting a same crop year after year provide
  opportunities for pests to multiply to
  uncontrollable size.
• Intercropping
• Confine insects living space
• Planting rows of hedges or trees in and around
  crop fields
• Act as barriers to insect invasions
• Provide habitats for insects enemies
• Serve as windbreaks to reduce soil erosion

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Alternative methods of insect control (Continued)

• Adjusting planting times
• Select the time window so that insect pests
  either starve to death before the crop is
  available or are consumed by their natural
  enemies.
• Applying insect sex attractants
• Either lure pests to traps containing toxic
  chemicals or attract natural predators into crop
  fields.
• The sex attractants work on only one species,
  requires little amount, do not cause genetic
  resistance.
• Expensive to produce
• Effective only against adults insects, do not
  control juvenile forms.
• Applying insect hormones

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Alternative methods of insect control (Continued)

• Biological control
• Introduce predators
• Genetic engineering
• Sterilization
• Breed and sterilize large numbers of the pest
  insect by radiation, release the sterilized pests
  in the target area, mate but produce no
  off-springs.
• Require the number of sterilized insects to be
  sufficient high.
• The procedure is expensive

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Study questions

• Why does DDT bioaccumulate in the food chain?
  Give an example of DDT bioaccumulation.
• How does DDT kill insects?
• How do organophosphates and carbamates kill
  insects?
• Why can insects easily develop resistance to
  pesticides?