Pesticides, Herbicides, Toxic organics, and Toxic metals

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Pesticides, Herbicides, Toxic organics, and Toxic
metals
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Reasons for Pesticide Use

• Insects, rodents, etc. are disease carriers.
• Insects compete with us for food It is estimated
  that 30 of agricultural crops are consumed by
  insects worldwide.
• In many situations it is NOT economical to use
  pesticides!

Swarms of locusts like this one in Morocco can
completely destroy crops.
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Bioaccumulation

• Bioaccumulation
• increase in concentration of a pollutant from the
  environment to the first organism in a food chain
  
• Biomagnification
• increase in concentration of a pollutant from one
  link in a food chain to another
• Pollutant characteristics
• long lived
• mobile
• soluble
• biologically active

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Persistent insecticides polychlorinated organics
some DDT info http//www.cs.stedwards.edu/chem/Ch
emistry/CHEM29/CHEM29/ddt/ddt.html
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Persistent insecticides polychlorinated organics
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DDT why was it used?

• 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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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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Pesticide causes and effects
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Bird populations decimated
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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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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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 life forms.
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Normal destruction of acetylcholine
Enzyme regenerated. Neuron signal transmission
completed
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Inhibition of acetylcholinesterase by
organophosphates
Organophosphate mimics acetylcholine in binding
with the enzyme
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Toxicity difference of parathion and malathion
Parathion and malathion 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 malathion hydrolysis enzyme is possessed by
mammals, but not by insects.
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Carbamates
General structure
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Inhibition of acetylcholinesterase 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 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 (often disasterous
  consequences! Ex cane toad in Australia
• 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