Pesticides

1
Pesticides

• Insecticides
• organochlorines
• organophosphates
• carbamates
• Herbicides
• triazines
• chloroacetamides
• phosphonates
• phenoxy
• Fungicides

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Pesticides

• 109 kg used in North America annually (all uses)
• 108 kg used in California for agriculture
• The EPA stopped tracking pesticide use in 2001
• 50 used for agriculture (North America)
• 85 used for agriculture worldwide
• insecticide cotton
• herbicide corn and soybeans

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Pesticides
From NIH
4
Pesticides

• Concerns
• exposure of the public to pesticides through food
  (chronic).
• 50 of the food eaten in the US has measurable
  levels of at least 1 pesticide.
• exposure of farm workers to pesticides (acute).
• exposure through home use of pesticides.
• Some toxicologists believe normal exposure to
  pesticides is not harmful.

5
Pesticides

• History
• 1000 years ago Greeks burned sulfur to create
  sulfur dioxide gas to control insects used up
  until the 1800s and sulfur dust is used today as
  a fungicide.
• Romans used arsenic to control insects used
  through WWII.
• Many inorganic compounds (NaF, B(OH)3) have been
  used throughout history.
• Natural oils from petroleum, fish, and whales,
  have also been used.
• Usually the doses needed were enough to be very
  toxic to humans.
• Organic compounds were synthesized for pesticide
  use beginning with WWII.

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Pesticides

• Organochlorines
• stable do not require frequent application
• highly toxic to insects, low toxicity to humans
• persistent in the environment
• bioaccumulate to toxic levels for animals high
  on the food chain

para-dichlorodiphenyltrichloroethane DDT

• hexachlorobenzene
• HCB
• agricultural fungicide
• banned in the US, however it is a byproduct of
  other processes.
• very persistent in the environment

dichlorodiphenyldichloroethane DDE
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Pesticides
Organochlorines

• major replacement for DDT
• highly toxic to fish lakes cannot be restocked
  for years
• relatively volatile transported through air
• currently banned in US but sparsely used in
  other countries
• LD50 50mg/kg

toxaphene

• flame retardant and insecticide
• still used in China and Australia against giant
  termites
• currently banned in US
• LD50 235mg/kg

mirex

• similar structure and effectiveness as DDT and
  DDE
• less persistent due to the reactive methoxy
  groups
• metabolization by organisms rather than
  bioaccumulation
• currently used in the US to control flies and
  mosquitoes
• LD50 5000mg/kg

methoxychlor
8
Pesticides
Organochlorines
endosulfan
From USGS

• Use is permitted since it is not as persistent as
  other organochlorines (sulfur/oxygen bond).
  However its degradation product is somewhat
  persistent.
• Close to being banned in the US.
• Highly toxic and endosulfan sulfate (degradation
  product) is also highly toxic.
• LD50 is 80-160 mg/kg in rats.
• Is blamed for the decline of several fish and
  amphibian species in areas of high use.

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Pesticides

• Organophosphates
• derivatives of phosphoric acid
• decompose within days to weeks after application
• break down to phosphoric acid, alcohols, and
  thiols
• metabolized quickly by animals no
  bioaccumulation
• tend to be more acutely toxic than
  organochlorines
• distributed widely both for agriculture and
  domestic uses
• Three types
• A
• B
• C

• tend to be the most volatile
• break down the fastest

• more stable than type A
• break down into type A
• some insects absorb type B and C better than type
  A
• generally, but not always less toxic than type A

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Pesticides
Organophosphates

• type A
• highly toxic LD50 25mg/kg
• used in fly strips and flea collars due to its
  volatility

dichlorvos

• type B
• extremely toxic LD50 3mg/kg
• non-specific can easily kill birds and fish and
  bees
• may be responsible for more human deaths than any
  other pesticide
• still used in developing countries

parathion

• type C
• slightly toxic LD50 985mg/kg
• widely used agriculturally and domestically
• used to spray communities to reduce mosquitoes
  and other pests

malathion
11
Pesticides

• Carbamates
• derivatives of carbamic acid
• dermal toxicity tends to be lower than for
  organophosphates
• break down within weeks
• decompose to CO2, alcohols, and amines
• highly toxic towards honeybees

carbamic acid
carbamate
12
Pesticides
Carbamates

• extremely toxic LD50 8mg/kg
• one crystal, mistaken for a seed by a bird, will
  kill the bird
• absorbed through plant roots and distributed to
  green vegetation
• effectively banned in the US this year

carbofuran

• somewhat toxic LD50 307mg/kg
• widely used in lawn and gardens
• extremely toxic toward honeybees
• somewhat soluble and tends to contaminate water
  supplies
• production in India caused Union Carbide disaster
  in Bhopal

carbaryl

• extremely toxic LD50 1mg/kg
• may only be handled by professional applicators
• banned for domestic use

aldicarb
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Pesticides
Organophosphates and Carbamates as Neurotoxins

• Both interfere with cholinesterase activity.
• Cholinesterase is an enzyme that breaks down
  acetylcholine.
• Acetylcholine is an important neurotransmitter.
• High levels of acetylcholine cause convulsions,
  irregular heartbeat, and death.
• Most poisoning occurs through acute exposure of
  people applying the insecticides.
• A particular problem in developing nations where
  safety regulations and bans on particularly toxic
  chemicals are not in place.
• Insecticides are also commonly used for
  committing suicide.
• 10,000 40,000 die annually, and millions suffer
  illness from insecticide exposure. (30 die in
  the US and 20,000 require medical care.)

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Pesticides
Triazines

• general formula of a herbicide triazine
• substitute a Cl, and 2 amines at the carbons

• represents 40 of all weed killer applied in the
  US
• used mostly on corn, since corn is resistant
• works by blocking the transformation of CO2 to
  sugars by photosynthesis
• tends to accumulate in water and has been banned
  in some countries
• LD50 2000mg/kg

atrazine
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Pesticides
From USGS
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Pesticides
Chloroacetamides

• derivatives of chloroacetic acid
• replace the OH with a tertiary amine

• usually used with atrazine in intensively planted
  areas
• LD50 1000mg/kg
• commonly found in groundwater near applied areas

alachlor
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Pesticides
Phosphonates

• commonly known as Roundup
• extensively used commercially and domestically
• inhibits the synthesis of essential amino acids
• LD50 5000mg/kg
• some crops have been genetically engineered to
  resist glyphosate
• absorbs to soil better than other herbicides
  (doesnt end up in the water as much)

glyphosate
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Pesticides
Phenoxy

• most widely used herbicide in the world
• kills broadleaf weeds, but not grass
• LD50 640mg/kg
• component of Agent Orange

2,4 D

• good for clearing brush used on roadsides
• LD50 500mg/kg
• component of Agent Orange
• manufacturing process usually introduces
  unacceptable amounts of the dioxin TCDD
• banned in the US

2,4,5-T
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Pesticides
Natural pesticides

• found in chrysanthemum flowers
• unstable in sunlight
• stable derivatives are usually used (pyrethroids)
• common in household insecticides
• extremely toxic to aquatic life
• LD50 1000mg/kg

pyrethrin

• derived from roots of tropical plants
• extremely toxic to fish
• LD50 400mg/kg

rotenone
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Pesticides
Integrated Pest Management

• The use of pesticides has decreased in many
  developed countries over the past decades due to
• chemical control the use of both synthetic and
  natural chemical pesticides.
• biological control reducing pest populations by
  introducing predators, parasites, or pathogens.
• cultural control using farming practices that
  minimize pests.
• host-plant resistance using plants that are
  resistant to pest attack, including genetically
  engineered plants.
• physical control using non-chemical methods to
  reduce pests
• regulatory control preventing the invasion of
  an area by new pests.