Pesticides

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Pesticides
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What are Pesticides?

• Substances used to control
• 1. Organisms which may adversely affect public
  health
• 2. Organisms which attack food and other
  materials essential to humanity

• Such organisms include
• vermin (e.g. rats, mice)
• insects and similar invertebrates
• nematode worms (e.g. hookworm, filaria)
• green plants (i.e. weeds)
• fungi (e.g. mildews)

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Chemical Pest and Disease Control in Crops -
Beginnings

• In 19th cent. introduction of
• Use of lime sulfur, sulfur dusting and Bordeaux
  mixture to control vine powdery mildew arsenates
  to control Colorado beetle and nicotine,
  pyrethrum and tar oil insecticides
• Late 19th and early 20th centuries characterized
  by
• increasing awareness of the possibilities of
  avoiding losses from pests
• the rise of firms specializing in pesticide
  manufacture
• the development of better application machinery.

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1942-62 Pesticides - the Panacea (1)

• Insecticidal properties of DDT discovered in 1942
• This followed by discovery of other chlorinated
  insecticides in late forties/early fifties
• aldrin, chlordane, dieldrin, endrin, heptachlor,
  methoxychlor, toxaphene

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1942-62 Pesticides - the Panacea (2)

• During WWII, discovery of organophosphorus
  compounds
• very powerful insecticides and acaricides, e.g.
  parathion
• low in cost, but toxic to humans and other
  mammals
• act as AChase inhibitors

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1942-62 Pesticides - the Panacea (3)

• By the application of these new compounds great
  benefits accrued
• crop losses were cut sharply
• locust attack was reduced to a manageable problem
• by killing the carriers of human disease,
  millions of lives were saved
• first major use of DDT in suppression of typhus
  epidemic in Naples, 1943-44
• anti-malaria campaigns

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Rise of Problems in using Pesticides

• In the early 1950s more resistant strains of
  pests arose
• in treating cotton crops standard doses of DDT,
  parathion, etc., had to be doubled or trebled
• The compounds destroyed natural predators and
  helpful parasites
• population explosions of previously harmless
  insects
• Presence of residues in food, Man and wild life
• biomagnification in carnivores
• Rachel Carsons Silent Spring (1962)

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Biomagnification Concentrations of DDT in
Freshwater Animals
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Indices of Bioaccumulation

• Bioconcentration factor (BCF)
• Ratio of tissue chemical residue to chemical
  concentration in water. (Normally measured at
  steady state where there is no food-chain
  exposure.)
• Bioaccumulation factor (BAF)
• Ratio of tissue chemical residue to chemical
  concentration in an external environmental phase
  (i.e. water, sediment, or food).
• Biomagnification
• The increase in tissue chemical residues at
  higher trophic levels, primarily as a result of
  dietary accumulation

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Creation of New Pests - Example 1 Canete
Valley, Peru

• Heliothis worms, previously no problem,
  multiplied
• Wasps keeping both pests in check were poisoned
  by the increasing pesticide doses
• By 1955, cotton yields were down to 330
  kg/.hectare

• DDT introduced in 1949
• cotton yields 500 kg/hectare
• By 1952, yield 750 kg/hectare
• but DDT-resistant boll weevils had appeared
• Toxaphene replaced DDT
• within 2 years ineffective against boll weevils

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Creation of New Pests - Example 2 Borneo

• In early 1960s, DDT sprayed on thatched roofs and
  vegetation round villages to kill mosquitoes and
  control malaria
• DDT killed flies and mosquitoes, but poisoned
  insects eaten by gecko lizards that inhabited
  houses
• Geckos accumulated so much DDT that they died

• Moths, previously controlled by lizards, ate palm
  thatch and caused roofs to collapse
• Village cats ate dead geckos and were themselves
  poisoned
• Correspondingly, rats multiplied, causing an
  epidemic of bubonic plague

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Integrated Approach to Pest Control

• Application of all methods of control in place of
  reliance on chemicals alone
• Biological control
• Chemical control
• Replacement of persistent pesticides with less
  persistent ones, such as malathion

• (Several countries have partly or wholly banned
  use of DDT)
• Use of systemic fungicides
• Improvements in spraying techniques
• ultra-low-volume technology results in marked
  reduction in dose/unit area

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Major Types of Pesticide

• Rodenticides (acting on rats, mice and other
  rodents) - e.g. warfarin and superwarfarin
  acutely lethal toxicants fumigants
• Insecticides e.g. organophosphates carbamates
  pyrethroids organochlorine compounds
• Herbicides e.g. chlorophenoxy compounds
  bipyridyls
• Fungicides e.g. organomercurials
  dithiocarbamates Bordeaux mixture sulfur

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Structural Classification of Organochlorine
Insecticides
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Toxicological Properties of Organochlorine
Insecticides (I)

• Low volatility, chemically stable, lipid soluble,
  slow rate of biotransformation and degradation
• Interact with nerve cells in insects causing
  hyperexcitability and death after a short
  exposure
• No equivalent effects in humans or other mammals
  at similar doses
• Lipophilic - therefore easily absorbed by living
  organisms from water or food and stored in fatty
  tissue may be released into milk during
  lactation or into blood following fasting

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Toxicological Properties of Organochlorine
Insecticides (II)

• Properties result in persistence in the
  environment, bioconcentration and
  biomagnification in various food chains plants -
  herbivores - predators
• Residues accumulate in humans and other organisms
  to levels well above those in the environment
• These residues may reach toxic levels in the body
  even though their environmental concentration and
  intrinsic toxicity are low

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Ecological Effects

• Organochlorine compounds have been shown to
  exhibit potent estrogenic and enzyme-inducing
  properties both in wildlife and laboratory
  animals which interfere with fertility and
  reproduction
• In avian species they interfere with steroid
  metabolism - inhibit mobilisation of calcium to
  produce strong eggshells
• Risk-benefit analysis?

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Organophosphorus Compounds and Carbamates

• Organophosphorus compounds include malathion,
  diazinon and parathion
• Carbamates include carbaryl, propoxur, and
  aldicarb
• Both inhibit the enzyme acetylcholinesterase,
  essential for nerve signal transmission, by
  forming covalent derivatives
• The inhibitory derivative formed with carbamates
  is slowly reversible
• Both are lethal to humans after relatively small
  exposure
• Both hydrolyse relatively easily - thus do not
  accumulate in environment, in exposed organisms,
  or in food organisms

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Bipyridyl Herbicides - Chemical Structures
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Bipyridyl Herbicides - Toxicity

• Non-selective contact herbicides
• kill plants by interfering with photosynthesis
• Bind very strongly to soil
• no leaching after spraying
• have little or no environmental effect in ground

• Paraquat in particular very toxic to humans
  (diquat less so)
• target organ lung - selectively accumulates
  paraquat (active transport)
• causes progresssive fibrosis of lungs and damages
  kidneys
• no antidote

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Proposed Mechanism for Paraquat Toxicity
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Chlorphenoxy Herbicides

• Broad spectrum herbicide introduced in 1946
• Mimic auxins (growth stimulators) in plant
• cause abnormal growth leading to death
• Acute oral LD50 values range from 300 to gt1000
  mg/kg in different animal species, including
  humans
• In accidental intoxications, patients report
  intestinal irritation, aching muscles and
  weakness.

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TCDD

• 2,4,5-T found to cause cleft palate and renal
  malformations in rats
• Effects now known to be produced not by 2,4,5-T
  but by a by-product contaminant TCDD
• known teratogen and suspected carcinogen
• can be formed during production

• Agent Orange
• n-butyl esters of 2,4-D and 2,4,5-T (5050
  mixture)
• used as defoliant during Vietnam War
• contained 47 ?g TCDD/g
• many claims of adverse health effects among
  veterans
• epidemiological studies on those handling this
  material inconclusive

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Structure of TCDD (2,3,7,8-Tetrachlorodibenzo-p-d
ioxin)
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POPs and PICs

• Persistent Organic Pollutants (POPs)
• UNEP (1997) proposed international action to
  reduce or eliminate production and discharges of
  initially 12 compounds in this category
• programme to identify alternatives and sources
  undertaken also inventories

• Prior Informed Consent (PIC) Procedure for
  certain hazardous chemicals in international
  trade
• Voluntary/compulsory procedure between states to
  help control imports of unwanted chemicals
• administered by Joint Secretariat of FAO/UNEP

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Self-Assessment Questions (1)

• What is meant by the term pesticide?
• What type of compound chemically is DDT?
• What is the mechanism of action of
  organophosphorus insecticides?
• What were the advantageous applications of the
  early use of the new insecticides?
• What problems arose in their use in the 1950s?
• What is meant by biomagnification?
• What is the bioconcentration factor for DDT in
  freshwater plankton? Why might it be so high?

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Self-Assessment Questions (2)

• Give examples of a currently used rodenticide,
  insecticide, herbicide and fungicide.
• What is the target organ for the toxicity of DDT?
• Which type of tissue is likely to have the
  highest concentration of DDT?
• What are the adverse effects on wildlife of the
  use of DDT?
• What is the likely target organ of
  organophosphates?
• Organophosphates and carbamates are much more
  toxic to humans than DDT. Why are they preferred
  as insecticides?

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Self-Assessment Questions (3)

• What are the advantages of bipyridyl herbicides?
• What is the target organ in the human for the
  toxicity of paraquat? By what toxic mechanism
  does it act?
• What is the mechanism of action of the
  chlorphenoxy herbicides?
• What is the significance of the acronyms POP and
  PIC with regard to toxic substances?