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INSECTICIDES

ADVANTAGES AND DISADVANTAGES

Advantages

Compared to other forms of control, insecticide use is highly effective, easily employed by farmers and in many cases there is no commercially viable alternative.

Disadvantages

  1. Resurgence of treated populations
    Pest populations quickly recover and bounce back, leading to repeated insecticide applications.
  2. Resistance. Large reproductive ability and short generation time help speed selection of resistant individuals and insecticides are than applied at ever increasing concentrations.
  3. Selective kill and environments alteration can lead to minor pests becoming major pests. Two-spotted mite problems on apples after DDT application.
  4. Residues can be long lived and dangerous.
  5. Insecticides and their applications can be costly and time consuming.

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The Perfect insecticide & ld 50

Properties of perfect insecticide

  1. High toxicity to target pest
  2. Selective toxicity so beneficial insects are not affected
  3. Low toxicity to plants other non target organisms
  4. No harmful residue
  5. Cheap and safe to manufacture
  6. Stabile under storage
  7. Non corrosive
  8. Residues readily and cheaply detectable

Measurement of toxicity

LD 50 means lethal dose required to kill 50% of the target animals.

LD 50 is stated with method of application (e.g. oral or dermal) and the animal concerned (e.g. fly, rat, human).

LD 50 is measured in mg of insecticide/kg of target animal or mg/g. Therefore a relatively low LD 50 means a high toxicity.

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INSECTICIDE CLASSIFICATION

Insecticides can be classified by -

  1. Stage or type of arthropod that the insecticide kills e.g. ovicides, larvicides, miticides.
  2. Route of entry
    1. Stomach poisons, including systemics - insecticide is ingested with food
    2. Contact poisons - enter through cuticle
    3. Fumigants - gases and dusts - entry spiracles and tracheae.
  3. Chemical nature of compound
    1. Inorganics - arsenals, fluorides
    2. Natural organics
      1. oils - used against scale insects and aphids
      2. botanical extracts e.g rotenone, nicotine, pyrethrin
    3. Synthetic organics
      e.g. chlorinated hydrocarbons, organophosphates, carbamates, pyrethroids
  4. Mode of action
    1. Physical poisons - dusts, fumigants, oils - asphyxiate insects
    2. Protoplasmic poisons - inorganics that destroy cells
    3. Metabolic inhibitors - block pathways e.g. rotenone, synergists (q.v.)
    4. Neuroactive agents - affect the transmission of nerve impulses across synapses
    5. Insect growth inhibitors e.g. juvenile hormone which inhibits moulting

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INSECTICIDE METABOLISM

Insecticides are generally lipophilic ('fat-loving') but excretory systems deal with hydrophilic ('water-loving') compounds.

Therefore elimination is achieved by

  1. Modifying the insecticide (usually a detoxification step)
  2. Combing the modified insecticide compound with another compound to make it hydrophilic so that it can be excreted. This is usually done in the liver of vertebrates and the fat body of insects.

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SYNERGISM

Synergism occurs when the resultant toxicity of a mixture of chemicals is greater than the additive total of the individual chemicals. i.e:

A ------------------------> 50% kill

B ------------------------> 0% kill

A + B -------------------> 90% kill

The synergist (B) is not toxic but it blocks the action of enzymes which can detoxify insecticides, thereby enhancing the effectiveness of the original insecticide.

Synergists (e.g. sesame oil, piperonyl butoxide), can be used to

  1. Reduce the necessary dose of insecticides
  2. Reduce the degree of resistance in resistant insects
  3. Permit the use of previously ineffecive insecticides

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NATURAL ORGANIC INSECTICIDES

  1. Botanical insecticides
    e.g. pyrethrins, rotenone, nicotine, ryania,
    • Extracted from plants
    • Broad spectrum pesticides
    • Low environmental persistence
  2. Antibiotic insecticides - abamectin, ivermectin, spinosad
    • Produced by fermentation from actinomycetes or streptomycetes (spinosad)
    • Usually the activity is selective to individual groups
    • Abamectin is an acaricide and also shows activity against some thrips
    • Ivermectin is mainly of veterinary use
    • Spinosad shows activity against certain Lepidoptera larvae and thrips
  3. Mineral oils
    e.g. Horticultural Mineral Oils
    • Highly refined, narrow distillation range oils
    • Phytotoxicity problems mainly due to sunlight oxidising unsaturated hydrocarbons
    • Show direct toxicity by asphyxiating small insects
    • Also behavioural effects in reducing oviposition by some Lepidoptera
    • In most cases there is a relatively narrow window when the target is susceptible
    • The volume of spray used is critical as good coverage is essential
    • No reported cases of resistance developing in target pest species
    • Relatively non-toxic to predators and parasites

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SYNTHETIC ORGANIC INSECTICIDES

  1. Chlorinated hydrocarbons
    e.g. DDT, lindane, dieldrin, chlordane, heptachlor
    • Wide spectrum action on many insect pests
    • Simple structure - easy to manufacture
    • Low mammalian toxicity
    • Prolonged stability and residual action

      The discoverer of the insecticidal use of DDT was awarded the Nobel Prize in 1937 and DDT has been calculated to be the substance responsible for saving the greatest number of human lives due to is successful use against insect borne diseases such as typhus, malaria and dengue fever.

      However, like other chlorinated hydrocarbons, its lipophilic properties and long residual life has meant that it has accumulated in the fat tissues of animals at the top of food chains. It has also been found to affect the laying down of calcium in the egg shells of birds, leading to their breakage, particularly in birds of prey.

  2. Carbamates
    e.g. Aldicarb, methomyl, carbaryl, pirimicarb, bendiocarb
    • Some show selective toxicity to individual groups
    • Lepidoptera larvae (carbaryl), aphids (pirimicarb)
    • Some are fairly safe for mammals (carbaryl, pirimicarb), others very toxic (aldicarb)
  3. Organophosphates
    e.g. Malathion, parathion, dichlorvos, metasystox
    • Less persistent in environment
    • Very toxic in small doses
    • High mammalian and bird toxicity
    • Selective toxicity
    • Contact and some systemic effect
  4. Pyrethroids
    e.g. Bioresmethrin, phenothrin, allethrin, permethrin, deltamethrin, cypermethrin
    • Quick "knock-down"
    • Active at low rates per hectare
    • Some have low environmental persistence - bioresmethrin, phenothrin allethrin
    • Others are photostable and persistent - permethrin, deltamethrin, cypermethrin
    • Low mammalian toxicity
    • Do not accumulate in food chains but are harmful to parasites and predators.
  5. Insect Growth Regulators
    Two groups:
    1. juvenile hormone mimics (JHM)- methoprene, fenoxycarb, pyriproxyfen
    2. chitin synthesis inhibitors - diflubenzuron, triflumuron, lufenuron, cyromazine
      • Selective against insects and crustacea
      • Affects juveniles and their use can result in failure of insect eggs to hatch
      • JHM break down readily, but the chitin synthesis inhibitors are very stable
      • Both groups are harmful to parasites, predators and bee larvae
  6. Nicotinoid insecticides
    e.g. imidacloprid
    • Systemic insecticide - can be applied as a foliar spray, root drench or seed dressing
    • Active against sap sucking insects - aphids, scale insects, whitefly
    • Also active against beetle larvae - chrysomelid larvae, scarab larvae
    • Moderate mammalian toxicity
    • Nicotine type odour
  7. Pyrazole and pyrrole insecticides
    e.g. fipronil, tebufenpyrad, chlorfenapyr
    • Relatively slow acting
    • Active at low rates per hectare
    • Activity is selective to individual groups
    • Fipronil is active against most diptera larvae, thrips and certain Lepidoptera
    • Chlorfenapyr is active against certain Lepidoptera, with some overlap to fipronil
    • Tebufenpyrad is an acaricide
    • Tend to be stable and hence persistent
    • Relatively low mammalian toxicity

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HISTORY OF INSECTICIDE USE

Year/Period Insecticide/s Used
2000 years ago Sulphur, oils
1000 years ago Arsenics in China
1650 Rotenone (derris dust) - botanical
1690 Tobacco (nicotine)
1860/1890 Paris green and other arsenates
Kerosene and oil emulsions
P yrethrum
N aphthalene, paradichlorobenzene
Bordeaux mix = lime and copper sulphate
Early 20th century Fluorides, creosote
1918 Aerial crop dusting
1938 Sesame oil as synergist
1940 Methyl bromide as fumigant
1940s DDT, other organochlorides
- wide spectrum, cheap, high residual, environmental problems
1950s Organophosphates
- less residual but high toxicityCarbamates
- less toxic to humans, faster breakdown
1960s Juvenile hormones, synthetic pyrethroids
Now IPM, biological control
Future Venom derivatives ?

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