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S2 L3 Insecticides - synthetic

S2 L3 Insecticides - synthetic. Anna Drew with slide contribution from Martin Wilks, Syngenta Diane Alston & Joel Coats. World Insecticide Market. “Old” classes (nerve poisons). ORGANOCHLORINES Most famous DDT ( D ichloro- D iphenyl- T richloroethane)

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S2 L3 Insecticides - synthetic

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  1. S2 L3 Insecticides - synthetic Anna Drew with slide contribution from Martin Wilks, Syngenta Diane Alston & Joel Coats

  2. World Insecticide Market

  3. “Old” classes (nerve poisons) • ORGANOCHLORINES • Most famous DDT • (Dichloro-Diphenyl-Trichloroethane) • 4,4'-(2,2,2-trichloroethane -1,1-diyl) bis(chlorobenzene) • 1874 synthesized • 1939 insecticidal properties discovered • 1942 first introduced for malaria control in India

  4. Although useful • tends to accumulate in body fat • not really excreted • tended to build up in food chains • eg birds -> thin egg shells endangering species • resistance developed • Banned in the US, UK etc • Use: extensive use in developing countries • mosquito, tsetsi fly control – malaria, typhus • Action: • delays closing sodium channels • inhibition of axonal Na+-, K+- and Mg2+-ATPase • interaction with GABAA receptor chloride ionophores

  5. Lindane • gamma hexa chloro benzene • benzene hexachloride • still used in Sri Lanka • similar to DDT • more rapidly metabolised • less stable • Other cycloalkanes: • dieldrin • aldrin • [Methoxychlor]

  6. ORGANOPHOSPHATES • Esters of phosphoric acid - examples: eg dichlorvos (orthophosphate) eg diazinon (orthothionphosphate) eg parathion eg chlorpyrifos

  7. Selection: • solubility • volatility • relative toxicity (insects v mammals) • stability • Action: • mimic acetylcholine by binding with acetylcholinesterase • prevents ACh breakdown • duration of toxicity depends on how quickly enzyme is rehydrolysed • generally more toxic to vertebrates • persist less in environment • Uses: before contact, now systemic • aphids, spiders • Problems: • Handling – absorbed through skin • Self-poisoning – major public health problem

  8. Mortality rates of poison admissions at Anuradhapura General Hospital, Sri Lanka (2.4.02 – 13.1.03)

  9. Target site

  10. CARBAMATES • Derivatives of carbamic acid – examples: • Action: • also inhibits AChE resulting in accumulation of ACh at neuromuscular junctions or synapses • causes rapid twitching of voluntary muscles and finally paralysis • Use: some contact, some systemic eg methomyl eg aldicarb eg carbaryl eg propoxur

  11. Dinitroorthocresol Dinocap • SUBSTITUTED PHENOLS • Good on woody plants and outdoors • 1950 fungicide as a winterwash • then acaricide (mites, ticks) • then foliage control of mildew on fruits & mites on apple trees • Very toxic, protection for handling • Highly toxic to birds, slightly toxic to fish CH3CHC6H13

  12. Why preferentially toxic to insects? • Ease of access to site of action differs • eg pyrethroids • easily absorbed through insect exoskeleton but not skin • Action at site differs • insect sodium channels 100x more sensitive than mammalian channels • the proportion of sodium channels affected and hence the degree of hyperexcitability is dose-dependent • the duration of the hyperexcitable state and hence the nature of the effect is structure dependent • Variability of metabolism • eg OPs • insects: convert S -> O forming much more active compounds • mammals: esterase cleaves off ester group and compound is much easier to excrete

  13. INSECT GROWTH REGULATORS • Class: benzoylureas • diflubenzuron, lufenuron, novaluron • Cyromazine (triazine) • Use: leafminers in vegetable crops & ornamentals • Action: interfere with chitin synthesis • act at the larval stage • greatest value for control of caterpillars, beetle larvae • cause ruptured cuticle or death by starvation • taken up more by ingestion than contact

  14. PYRAZOLES • Fipronil • Use: • systemic material with contact and stomach activity • control of soil and foliar insects eg rice water weevil • baits for cockroaches, ants, termites • effective against insects resistant or tolerant to pyrethroid, OP and carbamates • Action: • inhibitor at the gamma-aminobutyric acid (GABA) receptor • non-competitive blockers at the GABA-gated chloride channels in neurons

  15. OTHERS • Since 1995 29 new compounds, 57 brands - Utah • Chlorfenapyr • broad-spectrum insecticide • ? interferes with oxidative phosphorylation • Sulfluramid • Buprofezin • inhibits chitin synthesis • similar to benzoyl phenylureas • Diafenthiuron (thiourea) • inhibits ATPase in mitochondria

  16. Indoxacarb (oxadiazine) • blocks sodium channel in nerve axon • inhibits propagation of nerve potential • Metaflumizone • blocks sodium channel in nerves • Pymetrozine (pyridine azomethine) • inhibits feeding of sucking insects – aphids • neuromuscular effects, prevents insertion of insect stylets • Flonicamid (nicotine-derived) • antifeedant – mode of action undetermined • Clofentezine, Hexythiazox, Etoxazole • mite growth inhibitor – ovicide (kills eggs) – apply early

  17. Pyridazinones • inhibits mitochondrial electron transport • affects respiration • like rotenone • Acequinocyl • inhibits mitochondrial electron transport • affects respiration • different site of action to other METI compounds • Bifenazate (carbazate) • related to carbamates • neurotoxic but mechanism unknown

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