The world market for insecticides is dominated by compounds interfering with the nervous system of pest invertebrates, since this target organ usually provides rapid control. Insecticides acting on target sites such as acetylcholinesterase (organophosphates and methyl carbamates), voltage-gated sodium channels (pyrethroids), nicotinic acetylcholine receptors (neonicotinoids) and ligand-gated chloride channels (macrocyclic lactones and phenylpyrazoles) account for more than 75% of total insecticides sales. First introduced in 1944, organophosphorus compounds (OPs) are economically still the most successful and diverse chemical class of insecticide. More than 100 different active ingredients belonging to this class are known. One of the most successful OPs is considered to be chlorpyrifos. This non-systemic insecticide, used on both the foliage and the soil, affects the stomach and respiratory action of the pest. All OPs act by binding irreversibly to the enzyme, acetylcholinesterase (AChE). This prevents the hydrolysis of the neurotransmitter acetylcholine in the central nervous system (CNS) and leads to prolonged periods of nerve excitation. This then results in paralysis and subsequently death of the insects and their predators. OPs are used to control almost all pests including Lepidoptera (e.g. moths), Coleoptera (e.g. beetles), Diptera (e.g. flies, such as whiteflies, and mosquitoes), Hemiptera (e.g. aphids and leafhoppers). Additionally they control nematodes (parasitic worms) and mites.
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