The first synthetic organochlorine insecticide, DDT (dichlorodiphenyl-trichloroethane), discovered in Switzerland in 1939, was very effective and used extensively to control head and body lice, human disease vectors and agricultural pests, in the decades leading up to the 1970s. Benzene hexachlo-ride (BHC) and chlordane were discovered during World War II and toxaphene (and heptachlor) slightly later. Shortly thereafter, two cyclodiene organochlorines, aldrin and dieldrin, were introduced, followed by endrin, endosulfan, and isobenzan. All these insecticides acted by blocking an insect's nervous system, causing malfunction, tremors, and death. All organochlorines are relatively insoluble, persist in soils and aquatic sediments, can bio-concentrate in the tissues of invertebrates and vertebrates from their food, move up trophic chains, and affect top predators. These properties of persistence and bioaccumulation led eventually to the withdrawal of registration and use of organochlorine insectides, from 1973 to the late 1990s, in industrialized nations, although they continued to be used in developing countries.
Organophosphate insecticides originated from compounds developed as nerve gases by Germany during World War II. Thus, those developed as insecticides, such as tetraethyl pyrophosphate (TEPP) and parathion, had high mammalian toxicities. Scores of other organophosphates including deme-ton, methyl schradan, phorate, diazinon, disulfoton, dimethoate, trichlorophon, and mevinphos have been registered. In insects, as in mammals, they act by inhibiting the enzyme cholinesterase (ChE) that breaks down the neurotransmitter acetylcholine (ACh) at the nerve synapse, blocking impulses and causing hyperactivity and tetanic paralysis of the insect, then death. Some are systemic in plants and animals, but most are not persistent and do not bioaccumulate in animals or have significant environmental impacts.
Carbaryl, the first carbamate insecticide, acts on nervous transmissions in insects also through effects on cholinesterase by blocking acetylcholine receptors. Other carbamate insecticides include aldicarb, methiocarb, methomyl, carbofuran, bendiocarb, and oxamyl. In general, although they are broad-spectrum insecticides, of moderate toxicity and persistence, they rarely bioaccumulate or cause major environmental impacts.
Botanical insecticides include nicotine from tobacco, pyrethrum from chrysanthemums, derris from cabbage, rotenone from beans, sabadilla from lilies, ryania from the ryania shrub, limonene from citrus peel, and neem from the tropical neem tree. Most, other than nicotine, have low levels of toxicity in mammals and birds and create few adverse environmental effects.
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