Insecticides used prior to the 1940s were mostly inorganic compounds such as arsenicals. After World War II, DDT and other chlorinated pesticides came on the market. There is no question about the spectacular insect-controlling effects achieved on many crops, and populations of some pests that affect both public and veterinary health were greatly diminished. The shortcomings of these compounds, particularly their lack of selectivity and harmful environmental effects, were eventually realized, however, leading to the termination of their use by the late 1970s. Meanwhile, organophosphorus and carbamate insecticides gained in popularity and have established themselves as two of the major classes of insecticides. Many of them offer at least some degree of selectivity (malathion is particularly outstanding in this regard) and are less persistent in the environment. In more recent years, functional synthetic analogues of naturally occurring toxic chemicals were developed. Pyrethroids, for example, are essentially synthetic mimics of naturally occurring pyrethrins found in the flowers of species of chrysanthemum. The synthetic neonicotinoids mimic naturally occurring nicotine from tobacco plants. Useful microbial products were also developed in the 1980s and 1990s; examples are Bacillus thuringiensis (Bt) toxins, avermectins, and spinosyns. Modern insecticides used today are generally very selective, mostly affecting only the targeted pest insect. They are potent, requiring only small quantities to achieve their effects, and they are much less persistent in the environment.
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