There are currently around 600 pesticides, both natural and synthetic. Natural pesticides include both chemicals derived from plant sources and biological agents such as parasitic wasps, mites, bacteria, and chemicals contained within or exuded by plants or bacteria. While there is no inherent reason why natural products should be any safer than synthetic ones (after all, insect venoms and toxins and poisonous plants are natural), it appears that the risks do lie in their potential impact on the environment rather than on their effect in food. There are also increasing numbers of cases where plants have been given a gene which expresses a natural pesticide (see Bacillus thuringiensis, below).
At the time of writing, naturally derived pesticides make up less than 5% of the world pesticide market, but a great deal of work is being devoted to the screening of natural sources and this proportion will certainly increase. The most successful natural product development so far has been that of the pyrethrin insecticides, of which 33 are currently available.
The largest classes of pesticides are pyrethrins, organochlorines, organophosphates, and carba-mates, although there are many smaller classes with only one or two members. The chemical structures of the key members of the major groups are given in Table 1.
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