Under the name pesticides are grouped a large number of chemicals which are used to suppress or eliminate undesirable organisms.
Most of these chemicals are poisonous substances capable of damaging one type of organism drastically while causing none or only nominal damage to the desired one even if the two are in a close association. It is due to this property of exerting a selective toxic action on different species or group of species that these chemicals have acquired a very important role in a modern society.
In his endeavour to feed a rapidly growing population man had to develop sophisticated technology for raising vegetables, fruits, cerials and live-stock of which the control of various pathogens, insects, pests, nematodes, rodents etc. forms an integral part. In good old days the problem was not so severe.
Sprawling human establishments and practices of intensive agriculture have created additional resources for subsistance of various pathogens, insects and pests etc, while the disturbed prey predator relationship in a drastically altered ecosystem has weakened or eliminated the natural means of population regulation.
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The number of disease causing organisms and pestilent insects has been estimated to be well over 86000 apart from a large number of species of fungi, ticks, mites and nematodes. On global scale these could cause a loss of over 50% of total world foodstuff production. (Annonymous 1979).