The use of pesticides began in the 18th century & the first synthetic pesticide chemical appeared in the latter part of the 19th century, but the most important phase of their development occurred in the past five decades.

In U.S., the use of arsenical & botanically derived insecticides led the field until the middle 1940s when the synthetic organic insecticide chemicals began a remarkable rate of growth; now, synthetic chemicals dominate the field, greatly outstanding other types of insecticide chemicals.

In the last two decades, much effort has been on the development of organophosphorus chemicals, compounds which inhibit Cholinesterase & certain aliesterases in insects, mites & mammals. While many of these are highly toxic, such chemicals remain in or on the plants for a relatively short time, their effective period varying from several hours to a few days. Also, most of these compounds are metabolized in mammals to harmless products in a day or so. As a result, organophosphorus compounds have a favourable chronic toxicity pattern to man & useful animals tissues, including milk.

In the past decades, emphasis has been on the development of carbamate - type & thionophosphate - type insecticide chemicals. The carbamates generally inhibit cholinesterase, metabolize rapidly in mammals & show very little propensity for storage in meat & milk. Much work has been done & is still being done, to elucidate the biochemistry of these two types of insecticide chemicals, especially their fate in insects & mammals.