Three major types of insecticides are used by farmers: organochlorines, organophosphates, and carbamates. The organochlorine compounds are being replaced as it becomes evident that they cause extensive, possibly irreparable, harm to wildlife. The environmental impact of insecticides embraces the development of pesticide-resistant insect strains. Excessive reliance on this method of insect control has also had very serious effects directly on human beings through direct exposure to insect sprays and through the accumulation of dangerous compounds in food sources.
Production of specific insecticides has been restricted and their use curtailed. DDT was banned for most uses by the EPA in 1972, but some is still exported to tropical countries for mosquito control. Aldrin and dieldrin were banned for most uses in 1974. Heptachlor and chlordane were banned for crop use in 1975. Toxaphene, now the most widely used organochlorine, is under review. The total amount of insecticides applied to major crops has not increased appreciably in recent years, although use, measured in terms of acres treated, grew 32% from 1971 to 1976, largely because of increased corn applications.
In contrast with herbicides, which are usually applied directly to plants or the soil, two-thirds of the insecticides used in agriculture are applied by aircraft -- but only between 25 and 50% of this reaches the crop. A large proportion remains airborne and drifts or is lost through volatilization, leaching, and surface transport. Less than 1% actually comes in contact with an insect.