Pesticides are unique in position among toxic substances as they are deliberately added to
suppress or eliminate some form of life. Under ideal conditions the injuring action should be highly specific and affect only the target organisms. The toxicity should disappear after the purpose for which it was applied has been achieved.
However, none of these features are met with in most of the pesticides which are in common use these days. This has made the use of these synthetic organics extremely hazardous pollutants of the environment and the biosphere. Some of the characteristic features of pesticide application may be summarized as follows:
(1) The use of synthetic pesticides has become a necessity in various branches of our economy. Fast expanding human establishments, intensive agriculture and higher input rate of waste material into the environment have created additional resources for various insects, pest and other harmful pathogens to multiply. Natural means of population regulation through prey-predator interactions have become ineffective due to disturbed functioning of natural systems. We are faced with a large number and variety of unwanted organisms. The application of the synthetic pesticides seems to be the only effective solution.
(2) Most of the pesticides are violent poisons and their handling is hazardous. The selective action of pesticides is never perfect and many non-target organisms are affected by their toxicity - some of which may be useful organisms. For example elimination of some insects and bees which vitally aid in pollination of many plants could cause considerable damage to agricultural productivity (Pimental et al 1980). These non-target organisms may also include domestic animals, live-stock, poultry etc. Accidental exposures may lead to human casualties as well.
(3) It is difficult to prevent the circulation of these chemicals in the environment. They are usually applied with the help of some aerial or surface spraying device or simply dusted manually. Air, water and living organisms carry them too far off places. Pesticides applied in tropics may appear in arctic or subarctic regions. Some of these pesticides have already attained a global distribution (for example DDT).
(4) Most of these synthetic organics or their decomposition product persists in toxic state in the environment for long durations. Thus, once applied they continue to harm the non-target organisms for long periods of time. They are bio-accumulated and bio-magnified, features which cause problems at higher trophic levels in an ecosystem. Persistence of DDT has been recorded for periods as long as twenty five years.
(5) It is impossible to reduce the rate of application of pesticides. Their application should be in adequate doses so as to eliminate the entire population of unwanted organisms whiteout providing them any chance to develop resistance to the chemical employed.
However, in actual practice, uneven distribution of insecticides over the area under operation usually exposes many of the undesired organisms also to lower concentrations (sub-toxic or sub-lethal concentrations). Thus resistant populations are developed and subsequent applications have to be in higher and higher doses. Decreasing the rate of their application could defeat the very purpose for which they are applied.