The concept of using chemicals for controlling insect pests is not new ; however, its importance and popularity increased only after the discovery of the insecticidal properties of DDT. Originally, these chemicals were classified on the basis of their mode of entry in the bodies of insects,viz. stomach poisons, contact poisons and fumigants. However, this classification has become outdated, because most of the synthetic organic insecticides act both as stomach and contact poisons and some have also the added fumigant action. At present the generally acceptable classification is based on the chemical nature of the insecticides.

Classification of insecticides (based on their chemical nature)

The inorganic insecticides were widly used before the synthetic organic insecticides were developed and marketed. At present, hardly any of these insecticides are being used in India. There is a feeling in some quarters that we can perhaps advantageously use arsenicals against the leaf-feeding insects, especially where some parasities or predators are known to be active against them.

Insecticides of plant origin have also been regulated to the background by the synthetics and at present relatively very little quantities of insecticides, like Pyrethins and Nicotine sulphate, are used to control insect pests of agricultural importance. The problems of residues on insecticides on food and fodder crops has emphasized the necessity of using insecticides which are less toxic to mammals or breakdown into non-toxic components in a resonable short time. Thus the insecticides of plant origin can be usefully employed to control a number of insect pests, especially where the products are to be consumed within a short time after treatment (vegetable and fruits).

The use of synthetic organic insecticides has increased with leaps and bounds throughout the world. In India, only about 10,120 hectares of crops were treated with pesticides in 1946-47 ; this area increased to about 17.4 million hectares by 1965-66 and the target for 1973-74 was 80 million hectares. The various insecticides required for controlling insects during this period is estimated to be 40,850 tonnes.

Starting with the manufacture of BHC in 1958, India today produces technical grades of 39 pesticides ( insecticides, fungicides, rodenticides, molluscicides, nematocides and herbicides), including the following insecticides used for controlling crop pests:

BHC, DDT, Malathion, Parathion (Methyl), Toxaphene, Pyrethrum extract, Nicotine sulphate, Fenitrothion, Methyl-demeton, Phosphamidon and Dimethoate.

Practically all insecticides are toxic to man and other animals;however they differ in their degree of toxicity. Thus the hazards to man and his domestic animals depend on the values and nature of toxicity (oral or dermal).

The United States Environmental Protection Agency,Washington, has recognized three categories, viz. danger-poison, warning and caution on the basis of these values.

Available data on some of the common insecticides are presented in Table 1.

Table 1. Acute Oral LD50 Values of Some Common Insecticides for White Rats