Pesticides are substances which either kill or repel the pests. The pests are organisms which harm the plants as well as the human beings. The pests are responsible for destruction of crop plants in the fields as well as the grains during storage. The pesticides kill rats, insects, nematodes as well as fungi and weeds. They are classified as following types:
Insecticide- kills insects
Rodenticide- kills rodents
Nematicide- kills nematodes
Fungicide- kills fungi
Herbicide- kills weeds
During second world war, in the course of developing agents of chemical warfare, some of the chemicals created in the laboratory were found to be lethal to insects. As s matter of fact insects were widely used to test chemicals as agents of death for man. This finally led to establishment of a huge pesticide industry.
Today, pesticide pollution had reached all major aquatic and terrestrial ecosystems. It occurs everywhere. The pesticides have been recovered from most of the major river systems and even from streams of ground water flowing unseen through the earth. Residues of these chemicals linger in soil to which they have been applied a dozen years before.
The pesticides are of several types depending upon their chemical structure:
1. Organochlorines- These are chlorinated organic compounds like hydrocarbons. Some of the examples of chlorinated compounds are DDT, BHC, chlordance heptachlor endrin, aldrin and endosulphan. Since they are lipophilic they get accumulated in the fatty tissues of animals.
2. Organosulphates- This is the second major group of insecticides, the alkyl or organic phosphates, the most poisonous chemicals in the world. The chief hazard attending their use is that acute poisoning of people applying the sprays or accidentally coming in contact with it. Parathion and Malathion are two well known examples of this group. Some others are tetraethyl pyrophosphate (TEPP), diazinon, guthion, trithion, ethion, demeton, disyston etc. All are capable of damaging plants when applied in excess amounts if mixed with the incompatible material, or used at the sensitive stage of plant development or when, temperature are too high. Organic phosphates disappear from the environment far more rapidly than the chlorinated hydrocarbon chemicals.
3. Carbamates- They are esters of carbonic acid. They include carbaryl carbofuran, adicarb propoxur etc.
4. Pyrethoids and rotenon- Pyrethoids is a well known synthetic insecticide obtained from pyrethrum. The pyrethrum is a chemical derived from the flowers of Chrysanthemum. The insecticide is not poisonous. It is also non-flammable. It kills fleas, fies, mosquitoes, lice etc. Pyrethrum ointment is used in the treatment of scabies.
Roots of two leguminous climbers yield poisonous substance called rotenone, which is widely used as an insecticide. The plants are Lonchocarpus nicous and Derris spp. Rotenone is very poisnous insecticide. It is, however harmless for animals and human beings.
5. Triazines (Herbicides)- It includes the chemicals like simazine, atrazine etc. They are obtained from urea 2:4 (dicholorophenoxy acid) is a synthetic auxin and is widely used for eradication of weeds.
Mode of action of pesticides:
Insecticides attack the nervous system of the insects. The conduction of nerve impulse is often disrupted. The herbicides interfere with photolysis of water in photosynthesis. It also disturbs the translocation of organic substances in plants.
