Any plant or animal that occurs in such abundance as to pose a distinct threat to man or his interests is called a pest. And the chemicals used for mitigation, control or elimination of such plants or animals are known as pesticides. Today we have algaecides, defoliants, herbicides, plant growth regulators and fungicides in use to control the growth of undesirable plants which compete with crops or other useful plants. Attractants, insecticides, miticides or acaricides, molluscicides, nematocides, repellents and rodenticides are used to reduce parasitism and disease-transmitting organisms in animals, crops, plants, foods, textiles and human beings.
Most of the pesticides are chemical compounds and act in a similar fashion, i.e. by blocking some metabolic process. They, however, differ in composition, potency, mode of action, speed of effect. So different pesticides are used at different stages of infection.
Some of the common insecticides are lead arsenate and calcium arsenate which are basically stomach poisons. They act as poisons when ingested. Fluorine compounds such as sodium fluoride and cryolite are also used to kill insects. Chlorinated hydrocarbons like DDT, BHC, lindane, aldrin, endrin etc. are again toxic for insects. Organophosphates are currently the largest and most versatile class of insecticides. Carbamates are a relatively new group of insecticides that include some compounds such as carbamyl, methomyl and carbofuran. Fumigants such as hydrogen cyanide, nicotine etc., kill the pest by acting on its respiratory system.
Insecticides can also kill mites. So they are used as miticides. Some compounds such as aramite and kelthane are made to be used specifically against mites.
Repellents repulse animals rather than kill them. They are mainly used against mosquitoes, blood-sucking flies and chiggers. Some of the common repellents are dimethyl pathalate and deet.
Some of the pesticides induce sterilization in insects. Chemosterliants inhibit reproduction of insect pests rather than killing them directly. The commonly used chemosterliants are apholate, TEPA, metepa, aminopterin, amethopterin etc.
Fungicides are used to eliminate undesirable fungus growth. Some of the commonly used fungicides include lime sulphur, Bordeaux mixture, copper sulphate, mercuric chloride, cadmium chloride, phenyl mercuric acetate, ferbam, thiram, folpet, dichloran etc.
Herbicides are divided into two categories: nonselective which kill the undesirable plants and selective which kill only a certain group of plants. Some of the common ones include sodium arsenite, cacodylic acid, dalapon, atazine, barban etc.
Rodenticides are mainly used against vertebrate pests. The most toxic rodenticides are thallium sulphate, arsenic trioxide, arsenic sulphide, strychnine coumafene, sodium fluoroacetate etc.
The solid pesticides are available in powder form. Most of the liquid pesticides are sprayed by hydraulic sprayers. They can even be used as fog or smoke, or sprayed with the help of hand-operated sprayers in the houses.
Scientists have even developed biological methods for pest control. In this method a pest is kept under control by its natural enemies. For example, the use of a pet cat to catch mice in the house. Some medicinal plant products are also used nowadays as a pest-control measure.