These substances destroy the harmful agents, viz. insects, herbs (weeds), fungi and rodents rats), for crops.
These are respectively called as insecticides, herbicides, fungicides and rodenticides.
Insecticides : An insecticide may be a substance or mixture of substances used for killing insects. DDT2 (p, p'-dichlorodiphenyl trichloroethane), γ-BHC (gammexane, lindane or 666), chlordane, heptachlor, aldrin and dialdrin are halogenated compounds. Malathion and parathion are organic phosphates.
Herbicides : Herbs (weeds, unwanted small plants) take away a good part of fertilizers and water needed by crops. Important herbicides are 2, 4-dichlorophenoxyacetic acid (2, 4-D) and 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T).
Fungicides : Copper sulphate, copper naphthenate, 2, 4, 6-trichlorophenol and Bordeaux mixture (CuSO4 + lime water) are common examples.
Rodenticides : Rodents (rats) destroy food stocks and spread diseases.
Important rodenticides are sodium monochloroacetate, sodium fluoroacetate, zinc phosphide and thallium sulphate. These are mixed with the food of rats.
