Any living organism that hinders the growth of crops is a pest and a man-made chemical used to kill a pest is called a pesticide. Pesticides can be specific in their action. When they are used to kill insects, they are called insecticides; likewise a fungicide is used to eliminate unwanted fungi.
For many years, DDT (dichlorodiphenyltrichloroethane) was used to kill pests. It kills by destroying the nervous tissue of insects. DDT tends to remain active for a long time. It accumulates in animal tissues as it moves up the food chain. It can also enter our body. Therefore, its use is not advisable.
Arsenic compounds of lead and calcium are often used in fruit sprays. Since these compounds as well as many other insecticides are poisonous, it is important that all vegetables and fruits always be thoroughly washed before they are eaten.
Fungi like rusts, molds, smut and mildew can cause infection in plants. They are killed with the help of fungicides. Sulphur sprays and Bordeaux mix are common fungicides. A calculated use of pesticides is necessary.