pesticide, any toxic substance used to kill animals, fungi, or plants that cause economic damage to crop or ornamental plants or are hazardous to the health of domestic animals or humans. All pesticides interfere with normal metabolic processes in the pest organism and often are classified according to the type of organism they are intended to control. (See herbicide; insecticide; fungicide; fumigant.) The use of certain pesticides is controversial. The insecticide DDT, for example, which came into wide use in the 1940s, was later heavily restricted in the United States and elsewhere because of its adverse affects on the health