What is the definition of pesticides?

A pesticide is a chemical substance sprayed on crops to kill insects, pests and bacteria. The chemical agents called pesticides include herbicides (for weed control), insecticides, and fungicides. More than half the pesticides used in the United States are herbicides that control weeds. The United States Department of Agriculture (USDA) estimates indicate that 86 percent of U.S. agricultural land areas are treated with herbicides, 18 percent with insecticides, and 3 percent with fungicides. The amount of pesticide used on different crops also varies. For example, in the United States, about 67 percent of the insecticides used in agriculture are applied to two crops, cotton and corn; about 70 percent of the herbicides are applied to corn and soybeans, and most of the fungicides are applied to fruit and vegetable crops.

They kill "pests".