A cropduster spreading pesticide. A pesticide is a substance or mixture of substances used for preventing, controlling, or lessening the damage caused by a pest.[1] A pesticide may be a chemical substance, biological agent (such as a virus or bacteria), antimicrobial, disinfectant or device used against any pest. Pests include insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes (roundworms) and microbes that compete with humans for food, destroy property, spread or are a vector for disease or cause a nuisance. Many pesticides are poisonous to humans.