Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine industry and by consumers. Insecticides are claimed to be a major factor behind the increase in the 20th century's agricultural productivity. Nearly all Insecticides have the potential to significantly alter ecosystems, Many are toxic to humans or animals, some become concentrated as they spread along the food chain, Insecticides which have residual or long term activity, and contact Insecticides which have no residual activity.