Insecticides are chemical agents used in reducing or controlling populations of insects which feed on crops grown in fields or in greenhouses. Insecticides can be classified in different ways, depending on the route of insect exposure. Contact insecticides are toxic to insects when brought into direct contact, stomach poison insecticides act through ingestion, and respiratory insecticides through respiration (systemic insecticides). Insecticides can act either remaining on the plant surface, or be absorbed by plants to fight insects inside or on the underside of leaves. Systemic insecticides are incorporated by treated plants and are transported in the plant juices.