Insecticides can be classified by their main route of entry. 1) Contact insecticides harm or kill an insect when spray droplets directly contact the insect’s body. 2) Stomach poisons must be swallowed and absorbed into the digestive system. 3) Fumigants are insecticides which are toxic in gaseous form in the air. They are most often used to kill insects (and pathogens) in stored grain. 4) Systemic insecticides have the property of being absorbed into the sap stream of plants.