Chemical control involves the use of chemicals to kill pests and pathogen. Most agricultural scientists agree that chemical pesticides can be hazardous to crops, animals, humans, and the environment, especially if used improperly. However, they also agree that chemicals can be very useful in modern agriculture.
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. These insecticides are effective against sap suckers. Most insecticides are chemical, though some are made from local plants. Insecticides are complex and powerful. Therefore, it is important to know how chemicals behave and what they can do, and when and how to apply them.
While insecticides are surely useful in controlling insects and plant diseases, farmers must also understand some of the problems caused by insecticides. It is noticed that after an area has been repeatedly sprayed against a pest with the same chemical, one or more of the following situations arise.