Insecticide, any toxic substance that is used to kill insects. An organic insecticide is a pesticide that uses only natural components to kill bugs. In many cases, organic materials used to repel insects are also called organic insecticides. This demand of consumer has led farmers to grow crops that are plentiful in nutrients and has least chemical presence.  Insecticides can be classified in any of several ways, on the basis of their chemistry, their toxicological action, or their mode of penetration.