An insecticide is a substance 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 agricultural 20th century's productivity.ince allyl isothiocyanate is harmful to the plant as well as the insect, it is stored in the harmless form of the glucosinolate, separate from the myrosinase enzyme. In general, tree rosin is considered a natural insecticide.