Insecticides is a substance that kill insects by chemical action. Most insecticides are hazardous to human beings. Labels on insecticides containers list special precaution for safe use. Fruits and vegetables that have been treated with insecticides always should be washed and soak in a water with rock salt or mineral salt to remove the deep-seated dirt and insecticide on the skin. Application of insecticides varies from spraying by hand with aerosol cans to crop-dusting by airplane.

Classification of Insecticides according to the method of application and the way they enter the insect's body:

• Contact Insecticides - are sprayed or dusted on the insect's body. The poison is absorbed through the body wall. Most soft-bodied insects are vulnerable to contact insecticides.
• Fumigants are insecticidal gases. Insects that are hard to reach by sprays are killed when they breathe or are exposed to the gas. Fumigants are used by professional exterminators to kill the cockroaches, bedbugs and to kill beetles in grain bins. The soil may be fumigated to destroy grubs that attack roots.

• Residual Insecticides are applied to leaves, the bodies of livestock and pets, and to the screens and walls. Insects absorb deadly doses by touching the poisoned surface.
• Stomach Insecticides - are applied on the surface of plants, fabrics, and wood, or are added to baits. The insecticide is eaten, along with the food material, by insects that chew, such as ants, caterpillars, and grasshoppers.
• Systemic Insecticides are absorbed by plant tissues, so that when insects feed on the sap they are poisoned.

Benefits and Problems. Insecticides is an substance in the fight against disease-carrying insects and insects that damage crops and property, but at the same time insecticides also create problems of their own.

• Most species of insects have become resistant or immune especially to DDT, which had been the most effective insecticide.
• Biologists discovered that the inappropriate or excessive use of insecticides destroys many beneficial insects, birds, and small mammals. Example is the Bees
• Some insecticides, especially DDT, tend to stay in soil and water and can eventually accumulate in the body tissue of fish, wildlife, and humans. DDT also tends to combine with other chemicals in the soil, producing new toxic compounds that are more harmful than DDT alone.

These problems may be prevented or lessened by using selective chemicals and synthetic sterilants, pheromones, and growth-regulating hormones.

• Sterilants are a substance that does not kill an insect but does prevent it from reproducing. Selective use of sterilants may in time succeed in removing harmful species, including insecticides-tolerant ones, without injuring beneficial species.
• Pheromone is a substance that resembles the sexual secretion of a particular species of insect. The substance is used to attract an insect of that species to a trap, where it can be destroyed with an insecticide. This method may replace the wholesale spraying and dusting that endanger other creatures.
• Growth-regulating Hormone is a substance that prevents juvenile insects from maturing into adults.