Insecticide, a substance that destroys insects by chemical action. Some insecticides also destroy mites, ticks, and spiders. Because it kills, an insecticide differs from a repellent, such as DEET, citronella, or creosote, which merely keeps insects away. Insecticides are a form of pesticide; other pesticides include herbicides (weed killers) and fungicides (fungus killers).

Many insecticides are poisonous to human beings. Labels on insecticide containers list special precautions for safe use. Fruits and vegetables that have been treated with insecticides always should be washed before they are eaten.

There are several ways of applying insecticides, from spraying by hand with aerosol cans to crop dusting by airplane.

Types of Insecticides

Insecticides are classified according to the method of application and the way they enter the insect's body. Many insecticides take effect in more than one way.

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.

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 lurk out of reach of sprays are killed when they breathe the gas. Fumigants are used by professional exterminators to rid houses of cockroaches and bedbugs and to kill beetles in grain bins. The soil may be fumigated to destroy grubs that attack roots.

Residual Insecticides are applied to foliage, the bodies of livestock and pets, and to screens and walls. Insects absorb deadly doses by touching the poisoned surface.

Systemic Insecticides are absorbed by plant tissues, so that when insects feed on the sap they are poisoned.

Benefits and Problems

Insecticides are tremendously effective weapons in the fight against disease-carrying insects and insects that damage crops and property. At the same time, insecticides create problems of their own. For example, many species of insects have become tolerant (resistant or immune)—especially to DDT, which had been the most broadly effective insecticide. Also, biologists have found that the careless or excessive use of insecticides destroys many beneficial insects, birds, and small mammals. In addition, some insecticides, notably DDT, tend to persist 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 avoided, at least in part, by the use of selective chemicals and synthetic sterilants, pheromones, and growth-regulating hormones.

A Sterilant is a substance that does not kill an insect but does prevent it from reproducing. Selective use of sterilants may in time succeed in eliminating harmful species, including insecticide-tolerant ones, without injuring beneficial species.

A 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.

A Growth-regulating Hormone is a substance that prevents juvenile insects from maturing into adults.

History of Insecticides

The earliest insecticides were of natural origin; they included borax, sulfur, and extracts of the camphor, tobacco, and pyrethrum plants. In the late 19th century the widespread use of arsenic compounds such as Paris green proved the value of insecticides to agriculture.

Since the 1940's chemists have invented hundreds of insecticides by synthesizing compounds such as chlorinated hydrocarbons and organic phosphates. Examples are DDT, chlordane, parathion, and toxaphene. Sterilants and pheromones were developed in the 1960's. Because of the possible wide-ranging ill effects of such insecticides as DDT and chlordane, which persist in the environment, numerous federal and state regulations banning or restricting their use in the United States were introduced in the 1960's and 1970's.

In the 1990's, there was an increase in the development of natural insecticides, which are less harmful to the environment and humans than synthetic insecticides. In addition, a number of crop plants, such as corn, were genetically modified to have traits that kill or repel certain insects pests. The first type of corn genetically modified to have these traits was introduced for commecial planting in 1996.

For control measures taken against specific insect pests, see the articles on those insects.