insecticide, any toxic substance that is used to

kill insects. Such substances are used primarily to control pests that infest cultivated

plants or to eliminate disease-carrying insects in specific areas.

Insecticides can be classified in any of several ways, on the basis of their chemistry,

their toxicological action, or their mode of penetration. In the latter scheme, they are

classified according to whether they take effect upon ingestion (stomach poisons),

inhalation (fumigants), or upon penetration of the body covering (contact poisons). Most

synthetic insecticides penetrate by all three of these pathways, however, and hence are

better distinguished from each other by their basic chemistry. Besides the synthetics, some

organic compounds occurring naturally in plants are useful insecticides, as are some

inorganic compounds; some of these are permitted in organic farming applications. Most

insecticides are sprayed or dusted onto plants and other surfaces traversed or fed upon by

insects.

Modes of penetration

Stomach poisons are toxic only if ingested through the mouth and are most useful

against those insects that have biting or chewing mouth parts, such as caterpillars,

beetles, and grasshoppers. The chief stomach poisons are the arsenicals—e.g., Paris green

(copper acetoarsenite), lead arsenate, and calcium arsenate; and the fluorine compounds,

among them sodium fluoride and cryolite. They are applied as sprays or dusts onto the

leaves and stems of plants eaten by the target insects. Stomach poisons have gradually been

replaced by synthetic

insecticides, which are less dangerous to humans and other mammals.

Contact poisons penetrate the skin of the pest and are used against those arthropods,

such as aphids, that pierce the surface of a plant and suck out the juices. The contact

insecticides can be divided into two main groups: naturally occurring compounds and

synthetic organic ones. The naturally occurring contact insecticides include nicotine,

developed from tobacco; pyrethrum, obtained from flowers of Chrysanthemum cinerariaefolium

and Tanacetum coccineum; rotenone, from the roots of Derris species and related plants; and

oils, from petroleum. Though these compounds were originally derived mainly from plant

extracts, the toxic agents of some of them (e.g., pyrethrins) have been synthesized.

Natural insecticides are usually short-lived on plants and cannot provide protection

against prolonged invasions. Except for pyrethrum, they have largely been replaced by newer

synthetic organic insecticides as

technical products.

Fumigants are toxic compounds that enter the respiratory system of the insect through

its spiracles, or breathing openings. They include such chemicals as hydrogen cyanide,

naphthalene, nicotine, and methyl bromide and are used mainly for killing insect pests of

stored products or for fumigating nursery stock.

Synthetic insecticides

The synthetic contact insecticides are now the primary agents of insect control. In

general they penetrate insects readily and are toxic to a wide range of species. The main

synthetic groups are the chlorinated hydrocarbons, organic phosphates (organophosphates),

and carbamates.

Chlorinated hydrocarbons

The chlorinated hydrocarbons were developed beginning in the 1940s after the discovery

(1939) of the insecticidal properties of DDT. Other examples of this series are BHC,

lindane, Chlorobenzilate, methoxychlor, and the cyclodienes (which include aldrin,

dieldrin, chlordane, heptachlor, and endrin). Some of these compounds are quite stable and

have a long residual action; they are, therefore, particularly valuable where protection is

required for long periods. Their toxic action is not fully understood, but they are known

to disrupt the nervous system. A number of these insecticides have been banned for their

deleterious effects on the environment.
Organophosphates

The organophosphates are now the largest and most versatile class of insecticides. Two

widely used compounds in this class are parathion and malathion; others are Diazinon,

naled, methyl parathion, and dichlorvos. They are especially effective against sucking

insects such as aphids and mites, which feed on plant juices. The chemicals’ absorption

into the plant is achieved either by spraying the leaves or by applying solutions

impregnated with the chemicals to the soil, so that intake occurs through the roots. The

organophosphates usually have little residual action and are important, therefore, where

residual tolerances limit the choice of insecticides as

soil disinfectant

. They are generally much more toxic than the chlorinated hydrocarbons.

Organophosphates kill insects by inhibiting the enzyme cholinesterase, which is essential

in the functioning of the nervous system.

Carbamates

The carbamates are a group of insecticides that includes such compounds as carbamyl,

methomyl, and carbofuran. They are rapidly detoxified and eliminated from animal tissues.

Their toxicity is thought to arise from a mechanism somewhat similar to that for the

organophosphates.

Environmental contamination and resistance

The advent of synthetic insecticides in the mid-20th century made the control of

insects and other arthropod pests much more effective, and such chemicals remain essential

in modern agriculture despite their environmental drawbacks. By preventing crop losses,

raising the quality of produce, and lowering the cost of farming, modern insecticides and

fungicide increased crop

yields by as much as 50 percent in some regions of the world in the period 1945–65. They

have also been important in improving the health of both humans and domestic animals;

malaria, yellow fever, and typhus, among other infectious diseases, have been greatly

reduced in many areas of the world through their use.

But the use of insecticides has also resulted in several serious problems, chief among

them environmental contamination and the development of resistance in pest species. Because

insecticides are poisonous compounds, they may adversely affect other organisms besides

harmful insects. The accumulation of some insecticides in the environment can in fact pose

a serious threat to both wildlife and humans. Many insecticides as

formulation products are short-

lived or are metabolized by the animals that ingest them, but some are persistent, and when

applied in large amounts they pervade the environment. When an insecticide is applied, much

of it reaches the soil, and groundwater can become contaminated from direct application or

runoff from treated areas. The main soil contaminants are the chlorinated hydrocarbons such

as DDT, aldrin, dieldrin, heptachlor, and BHC. Owing to repeated sprayings, these chemicals

can accumulate in soils in surprisingly large amounts (10–112 kilograms per hectare [10–

100 pounds per acre]), and their effect on wildlife is greatly increased as they become

associated with food chains. The stability of DDT and its relatives leads to their

accumulation in the bodily tissues of insects that constitute the diet of other animals

higher up the food chain, with toxic effects on the latter. Birds of prey such as eagles,

hawks, and falcons are usually most severely affected, and serious declines in their

populations have been traced to the effects of DDT and its relatives. Consequently, the use

of such chemicals began to be restricted in the 1960s and banned outright in the 1970s in

many countries.