- Created: 21-01-22
- Last Login: 21-01-22
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Stomach poisons have gradually been replaced by synthetic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.
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.
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.
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.
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.