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Insecticide - chemical substance


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Stomach poisons have gradually been replaced by synthetic

insecticide
s, 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.
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