Insecticides

insecticides, chemical, biological, or other agents used to destroy insect pests; the term commonly refers to chemical agents only.

Chemical Insecticides

The modern history of chemical insecticides in the United States dates from 1867, when Paris green proved effective against the Colorado potato beetle. Within a decade Paris green and kerosene oil emulsion were being employed against a variety of chewing and sucking insects. In the early part of the 20th cent. fluorine compounds and plant-derived insecticides were developed. Except for plant derivatives such as nicotine, pyrethrin, and rotenone, early insecticides were almost all inorganic chemicals. The discovery in Europe in 1939 of the insecticidal value of DDT, a synthetic organic compound, led to the synthesis of thousands of organic molecules in a search for potent chemicals. Today several hundred chemical insecticidal agents are registered by the U.S. Environmental Protection Agency and licensed in more than 10,000 formulations. Promptly effective, easy to use, and readily available, chemicals have become the modern weapons of choice against insects, contributing to stable food and fiber productivity, to human and animal health, and to the comfort and quality of human life.

As early as the 1920s, insecticide use in the United States prompted concerns over residues in foodstuffs and calls for regulation. In the 1960s, with increasing worldwide interest in environmental protection, chemical insecticides became objects of scientific and popular protest. Critics charged that chemical insecticides were dangerous and self-defeating, provoking the development of resistance by target pests, sabotaging ecological systems, and poisoning people and other organisms as well as the environment. In response, governments have restricted or proscribed many of the most dangerous insecticides, including many chlorinated hydrocarbon standbys: DDT, benzene hexachloride, lindane, aldrin, dieldrin, chlordane, heptachlor, endrin, and toxaphene—all powerful, broad-spectrum contact and stomach poisons.

Chemists, meanwhile, have invented alternative insecticides that attack selectively instead of indiscriminately, and that break down into nontoxic substances in the environment. Organophosphates attack insect nervous systems, much like the chlorinated hydrocarbons, but are much quicker to break down into nontoxic substances. A large and versatile group, the organophosphates include parathion, with one of the highest mammalian toxicities, and Malathion, with one of the lowest. Carbamate insecticides, esters of carbanilic acid that kill insect larvae, nymphs, and adults on contact, have gained favor because they break down even more quickly than organophosphates and are less hazardous to humans. Among the carbamates is Sevin, or carbaryl, an N-methyl aromatic carbamate ester.

Alternatives: Biological Insecticides

The liabilities of chemical insecticides have encouraged interest in biological controls, which turn natural processes and mechanisms against pest insects and have few if any harmful side effects. Biological controls include using predators, parasites, and pathogens to kill target insects without harming other organisms. In another strategy, huge numbers of sterilized male insects are released to compete with fertile males for mates, diminishing the population of the next generation. Interest is growing in the use of synthetic insect hormones to disrupt pests' vital processes, such as growth; and synthetic pheromones, powerful insect sex attractants, to monitor pest populations, sabotage pest reproduction, and lure pests into traps. In practice, however, some of the environmentally attractive features of biological insecticides—their inherently slow and selective activity, their strict management requirements—can make them economically unattractive to farmers. Increasingly, therefore, biological and chemical methods are coordinated in Integrated Pest Management programs.

Bibliography

See R. Carson, Silent Spring (1962); A. Mallis, Handbook of Pest Control (7th ed. 1990); G. J. Marco et al., ed., Regulation of Agrochemicals (1991); R. L. Metcalf, Destructive and Useful Insects: Their Habits and Control (5th ed. 1992).

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Insecticides 

chemicals for the control of insect pests. Depending on the path by which they penetrate the insect organism, they are divided into four groups.

Stomach poisons, which enter the organism through the mouth, include most inorganic arsenic compounds (calcium, magnesium, barium, and lead arsenates; calcium arsenite), silicofluorides and fluorides of metals, thiodiphenylamine, and a group of special preparations (Eulans, mitin, irgan, and others), which are used for the protection of wool and furs from destruction by moths.

Contact poisons, which penetrate through the skin, include the organic compounds of phosphorus, chlorine, nitrogen, and sulfur and the pyrethrins and pyrethroids.

Systemic poisons, which are absorbed by the roots and leaves of plants, migrate within the vascular system with the nutrients, making the plants poisonous to parasitic insects; such poisons include metilmerkaptofos and fosfamid. Systemic (organic phosphorus) insecticides are also used to control ectoparasites of animals (the blood of the animal becomes poisonous to insects after the introduction of the preparation) and to eliminate rats (leading to the death of the animal, which is the reservoir of the infection, and of the parasitic carriers). Butadion is used in exceptional cases for delousing humans. A single dose renders human blood insecticidal for a period of two weeks.

Fumigants, or inhalation insecticides, enter the organism of insects in the vapor or gaseous state through the tracheal system during breathing. These materials include hexachlorobutadiene and dikhlorfos, as well as finely milled silicates and mineral oils, which disrupt the respiratory functions of insects. The accepted insecticide classification is arbitrary, since most insecticides are capable of penetrating the organism simultaneously by several paths. For this reason, some preparations are included in a particular group based on the principal path of entry.

Worldwide losses caused by insect pests are estimated at $30 billion in agriculture alone. For this reason, total insecticide production is increasing. At the same time, the world production of inorganic insecticides, such as arsenic and fluorine compounds, is decreasing because of their high toxicity; in several European countries their production has been completely discontinued. On the other hand, the overall production increase is due to the manufacture of new organic compounds. The worldwide line of available insecticides includes more than 200 names. Organic compounds of phosphorus, chlorine, and carbamic acid derivatives are the most commonly used materials. Some of the organic insecticides used in the USSR are listed in Table 1.

Insecticides are applied by spraying, dusting, fumigation, and impregnation. There is a variety of physical forms of these preparations, such as dusts, emulsions or suspensions, and wettable powders.

Depending on the degree of toxicity to humans and warmblooded animals, the insecticides are divided into four groups: strongly active (LD₅₀ up to 50), highly toxic (50–200), moderately toxic (200–1,000), and low-toxicity (above 1,000). The persistence of insecticide activity on plants or in the bodies of animals varies widely, from one day to several years. The rules concerning the storage, application, and transportation of insecticides should be strictly observed in order to prevent harmful effects, such as contamination of water reservoirs and poisoning of bees, bumblebees, other pollinating insects, and parasitic and predatory insects, and accumulation in animal and vegetable products and in feeds.

REFERENCES

Mel’nikov, N. N. Khimiia pestitsidov. Moscow, 1968.
Chemie der Pflanzenschutz- und Schadlingsbekämpfungsmittel, vol. 1. Edited by R. Wegler. Berlin, 1970.

Table 1. Most important organic insecticides used in the USSR (LD50 is the average dose, in mg/kg liveweight, at which 50% of the affected anima1s die)
	Chemical name	LD50	Form	Purpose
Chloroorganic insecticides
HCBD .............	Hexachlorobutadiene	200–250	Liquid	Soil fumigant for control ot phylloxera and vine louse
HCCH (lindane) .............	γ-hexachlorocyclohexane	125	Suspensions, powders, aerosoles, etc.	For control of locust pests and larvae of the click beetle, nocturnal ground beetle, cabbage and cotton moths, and silver γ moth
Heptachlor ............	1,4,5,6,7,8,8-heptachloro-4,7-endomethylene- 3a,4,7a-tetrahydrindene	60–135	60% emulsion concentrate	Seed disinfectant for all crops, excluding edible root crops; for control of gray weevils and beet pests, flea beetles, and larvae of the click beetle, nocturnal ground beetle, and fly
Polychlorocamphene ....	[Mixture of polychloroterpenes]	60–200	50% emulsion concentrate	Treatment of potato plants against Colorado beetle and of sugar beet against sugar beet flea beetles and beet pests
Polychloropinene .............	[Same as polychlorocamphene]	350	20% and 50% oil solution; 65% emulsion concentrate	Treatment of potato plants against Colorado beetle and of sugar beet against sugar beet flea beetles and beet pests Same as polychlorocamphene
Organic phosphorus insecticides
Karbofos (malathion) ........	O.O-dimethyl-S(1,2-dicarbethoxyethyl) phosphorodithioate	500–1,500	35% emulsion concentrate	For control of aphids
Metalos (methyl parathion) ............	O,O-dimethyl-O 4-nitrophenyl-thiophosphate	25–50	′20% emulsion concentrate; 2.5% dust	For control of shield bugs, aphids, thrips, mealybugs, and other pests
Metilmerkaptofos (Metasystox) ..................	[Mixture of O,O-dimethyl-ethylthiophosphate with its thio isomer (70:30))	80–100	30% emulsion concentrate	For control of aphids
Metilnitrofos ........	[Mixture of 0,O-dimethyl-O-4-nitro-3-methylphenyl-thiophosphate with its 6-nitro isomer (70:30)]	400–1,000	Same as metilmerkaptofos	Same as metilmerkaptofos
Trikhlormetafos-3 .......	O-methyl-O-ethyl-2,4,5-trichloro-phenylthiophosphate	330–800	30–50% emulsion concentrate	For control of jumping plant lice and hoppers, aphids, scale insects, fly larvae, and other pests
Fosfamitd (Rogor) .........	O,O-dimethyl-S-(N-methylcarbamoyl-methyl)-dithiophosphate	200–250	40% emulsion concentrate	For control of sucking and gnawing pests
Khlorofos (trikhlorfon) .........	O,O-dimethyl-2,2,2-trichloro-1-hydroxy-ethylphosphonate	630	80% wettable powder,7% granulated powder	For control of shield bugs, caterpillars and larvae, aphids, thrips, horseflies, and houseflies
Carbamic acid derivatives
Sevin (naphtylcarbamate, Carbaryl ......	N-methyl-1 -naphthyl-carbamate	560	85% wettable powder	For control of codling moths, plum fruit moths, tortrix moths, black-veined white butterflies, aphids, bugs on Cruciferae fruit plants, Colorado beetle, and many other pests
Plant insecticides
Anabasine ..............	(2-piperidyl)-pyridine	70–100	Aqueous solution	Used mainly to control aphids and thrips in bean crops, flax, tobaceo, sugar beet, hops, and fruit and berry crops
Pyrethrin (powdered Dalmatia chrysanthemum) ............	[Same as anabasine]	Same AS anabasine	Powder	For control of bedbugs, lice, and other insects

Berim, N. G. Biologicheskie osnovy primeneniia insektitsidov. Leningrad, 1971.

N. N. MEL’NIKOV