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Insecticides

Insecticides


Insecticides are natural or synthetic chemicals used to manage insects pests; they are important for disease control and providing food and fiber for a growing world population. Insect control with chemicals began about 2,000 years ago with the use of natural products, whereas the age of synthetic insecticides began with the introduction of dichlorodiphenyl trichloroethane (DDT) in the 1940s. Here the major classes of insecticides are covered, and important example compounds provided. The discussion is organized topically by mode of action/target tissue. In addition to synthetic materials, natural products are addressed because their use is increasing in "organic farming." This entry then explores the development of plants or viruses genetically engineered to produce insect-selective toxins . The harmful effects of insecticides on humans will also be discussed, as well as some nonchemical control techniques.

Compounds Affecting Nerves

The pyrethroids are composed of natural pyrethrins, which are isolated from chrysanthemum flowers, as well as newer synthetic materials. Older pyrethroids (e.g., pyrethrins and tetramethrin) degrade too rapidly in the environment to be used in agriculture. They are used in buildings, and because of their general safety, they are even applied to humans to control lice. Newer pyrethroids have greater chemical stability (e.g., permethrin and deltamethrin), which allows their use on many types of field crops. Another important use of permethrin is application to mosquito netting. Intoxication by pyrethroids develops rapidly (in 1 to 2 minutes), and involves a rapid loss of normal posture and movement called "knockdown." Pyrethroids affect nerve impulse generation throughout the entire nervous system. Multiple nerve impulses occur when only a single one was expected, and there is an increased release of chemical neurotransmitters as well. These actions result in convulsions, prostration, and death.

Sabadilla, an extract from the seeds of a tropical lily, is used in home gardens and organic farming operations. It degrades rapidly in the environment, and causes signs of intoxication, and has a mode of action similar to that of pyrethroids. Sabadilla extract has low toxicity to mammals.

The tobacco compound nicotine has been used as an insecticide for over 200 years. It is especially effective against sucking insects, such as aphids, and has excellent contact activity . Related compounds are neonicotinoids (e.g., imidacloprid), which have similar insecticidal activity, but are less toxic to mammals. Nicotine and imidacloprid mimic the action of acetylcholine , which is the major excitatory neurotransmitter in an insect's central nervous system. The action of acetylcholine is stopped by the enzyme acetylcholinesterase, which rapidly breaks down acetylcholine. Nicotine and imidacloprid are also neuroexcitatory, but do so persistently, since they are not affected by acetylcholinesterase. Overstimulation of the nervous system often leads to convulsions, paralysis, and death.

The organophosphorus (OP) and carbamate insecticides are used to control a wide variety of insect pests. The acute toxicity of the OPs and carbamates varies, and many of them have high mammalian toxicity . These compounds react chemically with the active site of acetylcholinesterase, producing a blocked enzyme that cannot degrade acetylcholine. The concentration of acetylcholine then builds up and hyperexcitation occurs. The signs of intoxication include restlessness, tremors, convulsions, and paralysis. Blockage of acetylcholinesterase by OPs is persistent, and recovery of the enzyme takes many hours or even days. The mode of action of the carbamates

is similar, except that enzyme blockage is less stable and recovers in a matter of minutes. Among insects, carbamates are particularly toxic to hymenoptera, such as honeybees.

Organochlorines represent one of the oldest groups of synthetic insecticides, with only biodegradable materials such as lindane and endosulfan still used in pest control. High mammalian toxicity was common with organochlorines, but a newer compound, fipronil, has improved selective toxicity toward a variety of insect pests. These insecticides cause hyperexcitability and convulsions by blocking the inhibitory neurotransmitter γ aminobutyric acid (GABA). Normally, GABA has a dampening effect that maintains normal nerve activity. Blocking the effects of GABA removes inhibition, leading to hyperexcitation of the nervous system and convulsions.

Deet is an important insect repellant. This compound is applied to skin or clothing, and repels biting flies (e.g., blackflies and mosquitoes). Deet acts on the sensory nerves, causing insects to avoid treated surfaces.

Compounds Affecting Muscles

Ryania consists of the powdered stem of the tropical shrub, Ryania speciosa. The extract contains ryanodine and related compounds, and has a low toxicity to mammals. The powder is used as a stomach poison on vegetables and fruit. Ryanodine induces paralysis in insects by direct action on the muscles, resulting in sustained contraction and paralysis.

Avermectins are a group of closely related compounds isolated from the fungus Streptomyces avermitilis that are used to control the parasites of humans and animals, as well as arthropod pests in crops. They have fairly high mammalian toxicity, but their movement into treated leaves, oral activity against insect pests, and rapid breakdown in sunlight are all favorable properties. In insects and worms poisoned by avermectin, inactivity and flaccid paralysis occur from its relaxing effect on muscles.

Compounds Disrupting Energy Metabolism

These compounds vary, from the natural product rotenone (from Derris or Lonchocarpus root, used to control vegetable and fruit insects) to the synthetics sulfluramid and hydramethylnon (used to control mites and cockroaches). Interestingly, the highest acute toxicity to mammals is caused by the natural product rotenone. These compounds affect the production of adenosine triphosphate (ATP) , the energy storage molecule of the cell that is produced by mitochondria, the "powerhouse" of the cell. The disruption of energy metabolism and the subsequent loss of ATP result in a slowly developing toxicity, and the effects of all these compounds include inactivity, paralysis, and death.

Insect Growth Regulators

Insects exposed to diflubenzuron and related compounds are unable to form normal cuticle (skin) because their ability to synthesize it is lost. Thus, the cuticle becomes thin and brittle, and is unable to support the insect or to withstand molting, a process requiring the shedding of the old cuticle, as in metamorphosis. Diflubenzuron and other chitin synthesis inhibitors have extremely low mammalian toxicity and are used against termites.

Methoprene and fenoxycarb mimic the action of insect juvenile hormone in molting and reproduction, and have low toxicity to mammals. Exposure at molting produces deformed insects having mixed larval/pupal or larval/adult morphologies, and they disrupt reproductive physiology in adults to effectively serve as a method of birth control.

Tebufenozide acts by mimicking the effects of the insect hormone ecdysone, which along with juvenile hormone, controls the initiation of a molt. Exposure to tebufenozide induces a premature molt that traps the insect in its old cuticle. This compound is especially effective against caterpillars.

Toxins from Bacillus thuringiensis

The bacterium Bacillus thuringiensis forms an internal crystal that contains a number of insecticidal protein toxins. When eaten by the insect, the crystal dissolves in the midgut, the toxin mixture is released, and the proteins are cleaved into active forms. The toxins bind specifically to midgut cells and assemble a pore that leads to disintegration of the cells, gut paralysis, and death. B. thuringiensis strains have toxins specific for caterpillars, beetles, or flies. They have little or no effect on mammals.

Human Toxicity of Insecticides

In mammals DDT and related organochlorines have effects on the endocrine system, including the disruption of thyroid hormone synthesis and mimicking of the effects of estrogen . Liver cancer has also been observed in mice exposed to these substances, and there has been one claimed association between exposure to DDT and breast cancer. Epidemiological studies show a consistent connection between exposure to pesticides and the occurrence of Parkinson's disease in rural populations. A well-documented effect of some OPs is organophosphorus-induced delayed neuropathy , a slowly developing degeneration of the leg nerves that results in irreversible limping. A specific hazard of pyrethroids is paresthesia, a tingling or burning sensation in exposed skin.

Nonchemical Control Methods

There is considerable interest in developing genetically enhanced, insect-specific viruses or crop plants that would replace conventional chemical insecticides. Corn, cotton, and potatoes have been engineered to express B. thuringiensis toxins to control chewing insects. Although this approach has worked effectively for controlling some pests, others not targeted by the B. thuringiensis toxin must be controlled by other means.

Sex pheromones, chemicals that attract one sex of an insect to the other, also have uses in pest control. They are often utilized with traps to monitor the number of pest insects in an area, and when applied in the field at higher levels, they can disrupt reproduction or egg laying.

Biological control involves the introduction of predators and parasitoids to attack pests. The extent of control using this technique varies and can be quite good in some cases, but unforeseen ecological impacts occur when imported species attack nontarget organisms.

Chemical insecticides remain an important tool for managing insect pests of humans, animals, and food and fiber crops. Compounds that are persistent in the environment are no longer used, and the amounts sprayed have dropped from kilograms per acre to grams per acre of active ingredient. Future compounds and technologies will seek to maintain high levels of effectiveness with a reduced impact on the environment and human health.

see also Agricultural Chemistry; Herbicides; Pesticides.

Jeffrey R. Bloomquist

Bibliography

Budavari, Susan, ed. (1996). The Merck Index, 12th edition. Whitehouse Station, NJ: Merck & Co.

Massaro, Edward J., ed. (2002). Handbook of Neurotoxicology, Vol. 1. Totowa, NJ: Humana Press.

Meister, Richard T., ed. (2000). Farm Chemicals Handbook. Willoughby, OH: Meister Publishing.

Pedigo, Larry P. (1989). Entomology and Pest Management. New York: Macmillan.

Internet Resources

Miller, Terry L. "ExToxNetThe Extension Toxicology Network." Available from <http://ace.orst.edu/info/extoxnet/>.

Ware, George W. "An Introduction to Insecticides." Available from <http://www.ipmworld.umn.edu>.

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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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insecticide

insecticide A chemical substance that is toxic to insects and is used to control infestations of insect pests. Insecticides may be derived from substances that occur naturally in plants (e.g. pyrethrum, prepared from the flowers of Chrysanthemum cinerariaefolium, and derris (or rotenone), prepared from the roots of Derris elliptica) or are synthesized industrially.

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insecticide

insecticide (in-sekt-i-syd) n. a preparation used to kill destructive or disease-carrying insects. Some insect powders contain organic phosphorus compounds and fluorides; when ingested accidentally they may cause damage to the nervous system. The use of such compounds is generally under strict control. See also DDT.

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insecticide

in·sec·ti·cide / inˈsektiˌsīd/ • n. a substance used for killing insects. DERIVATIVES: in·sec·ti·cid·al / -ˌsektiˈsīdl/ adj.

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insecticide

insecticide See pesticide.

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insecticide

insecticidebackside, trackside •bedside • airside •Tayside, wayside •lakeside • stateside • graveside •quayside, seaside, Teesside •beachside • hillside • ringside •suicide • herbicide • regicide •fungicide • filicide • Barmecide •homicide •germicide, spermicide •tyrannicide • parricide •fratricide, matricide, patricide •uxoricide • countryside • infanticide •insecticide • pesticide • parasiticide •mountainside • Merseyside •Tyneside •dioxide, dockside, hydroxide, monoxide, oxide, peroxide •alongside •diopside, topside •broadside • downside • roadside •poolside • upside • nearside •fireside • Humberside • underside •genocide • waterside • riverside •silverside • overside •kerbside (US curbside) • Burnside

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Insecticides

Insecticides

History: yesterday and today

Nature of insecticices

Kinds of insecticides

Benefits of insecticide use

Damages caused by insecticide use

Resources

An insecticide is a substance used by humans to gain some advantage in the struggle with various insects that are considered pests. In the sense used here, a pest insect is considered undesirable, from the human perspective, because: (a) it is a vector that transmits disease-causing pathogens to humans (such as those causing malaria or yellow fever), or other diseases to livestock or crop plants; or (b) it causes a loss of the productivity or economic value of crop plants, domestic animals, or stored foodstuffs. The abundance and effects of almost all insect pests can be managed through the judicious use of insecticides.

However, the benefits of insecticide use are partly offset by important damages that may result. There are numerous cases of people being poisoned by accidental exposures to toxic insecticides. More commonly, ecological damage may be caused by the use of insecticides, sometimes resulting in the deaths of large numbers of wildlife.

History: yesterday and today

Humans have been using insecticides for thousands of years. The Egyptians used unspecified chemicals to combat fleas in their homes about 3,500 years ago, and arsenic has been used as an insecticide in China for at least 2,900 years. Today of course, insecticide use is much more prevalent. During the 1990s, more than 300 insecticides were available, in hundreds of different formulations and commercial products (which may involve similar formulations manufactured by different companies).

Nature of insecticices

Almost all insecticides are chemicals. Some are natural biochemicals extracted from plants, while others are inorganic chemicals based on toxic metals or compounds of arsenic. However, most modern insecticides are organic chemicals that have been synthesized by chemists. The costs of developing a new insecticide and testing it for its usefulness, toxicology, and environmental effects are huge, equivalent to tens of millions of dollars. However, if an insecticide effective against an important pest is discovered, the profits are also potentially huge.

Kinds of insecticides

Insecticides are an extremely diverse group of chemicals, plus additional formulations based on living microorganisms. The most important groups of insecticides are described below.

  • Inorganic insecticides are compounds containing arsenic, copper, lead, or mercury. They are highly persistent in terrestrial environments, being slowly dispersed by leaching and erosion by wind and water. Inorganic insecticides are used much less than in the past, having been widely replaced by synthetic organics. Examples of insecticides include Paris green (a mixture of copper compounds), lead arsenate, and calcium arsenate.
  • Natural organic insecticides are extracted from plants. They include nicotine extracted from tobacco (usually applied as nicotine sulphate), pyrethrum extracted from daisylike plants, and rotenone from several tropical shrubs.
  • Chlorinated hydrocarbons (or organochlorines) are synthetic insecticides, including DDT (dichlorodiphe-nyltrichloroethane) and its relatives DDD (dichloro-diphenyl-dichloroethane) and methoxychlor, lindane, and cyclodienes such as chlordane, heptachlor, aldrin, and dieldrin. Residues of organochlorines are quite persistent in the environment, having a half-life of about 10 years in soil. They are virtually insoluble in water, but are highly soluble in fats and lipids. Their persistence and strongly lipophilic nature causes organochlorines to bio-concentrate and to further food-web magnify in high concentrations in species at the top of food webs.
  • Organophosphate insecticides include fenitrothion, malathion, parathion, and phosphamidon. These are not very persistent in the environment, but most are extremely toxic to arthropods and also to non-target fish, birds, and mammals.
  • Carbamate insecticides include aldicarb, aminocarb, carbaryl, and carbofuran. They have a moderate persistence in the environment, but are highly toxic to arthropods, and in some cases to vertebrates.
  • Synthetic pyrethroids are analogues of natural pyrethrum, and include cypermethrin, deltamethrin, permethrin, synthetic pyrethrum and pyrethrins, and tetramethrin. They are highly toxic to invertebrates and fish, but are of variable toxicity to mammals and of low toxicity to birds.
  • More minor groups of synthetic organic insecticides include the formamidines (e.g., amitraz, formeta-nate) and dinitrophenols (e.g., binapacryl, dinocap).
  • Biological insecticides are formulations of microbes that are pathogenic to specific pests, and consequently have a relatively narrow spectrum of activity in ecosystems. An example is insecticides based on the bacterium Bacillus thuringiensis (or B.t ). There are also insecticides based on nuclear polyhedrosis virus (NPV) and insect hormones.

Benefits of insecticide use

Humans have attained important benefits from many uses of insecticides, including: (1) increased yields of crops because of protection from defoliation and diseases; (2) prevention of much spoilage of stored foods; and (3) prevention of certain diseases, which conserves health and has saved the lives of millions of people and domestic animals. Pests destroy an estimated 37% of the potential yield of plant crops in North America. Some of this damage can be reduced by the use of insecticides. In addition, insecticide spraying is one of the crucial tools used to reduce the abundance of mosquitoes and other insects that carry certain diseases (such as malaria) to humans. The use of insecticides to reduce the populations of these vectors has resulted in hundreds of millions of people being spared the deadly or debilitating effects of various diseases.

This is not to say that more insecticide use would yield even greater benefits. In fact, it has been argued that pesticide use in North America could be decreased by one-half without causing much of a decrease in crop yields, while achieving important environmental benefits through fewer ecological damages. In fact, three European countries (Sweden, Denmark, and the Netherlands) passed legislation in the 1990s, which required at least a 50% reduction in agricultural pesticide use by the year 2000. Similar actions have been proposed and adopted in North America in the 2000s.

Because of the substantial benefits of many uses of pesticides, their use has increased enormously since the 1950s. For example, pesticide usage increased by ten-fold in North America between 1945 and 1989, although it leveled off during the 1990s and early-2000s. Pesticide usage (including insecticides) is now a firmly integrated component of the technological systems used in modern agriculture, forestry, horticulture, and public-health management in most parts of the world.

Damages caused by insecticide use

The considerable benefits of many uses of insecticides are partially offset by damages caused to ecosystems and sometimes to human health. Each year about one million people are poisoned by pesticides (mostly by insecticides), including 20,000 fatalities. Although developing countries only account for about 20% of global pesticide use, they sustain about one-half of the poisonings. This result is because highly toxic insecticides are used in many developing countries, but with poor enforcement of regulations, illiteracy, and inadequate use of protective equipment and clothing. The most spectacular case of pesticide-related poisoning occurred in 1984 at Bhopal, India. About 2,800 people were killed and 20,000 seriously poisoned when a factory accidentally released 44 tons (40 tonnes) of vapors of methyl isocyanate to the atmosphere. (Methyl iso-cyanate is a precursor chemical used to manufacture carbamate insecticides.)

In addition, many insecticide applications cause ecological damage by killing non-target organisms (that is, organisms that are not pests). These damages are particularly important when broad-spectrum insecticides (i.e., that are not toxic only to the pest) are sprayed over a large area, such as an agricultural field or a stand of forest. Broadcast sprays of this sort expose many non-target organisms to the insecticide and cause unintended but unavoidable mortality. For instance, broadcast insecticide spraying causes non-target mortality to numerous arthropods other than the pest species, and birds, mammals, and other creatures may also be poisoned. The non-target mortality may include predators and competitors of the pest species, which may cause secondary damage by releasing the pest from some of its ecological controls.

Some of the best-known damage caused by insecticides involves DDT and related organochlorines, such as DDD, dieldrin, aldrin, and others. These chemicals were once widely used in North America and other industrialized countries, but their use was banned in the early 1970s. DDT was first synthesized in 1874, and its insecticidal properties were discovered in 1939. The first successful uses of DDT were during World War II (19391945), in programs to control body lice, mosquitoes, and other vectors of human diseases. DDT was quickly recognized as an extremely effective insecticide, and immediately after the war it was widely used in agriculture, forestry, and spray programs against malaria. The manufacturing and use of DDT peaked in 1970, when 385 million lb (175 million kg) were produced globally. At about that time, however, developed countries began to ban most uses of DDT. This action was taken because of ecological damages that were being caused by its use, including the contamination of humans and their agricultural food web, and the possibility that this was causing human diseases. However, the use of DDT has continued in less-developed countries, especially in the tropics, and mostly in programs against mosquito vectors of diseases.

Two physical-chemical properties of DDT and other organochlorines have an important influence on their ecological damages: their persistence and high solubility in fats. Chlorinated hydrocarbons are highly persistent in the environment because they are not easily degraded by microorganisms or physical agents such as sunlight or heat. DDT has a typical half-life in soil of about three years. In addition, DDT and related organochlorines are extremely insoluble in water, so they cannot be diluted into this abundant solvent. However, these chemicals are highly soluble in fats or lipids (i.e., they are lipophilic), which mostly occur in organisms. Consequently, DDT and related organochlorines have a powerful affinity for organisms, and therefore bio-concentrate into organisms in strong preference to the non-living environment. Moreover, organisms are efficient at assimilating any organochlorines present in their food. As a result, predators at the top of the food web develop the highest residues of organochlorines, particularly in their fatty tissues (this is known as food-web magnification). Both bio-concentration and food web magnification tend to be progressive with age, that is, the oldest individuals in a population are most contaminated. Although organochlorine residues are ubiquitous in the biosphere, much higher concentrations typically occur in animals that live close to areas where these chemicals have been used, such as North America.

Intense exposures to DDT and other organo-chlorines cause important ecological damages, including poisonings of birds. In some cases, bird kills were caused directly by the spraying of DDT in urban areas during the 1950s and 1960s to kill the beetle vectors of Dutch elmdisease. So much bird mortality occurred in sprayed neighborhoods that there was a marked reduction of bird songhence the title of Rachael Carsons (1962) book: Silent Spring, which is often considered a harbinger of the modern environmental movement in North America.

In addition to the direct toxicity of chlorinated hydrocarbons, more insidious damage was caused to birds and other wildlife over large regions. Mortality to many species was caused by longer-term, chronic toxicity, often occurring well away from sprayed areas. It took years of population monitoring and ecotoxicological research before organochlorines were identified as the causes of these damages. In fact, the chronic poisoning of birds and other wildlife can be considered an unanticipated problem that occurred because scientists (and society) had not had experience with the longer-term effects of persistent, bio-accumulating organochlorines.

Species of raptorial birds were among the most prominent victims of organochlorine insecticides. These birds are vulnerable because they feed at the top of their food web, and therefore accumulate organochlorines to high concentrations. Breeding populations of various raptors suffered large declines. In North America, these included the peregrine falcon (Falco peregrinus ), osprey (Pandion haliaetus ), bald eagle (Haliaeetus leucocephalus ), and golden eagle (Aquila chrysaetos ). In all cases, these birds were exposed to a combination (cocktail) of organochlorines that included the insecticides DDT, DDD (both of which are metabolized to DDE (dichlorodiphenyl-dichloroethylene) in organisms), aldrin, dieldrin, and heptachlor, as well as PCBs, a non-insecticide with many industrial uses. Research has suggested that DDT was the more important toxin to birds in North America, while cyclodienes (particularly dieldrin) were more important in Britain.

Damage caused to predatory birds was largely associated with chronic effects on reproduction, rather than toxicity to adults. Reproductive damages included the production of thin eggshells that could break under the weight of an incubating parent, high death rates of embryos and nestlings, and abnormal adult behavior. These effects all contributed to decreases in the numbers of chicks raised, which resulted in rapid declines in the sizes of populations of the affected birds.

Since the banning of most uses of DDT and other organochlorines in North America, their residues in wildlife have been declining. This has allowed previously affected species to increase in abundance. In 1999, for example, the U.S. Fish and Wildlife Service removed the peregrine falcon from the list of species considered endangered. Although the population recovery of the peregrine falcon was aided by a program of captive-breeding and release, its recovery would not have been possible if their exposure to organochlorines in wild habitats had not been first dealt with. As of February 2006, the American bald eagle is being considered for removal from the endangered species list after the federal government said in 1999 it was no longer in danger of extinction.

DDT and related organochlorine insecticides have largely been replaced by organophosphate and carbamate chemicals. These chemicals poison insects and other arthropods by inhibiting a specific enzyme, acetylcholine esterase (AChE), which is critical in the transmission of neural impulses. Vertebrates such as amphibians, fish, birds, and mammals are also highly sensitive to poisoning of their cholinesterase enzyme system. In all of these animals, acute poisoning of the AChE function by organophosphate and carbamate insecticides can cause tremors, convulsions, and ultimately death to occur.

Carbofuran is a carbamate insecticide that caused much bird mortality during its routine agricultural usage. For this reason, the further use of this chemical was banned in North America during the late 1990s. In 1996, it was discovered that agricultural use of the organophosphate monocrotophos against grasshoppers in Argentina was killing large numbers of Swainsons hawks (Buteo swainsoni ). This raptor breeds in the western United States and Canada and winters on the pampas of South America. Populations of Swainsons hawks had been declining for about fifteen years, and it appears the cause was poorly regulated use of monocrotophos on their wintering grounds. Because of risks of ecological damages caused by its use, monocrotophos has been banned in the United States. It was never registered for use in Canada, but it could be legally used in Argentina. These are two examples of non-organochlorine insecticides that cause important ecological damages. In 2006, an international agreement between industry, environmentalists, and government officials protect the Swainsons hawks from poisoning by monocrotophos in Argentina.

Of course, not all insecticides cause these kinds of serious ecological damages. For example, the toxicity of the bacterial insecticide B.t. is largely limited to moths, butterflies, beetles, and fliesit is essentially

KEY TERMS

Bioconcentration The occurrence of chemicals in much higher concentrations in organisms than in the ambient environment.

Broad-spectrum pesticide A pesticide that is not toxic only to the pest but other plant and animal species as well.

Ecotoxicology The study of the effects of toxic chemicals on organisms and ecosystems. Ecotoxicology considers both direct effects of toxic substances and also the indirect effects caused, for example, by changes in habitat structure or the abundance of food.

Food-web magnification The tendency for top predators in a food web to have the highest residues of certain chemicals, especially organochlorines.

Non-target organism Organisms that are not pests, but which may be affected by a pesticide treatment.

Pest Any organism judged to be significantly interfering with some human purpose.

non-toxic to most other invertebrates or vertebrate animals. Other relatively pest-specific insecticides are being developed and are increasing rapidly in use, often in conjunction with a so-called integrated pest management (or IPM) system. In IPM, insecticides may be used as a method of last resort, but heavy reliance is also placed on other methods of pest management. These include the cultivation of pest-resistant crop varieties, growing crops in rotation, modifying the habitat to make it less vulnerable to infestation, and other practices that reduce the overall impacts of pest insects.

The continued development of pest-specific insecticides and IPM systems will further reduce societys reliance on broad-spectrum insecticides and other damaging pesticides. Until this happens, however, the use of relatively damaging, broad-spectrum insecticides will continue in North America. In fact, the use of these chemicals is rapidly increasing globally, because they are becoming more prevalent in less-developed countries of tropical regions.

Resources

BOOKS

den Hond, Frank, Peter Groenewegen, and Nico M. van Straalen, eds. Pesticides: Problems, Improvements, Alternatives. Malden, MA: Blackwell Publishing, 2003.

Matthews, G.A. Pesticides: Health, Safety, and the Environment. Oxford, UK, and Ames, IA: Blackwell Publishing, 2003.

Molles, Manuel C. Ecology: Concepts and Applications. Boston, MA: McGraw-Hill, 2005.

Smith, Thomas M. Elements of Ecology. San Francisco, CA: Benjamin Cummings, 2008.

Ware, G.W. The Pesticide Book. 5th Ed. Fresno, CA: Thomson Publications, 2000.

Wilson, Michael F., ed. Optimising Pesticide Use. England: Wiley, 2003.

OTHER

Ware, George W., and David M. Whitacre, Radcliffes IPM World Textbook. An Introduction to Insecticides, 4th ed. 2004. <http://ipmworld.umn.edu/chapters/ware.htm> (accessed November 15, 2006).

Bill Freedman

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Insecticides

Insecticides


Introduction

An insecticide is a substance used by humans to gain some advantage in the struggle with various insects that are considered "pests." In the sense used here, a pest insect is considered undesirable, from the human perspective, because: (a) it is a vector that transmits diseasecausing pathogens to humans (such as those causing malaria or yellow fever ), or other diseases to livestock or crop plants; or (b) it causes a loss of the productivity or economic value of crop plants, domestic animals, or stored foodstuffs. The abundance and effects of almost all insect pests can be managed through the judicious use of insecticides.

However, the benefits of insecticide use are partly offset by important damages that may result. There are numerous cases of people being poisoned by accidental exposures to toxic insecticides. More commonly, ecological damage may be caused by the use of insecticides, sometimes resulting in the deaths of large numbers of wildlife .

Humans have been using insecticides for thousands of years. The Egyptians used unspecified chemicals to combat fleas in their homes about 3,500 years ago, and arsenic has been used as an insecticide in China for at least 2,900 years. Today of course, insecticide use is much more prevalent. During the 1990s, more than 300 insecticides were available, in hundreds of different formulations and commercial products (which may involve similar formulations manufactured by different companies).

Almost all insecticides are chemicals. Some are natural biochemicals extracted from plants, while others are inorganic chemicals based on toxic metals or compounds of arsenic. However, most modern insecticides are organic chemicals that have been synthesized by chemists. The costs of developing a new insecticide and testing it for its usefulness, toxicology , and environmental effects are huge, equivalent to at least $20-30 million. However, if an insecticide effective against an important pest is discovered, the profits are also potentially huge.


Kinds of insecticides

Insecticides are an extremely diverse group of chemicals, plus additional formulations based on living microorganisms . The most important groups of insecticides are described below.

  • Inorganic insecticides are compounds containing arsenic, copper , lead , or mercury. They are highly persistent in terrestrial environments, being slowly dispersed by leaching and erosion by wind and water . Inorganic insecticides are used much less than in the past, having been widely replaced by synthetic organics. Examples of insecticides include Paris green (a mixture of copper compounds), lead arsenate, and calcium arsenate.
  • Natural organic insecticides are extracted from plants. They include nicotine extracted from tobacco (usually applied as nicotine sulphate), pyrethrum extracted from daisy-like plants, and rotenone from several tropical shrubs.
  • Chlorinated hydrocarbons (or organochlorines) are synthetic insecticides, including DDT and its relatives DDD and methoxychlor, lindane, and cyclodienes such as chlordane , heptachlor, aldrin, and dieldrin. Residues of organochlorines are quite persistent in the environment, having a half-life of about 10 years in soil . They are virtually insoluble in water, but are highly soluble in fats and lipids. Their persistence and strongly lipophilic nature causes organochlorines to bio-concentrate and to further food-web magnify in high concentrations in species at the top of food webs.
  • Organophosphate insecticides include fenitrothion, malathion, parathion, and phosphamidon. These are not very persistent in the environment, but most are extremely toxic to arthropods and also to non-target fish , birds , and mammals .
  • Carbamate insecticides include aldicarb, aminocarb, carbaryl, and carbofuran. They have a moderate persistence in the environment, but are highly toxic to arthropods, and in some cases to vertebrates .
  • Synthetic pyrethroids are analogues of natural pyrethrum, and include cypermethrin, deltamethrin, permethrin, synthetic pyrethrum and pyrethrins, and tetramethrin. They are highly toxic to invertebrates and fish, but are of variable toxicity to mammals and of low toxicity to birds.
  • More minor groups of synthetic organic insecticides include the formamidines (e.g., amitraz, formetanate) and dinitrophenols (e.g., binapacryl, dinocap).
  • Biological insecticides are formulations of microbes that are pathogenic to specific pests, and consequently have a relatively narrow spectrum of activity in ecosystems. An example is insecticides based on the bacterium Bacillus thuringiensis (or B.t.). There are also insecticides based on nuclear polyhedrosis virus (NPV) and insect hormones.

Benefits of insecticide use

Humans have attained important benefits from many uses of insecticides, including: (1) increased yields of crops because of protection from defoliation and diseases; (2) prevention of much spoilage of stored foods; and (3) prevention of certain diseases, which conserves health and has saved the lives of millions of people and domestic animals. Pests destroy an estimated 37% of the potential yield of plant crops in North America . Some of this damage can be reduced by the use of insecticides. In addition, insecticide spraying is one of the crucial tools used to reduce the abundance of mosquitoes and other insects that carry certain diseases (such as malaria) to humans. The use of insecticides to reduce the populations of these vectors has resulted in hundreds of millions of people being spared the deadly or debilitating effects of various diseases.

This is not to say that more insecticide use would yield even greater benefits. In fact, it has been argued that pesticide use in North America could be decreased by one-half without causing much of a decrease in crop yields, while achieving important environmental benefits through fewer ecological damages. In fact, three European countries (Sweden, Denmark, and the Netherlands) passed legislation in the 1990s requiring at least a 50% reduction in agricultural pesticide use by the year 2000, and similar actions may eventually be adopted in North America.

Because of the substantial benefits of many uses of pesticides , their use has increased enormously since the 1950s. For example, pesticide usage increased by 10-fold in North America between 1945 and 1989, although it leveled off during the 1990s. Pesticide usage (including insecticides) is now a firmly integrated component of the technological systems used in modern agriculture, forestry , horticulture , and public-health management in most parts of the world.


Damages caused by insecticide use

The considerable benefits of many uses of insecticides are partially offset by damages caused to ecosystems and sometimes to human health. Each year about one million people are poisoned by pesticides (mostly by insecticides), including 20,000 fatalities. Although developing countries only account for about 20% of global pesticide use, they sustain about half of the poisonings. This is because highly toxic insecticides are used in many developing countries, but with poor enforcement of regulations, illiteracy, and inadequate use of protective equipment and clothing. The most spectacular case of pesticide-related poisoning occurred in 1984 at Bhopal, India. About 2,800 people were killed and 20,000 seriously poisoned when a factory accidentally released 44 tons (40 tonnes) of vapors of methyl isocyanate to the atmosphere. (Methyl isocyanate is a precursor chemical used to manufacture carbamate insecticides.)

In addition, many insecticide applications cause ecological damage by killing non-target organisms (that is, organisms that are not pests). These damages are particularly important when broad-spectrum insecticides (i.e., that are not toxic only to the pest) are sprayed over a large area, such as an agricultural field or a stand of forest. Broadcast sprays of this sort expose many non-target organisms to the insecticide and cause unintended but unavoidable mortality. For instance, broadcast insecticide spraying causes non-target mortality to numerous arthropods other than the pest species, and birds, mammals, and other creatures may also be poisoned. The non-target mortality may include predators and competitors of the pest species, which may cause secondary damage by releasing the pest from some of its ecological controls.

Some of the best-known damage caused by insecticides involves DDT and related organochlorines, such as DDD, dieldrin, aldrin, and others. These chemicals were once widely used in North America and other industrialized countries, but their use was banned in the early 1970s. DDT was first synthesized in 1874, and its insecticidal properties were discovered in 1939. The first successful uses of DDT were during the Second World War, in programs to control body lice , mosquitoes, and other vectors of human diseases. DDT was quickly recognized as an extremely effective insecticide, and immediately after the war it was widely used in agriculture, forestry, and spray programs against malaria. The manufacturing and use of DDT peaked in 1970, when 385 million lb (175 million kg) were produced globally. At about that time, however, developed countries began to ban most uses of DDT. This action was taken because of ecological damages that were being caused by its use, including the contamination of humans and their agricultural food web, and the possibility that this was causing human diseases. However, the use of DDT has continued in less-developed countries, especially in the tropics, and mostly in programs against mosquito vectors of diseases.

Two physical-chemical properties of DDT and other organochlorines have an important influence on their ecological damages: their persistence and high solubility in fats. Chlorinated hydrocarbons are highly persistent in the environment because they are not easily degraded by microorganisms or physical agents such as sunlight or heat . DDT has a typical half-life in soil of about three years. In addition, DDT and related organochlorines are extremely insoluble in water, so they cannot be "diluted" into this abundant solvent. However, these chemicals are highly soluble in fats or lipids (i.e., they are lipophilic), which mostly occur in organisms. Consequently, DDT and related organochlorines have a powerful affinity for organisms, and therefore bio-concentrate into organisms in strong preference to the non-living environment. Moreover, organisms are efficient at assimilating any organochlorines present in their food. As a result, predators at the top of the food web develop the highest residues of organochlorines, particularly in their fatty tissues (this is known as food-web magnification). Both bio-concentration and food-web magnification tend to be progressive with age, that is, the oldest individuals in a population are most contaminated. Although organochlorine residues are ubiquitous in the biosphere , much higher concentrations typically occur in animals that live close to areas where these chemicals have been used, such as North America.

Intense exposures to DDT and other organochlorines cause important ecological damages, including poisonings of birds. In some cases, bird kills were caused directly by the spraying of DDT in urban areas during the 1950s and 1960s to kill the beetle vectors of Dutch elm disease. So much bird mortality occurred in sprayed neighborhoods that there was a marked reduction of bird song—hence the title of Rachael Carson's (1962) book: Silent Spring, which is often considered a harbinger of the modern environmental movement in North America.

In addition to the direct toxicity of chlorinated hydrocarbons, more insidious damage was caused to birds and other wildlife over large regions. Mortality to many species was caused by longer-term, chronic toxicity, often occurring well away from sprayed areas. It took years of population monitoring and ecotoxicological research before organochlorines were identified as the causes of these damages. In fact, the chronic poisoning of birds and other wildlife can be considered an unanticipated "surprise" that occurred because scientists (and society) had not had experience with the longer-term effects of persistent, bio-accumulating organochlorines.

Species of raptorial birds were among the most prominent victims of organochlorine insecticides. These birds are vulnerable because they feed at the top of their food web, and therefore accumulate organochlorines to high concentrations. Breeding populations of various raptors suffered large declines. In North America these included the peregrine falcon (Falco peregrinus), osprey (Pandion haliaetus), bald eagle (Haliaeetus leucocephalus), and golden eagle (Aquila chrysaetos). In all cases, these birds were exposed to a "cocktail" of organochlorines that included the insecticides DDT, DDD (both of which are metabolized to DDE in organisms), aldrin, dieldrin, and heptachlor, as well as PCBs, a non-insecticide with many industrial uses. Research has suggested that DDT was the more important toxin to birds in North America, while cyclodienes (particularly dieldrin) were more important in Britain.

Damage caused to predatory birds was largely associated with chronic effects on reproduction, rather than toxicity to adults. Reproductive damages included the production of thin eggshells that could break under the weight of an incubating parent, high death rates of embryos and nestlings, and abnormal adult behavior . These effects all contributed to decreases in the numbers of chicks raised, which resulted in rapid declines in the sizes of populations of the affected birds.

Since the banning of most uses of DDT and other organochlorines in North America, their residues in wildlife have been declining. This has allowed previously affected species to increase in abundance. In 1999, for example, the U.S. Fish and Wildlife Service removed the peregrine falcon from the list of species considered endangered. Although the population recovery of the peregrine falcon was aided by a program of captive-breeding and release, its recovery would not have been possible if their exposure to organochlorines in wild habitats had not been first dealt with.

DDT and related organochlorine insecticides have largely been replaced by organophosphate and carbamate chemicals. These chemicals poison insects and other arthropods by inhibiting a specific enzyme , acetylcholine esterase (AChE), which is critical in the transmission of neural impulses. Vertebrates such as amphibians , fish, birds, and mammals are also highly sensitive to poisoning of their cholinesterase enzyme system. In all of these animals, acute poisoning of the AChE function by organophosphate and carbamate insecticides can cause tremors, convulsions, and ultimately death to occur.

Carbofuran is a carbamate insecticide that caused much bird mortality during its routine agricultural usage. For this reason, the further use of this chemical was banned in North America during the late 1990s. In 1996, it was discovered that agricultural use of the organophosphate monocrotophos against grasshoppers in Argentina was killing large numbers of Swainson's hawks (Buteo swainsoni). This raptor breeds in the western United States and Canada and winters on the pampas of South America . Populations of Swain-son's hawks had been declining for about 10 years, and it appears the cause was poorly regulated use of monocrotophos on their wintering grounds. Because of risks of ecological damages caused by its use, monocrotophos has been banned in the United States and was never registered for use in Canada, but it could be legally used in Argentina. These are two examples of non-organochlorine insecticides that cause important ecological damages.

Of course, not all insecticides cause these kinds of serious ecological damages. For example, the toxicity of the bacterial insecticide B.t. is largely limited to moths , butterflies , beetles , and flies—its is essentially non-toxic to most other invertebrates or vertebrate animals. Other relatively pest-specific insecticides are being developed and are increasing rapidly in use, often in conjunction with a so-called "integrated pest management" (or IPM) system. In IPM, insecticides may be used as a method of last resort, but heavy reliance is also placed on other methods of pest management. These include the cultivation of pest-resistant crop varieties, growing crops in rotation , modifying the habitat to make it less vulnerable to infestation, and other practices that reduce the overall impacts of pest insects.

The continued development of pest-specific insecticides and IPM systems will further reduce society's reliance on broad-spectrum insecticides and other damaging pesticides. Until this happens, however, the use of relatively damaging, broad-spectrum insecticides will continue in North America. In fact, the use of these chemicals is rapidly increasing globally, because they are becoming more prevalent in less-developed countries of tropical regions.


Resources

books

Freedman, B. Environmental Ecology. 2nd Ed. San Diego, CA: Academic Press, 1995.

Thomson, W.T. Agricultural Chemicals, Book I, Insecticides. Fresno, CA: Thomson Publications, 1992.

Ware, G.W. The Pesticide Book. 5th ed. Fresno, CA: Thomson Publications, 2000.

periodicals

Pimentel, D., H. Acquay, M. Biltonen, P. Rice, M. Silva, J. Nelson, V. Lipner, S. Giordano, A. Horowitz, and M. D'A-mare. "Environmental End Economic Costs of Pesticide Use." Bioscience 42 (1992): 750-760.

other

Ware, G.W. An Introduction to Insecticides. 3rd ed. University of Arizona. 2000. <http://ipmworld.umn.edu/chapters/ware.htm>.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bioconcentration

—The occurrence of chemicals in much higher concentrations in organisms than in the ambient environment.

Broad-spectrum pesticide

—A pesticide that is not toxic only to the pest but other plant and animal species as well.

Ecotoxicology

—The study of the effects of toxic chemicals on organisms and ecosystems. Ecotoxicology considers both direct effects of toxic substances and also the indirect effects caused, for example, by changes in habitat structure or the abundance of food.

Food-web magnification

—The tendency for top predators in a food web to have the highest residues of certain chemicals, especially organochlorines.

Non-target organism

—Organisms that are not pests, but which may be affected by a pesticide treatment.

Pest

—Any organism judged to be significantly interfering with some human purpose.

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