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Pesticides are substances or a mixture of substances, of chemical or biological origin, used by human society to mitigate or repel pests such as bacteria, nematodes, insects, mites, mollusks, birds, rodents, and other organisms that affect food production or human health. They usually act by disrupting some component of the pest's life processes to kill or inactivate it. In a legal context, pesticides also include substances such as insect attractants, herbicides, plant defoliants, desiccants, and plant growth regulators.

History of Pesticides

The concept of pesticides is not new. Around 1000 b.c.e. Homer referred to the use of sulfur to fumigate homes and by 900 c.e. the Chinese were using arsenic to control garden pests. Although major pest outbreaks have occurred, such as potato blight (Phytopthora infestans ), which destroyed most potato crops in Ireland during the mid-nineteenth century, not until later that century were pesticides such as arsenic, pyrethrum, lime sulfur, and mercuric
chloride used. Between this period and World War II, inorganic and biological substances, such as Paris green, lead arsenate, calcium arsenate, selenium compounds, limesulfur, pyrethrum, thiram, mercury, copper sulfate, derris, and nicotine were used, but the amounts and frequency of use were limited, and most pest control employed cultural methods such as rotations, tillage, and manipulation of sowing dates. After World War II the use of pesticides mushroomed, and there are currently more than 1,600 pesticides available and about 4.4 million tons used annually, at a cost of more than $20 billion. The United States accounts for more than 25 percent of this market.

Older Insecticides

The first synthetic organochlorine insecticide, DDT (dichlorodiphenyl-trichloroethane), discovered in Switzerland in 1939, was very effective and used extensively to control head and body lice, human disease vectors and agricultural pests, in the decades leading up to the 1970s. Benzene hexachloride (BHC) and chlordane were discovered during World War II and toxaphene (and heptachlor) slightly later. Shortly thereafter, two cyclodiene organochlorines, aldrin and dieldrin, were introduced, followed by endrin, endosulfan, and isobenzan. All these insecticides acted by blocking an insect's nervous system, causing malfunction, tremors, and death. All organochlorines are relatively insoluble, persist in soils and aquatic sediments, can bioconcentrate in the tissues of invertebrates and vertebrates from their food, move up trophic chains, and affect top predators. These properties of persistence and bioaccumulation led eventually to the withdrawal of registration and use of organochlorine insectides, from 1973 to the late 1990s, in industrialized nations, although they continued to be used in developing countries.

Organophosphate insecticides originated from compounds developed as nerve gases by Germany during World War II. Thus, those developed as insecticides, such as tetraethyl pyrophosphate (TEPP) and parathion, had high mammalian toxicities. Scores of other organophosphates including demeton, methyl schradan, phorate, diazinon, disulfoton, dimethoate, trichlorophon, and mevinphos have been registered. In insects, as in mammals, they act by inhibiting the enzyme cholinesterase (ChE) that breaks down the neurotransmitter acetylcholine (ACh) at the nerve synapse, blocking impulses and causing hyperactivity and tetanic paralysis of the insect, then death. Some are systemic in plants and animals, but most are not persistent and do not bioaccumulate in animals or have significant environmental impacts.

Carbaryl, the first carbamate insecticide, acts on nervous transmissions in insects also through effects on cholinesterase by blocking acetylcholine receptors. Other carbamate insecticides include aldicarb, methiocarb, methomyl, carbofuran, bendiocarb, and oxamyl. In general, although they are broad-spectrum insecticides, of moderate toxicity and persistence, they rarely bioaccumulate or cause major environmental impacts.

Botanical insecticides include nicotine from tobacco, pyrethrum from chrysanthemums, derris from cabbage, rotenone from beans, sabadilla from lilies, ryania from the ryania shrub, limonene from citrus peel, and neem from the tropical neem tree. Most, other than nicotine, have low levels of toxicity in mammals and birds and create few adverse environmental effects.

Newer Insecticides

Synthetic pyrethroid insecticides, with structures based on the natural compound pyrethrum, were introduced in the 1960s and include tetramethrin, resmethrin, fenvalerate, permethrin, lambda-cyalothrin, and deltamethrin, all used extensively in agriculture. They have very low mammalian toxicities and potent insecticidal action, are photostable with low volatilities and persistence. They are broad-spectrum insecticides and may kill some natural enemies of pests. They do not bioaccumulate and have few effects on mammals, but are very toxic to aquatic invertebrates and fish.

In recent years, new classes of insecticides have been marketed, none of which are persistent or bioaccumulate. They include juvenile hormone mimics, synthetic versions of insect juvenile hormones that act by preventing immature stages of the insects from molting into an adult, and avermectins, natural products produced by soil microorganisms, insecticidal at very low concentrations. Bacillus thuringiensis toxins are proteins produced by a bacterium that is pathogenic to insects. When activated in the insect gut, they destroy the selective permeability of the gut wall. The first strains were toxic only to Lepidoptera, but strains toxic to flies and beetles have since been developed. B. thuringiensis has been incorporated into plants genetically.


Soil nematocides , such as dichlopropene, methyl isocyanate, chloropicrin, and methyl bromide, are broad-spectrum soil fumigants. Others, aldicarb, dazomet, and metham sodium, act mainly through contact. All have very high mammalian toxicities and can kill a wide range of organisms from both the plant and animal kingdoms. Although transient in soil, they may have drastic ecological effects on soil systems.


Two molluscicides , metaldehyde and methiocarb, are used as baits against slugs and snails. Although of high mammalian toxicity, they cause few problems other than the occasional accidental death of wild mammals. Several molluscicides, used to control aquatic snails, N -trityl morpholine, copper sulfate, niclosamine, and sodium pentachlorophenate, are toxic to fish.


Hormone-type herbicides such as 2,4,5-T; 2,4-D; and MCPA; were discovered during the 1940s. They do not persist in soil, are selective in their toxicity to plants, are of low mammalian toxicity, cause few direct environmental problems, but are relatively soluble and reach waterways and groundwater. Contact herbicides, which kill weeds through foliage applications, include dintrophenols, cyanophenols, pentachlorophenol, and paraquat. Most are nonpersistent, but triazines can persist in the soil for several years, are slightly toxic to soil organisms and moderately so to aquatic organisms. Herbicides cause few direct environmental problems other than their indirect effects, in leaving bare soil, which is free of plant cover and susceptible to erosion.


Many different types of fungicides are used, of widely differing chemical structures. Most have relatively low mammalian toxicities, and except for carbamates such as benomyl, a relatively narrow spectrum of toxicity to soil-inhabiting and aquatic organisms. Their greatest environmental impact is toxicity to soil microorganisms, but these effects are short term.

Effects on the Terrestrial Environment

Pesticides are biocides designed to be toxic to particular groups of organisms. They can have considerable adverse environmental effects, which may be extremely diverse: sometimes relatively obvious but often extremely subtle and complex. Some pesticides are highly specific and others broad spectrum; both types can affect terrestrial wildlife, soil, water systems, and humans.

Pesticides have had some of their most striking effects on birds, particularly those in the higher trophic levels of food chains, such as bald eagles, hawks, and owls. These birds are often rare, endangered, and susceptible to pesticide residues such as those occurring from the bioconcentration of organochlorine insecticides through terrestrial food chains. Pesticides may kill grain- and plant-feeding birds, and the elimination of many rare species of ducks and geese has been reported. Populations of insect-eating birds such as partridges, grouse, and pheasants have decreased due to the loss of their insect food in agricultural fields through the use of insecticides.

Bees are extremely important in the pollination of crops and wild plants, and although pesticides are screened for toxicity to bees, and the use of pesticides toxic to bees is permitted only under stringent conditions, many bees are killed by pesticides, resulting in the considerably reduced yield of crops dependent on bee pollination.

The literature on pest control lists many examples of new pest species that have developed when their natural enemies are killed by pesticides. This has created a further dependence on pesticides not dissimilar to drug dependence. Finally, the effects of pesticides on the biodiversity of plants and animals in agricultural landscapes, whether caused directly or indirectly by pesticides, constitute a major adverse environmental impact of pesticides.

Effects on the Aquatic Environment

The movement of pesticides into surface and groundwater is well documented. Wildlife is affected, and human drinking water is sometimes contaminated beyond acceptable safety levels. Sediments dredged from U.S. waterways are often so heavily contaminated with persistent and other pesticide residues that it becomes problematic to safely dispose of them on land.

A major environmental impact has been the widespread mortality of fish and marine invertebrates due to the contamination of aquatic systems by pesticides. This has resulted from the agricultural contamination of waterways through fallout, drainage, or runoff erosion, and from the discharge of industrial effluents containing pesticides into waterways. Historically, most of the fish in Europe's Rhine River were killed by the discharge of pesticides, and at one time fish populations in the Great Lakes became very low due to pesticide contamination. Additionally, many of the organisms that provide food for fish are extremely susceptible to pesticides, so the indirect effects of pesticides on the fish food supply may have an even greater effect on fish populations. Some pesticides, such as pyrethroid insecticides, are extremely toxic to most aquatic organisms. It is evident that pesticides cause major losses in global fish production.

Effects on Humans

The most important aspect of pesticides is how they affect humans. There is increasing anxiety about the importance of small residues of pesticides, often suspected of being carcinogens or disrupting endocrine activities, in drinking water and food. In spite of stringent regulations by international and national regulatory agencies, reports of pesticide residues in human foods, both imported and home-produced, are numerous.

Over the last fifty years many human illnesses and deaths have occurred as a result of exposure to pesticides, with up to 20,000 deaths reported annually. Some of these are suicides, but most involve some form of accidental exposure to pesticides, particularly among farmers and spray operators in developing countries, who are careless in handling pesticides or wear insufficient protective clothing and equipment. Moreover, there have been major accidents involving pesticides that have led to the death or illness of many thousands. One instance occurred in Bhopal, India, where more than 5,000 deaths resulted from exposure to accidental emissions of methyl isocyanate from a pesticide factory.

Testing and Reclassification

New pesticides require extensive laboratory and field testing and may take about five years to reach market. A pesticide company has to identify uses, test effectiveness, and provide data on chemical structure, production, formulation, fate, persistence, and environmental impacts. The product is tested in the laboratory, greenhouse, and field under different environmental conditions. After several years of testing, the company submits a registration data package to the U.S. Environmental Protection Agency (EPA). Data include studies on acute, chronic, reproductive, and developmental toxicity to mammals, birds, and fish, the pesticide's environmental fate, rates of degradation, translocations to other sites, and ecological studies on its harmful effects to, and on, nontarget plants and animals.

After its review by government and other scientists, the EPA grants registration of the product for certain uses, with agreed label data and directions for use. About 1 in 35,000 chemicals survives from initial laboratory testing to the market, a process that generally takes several years, and involves more than 140 tests.

The continued use of a pesticide is supervised by the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), enacted in 1947 and modified many times since. A review may be called for when new evidence indicates possible unreasonable risks to human health or the environment, including toxicity or ill health to humans or animals, hazards to nontarget organisms, and risks to endangered species and suggests that the risks may outweigh the benefits of continued registration. After review, the EPA may take no action, alter the pesticide label to minimize risk, reclassify the approved uses or eliminate specific uses, or cancel or suspend the pesticide's registration entirely.

Pesticides and Food Safety

Pesticides are used on food crops and meat produced from domestic animals. The residues contained within domestically produced food are monitored closely by the EPA, whereas those for imported food are tracked by the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture (USDA). Scientists determine the highest dose of a pesticide that might be ingested by animals (birds and mammals, including humans) to cause adverse health effects but not death; this is called the maximum tolerated dose (MTD). They also determine the no-observable-effect level (NOEL) and identify the amount of pesticides that may be safely consumed by humans, in terms of milligrams per kilogram of body weight, over a seventy-year lifetime. In calculating an acceptable exposure for a pesticide, scientists usually include a safety factor of one hundred below the NOEL, assuming a lifetime of exposure to the pesticide. Such calculations take for granted that a pesticide is applied to all labeled crops, at recommended rates, and that the treated food will be consumed daily for a lifetime. Pesticides that have been demonstrated to cause cancer in laboratory animals are not granted tolerance, or approved for application to food crops, based on legislation from Section 409, the socalled Delaney clause, of the federal Food, Drug and Cosmetic Act.

The Food and Drug Administration (FDA) and USDA, in addition to many states, have monitoring programs for pesticide residues in food. They sample approximately 1 percent of the national food supply. For every pesticide, the FDA conducts a total diet study (a market-based survey) to more accurately assess the exposure of the human population to pesticides. Similar calculations are made for exposure to pesticides that may reach drinking water through percolation into groundwater or runoff into waterways.

These adverse effects of pesticides on humans and wildlife have resulted in research into ways of reducing pesticide use. The most important of these is the concept of integrated pest management (IPM), first introduced in 1959. This combines minimal use of the least harmful pesticides, integrated with biological and cultural methods of minimizing pest losses. It is linked with using pesticides only when threshold levels of pest attacks have been identified. There is also a move toward sustainable agriculture which aims to minimize use of pesticides and fertilizers based on a systems approach.

see also Agriculture; Bioaccumulation; Carson, Rachel; DDT (Dichlorodiphenyl trichloroethane); Endocrine Disruption; Integrated Pest Management; Persistent Bioaccumulative and Toxic Chemicals (PBTs); Persistent Organic Pollutants (POPs); Water Pollution.


bohmart, b.l. (1997). the standard pesticide users guide, 4th edition. london: prentice-hall international.

carson, rachel. (1963). silent spring. london: hamish hamilton.

ekstrom, c., ed. (1994). world directory of pesticide control organizations. farnham, u.k.: british crop protection council.

leng, m.l.; leovey, e.m.k.; and zubkoff, p.l., eds. (1995). agrochemical environmental fate: state of the art. boca raton, fl: crc press.

pimentel, d., lehman, h., eds. (1993). the pesticide question: environment, economics, and ethics. new york: chapman and hall.

rand, g.m., ed. (1995). fundamentals of aquatic toxicology: effects, environmental fate and risk assessment. washington, d.c.: taylor and francis.

smith, r.p. (1992). a primer of environmental toxicology. philadelphia: lea and febiger.

ware, g.w. (1994). the pesticide book, 4th edition. fresno, ca: thomson publications.

internet resource

u.s. environmental protection agency web site. "pesticides." available from

Clive A. Edwards

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The word "pesticide" is a broad term that refers to any device, method, or chemical that kills plants or animals that compete for humanity's food supply or are otherwise undesirable. Pesticides include insecticides, fungicides, herbicides, nematocides (used to kill nematodes, elongated cylindrical worms), and rodenticides. Of these various pesticides, insecticides have a longer and more noteworthy history, perhaps because the number of insects labeled "pests" greatly exceeds the number of all other plant and animal "pests" combined. Hence, this article focuses on the use of agricultural insecticides.

Since they first began cultivating crops (around 7000 b.c.) if not before, humans have devised methods to prevent insects from eating or otherwise destroying precious crops. Some cultures relied on the practice of planting during certain phases of the moon. Other early agricultural practices that indirectly kept insect populations low were rotating crops; planting small, varied crops; and selecting naturally resistant plants. People picked bugs off plants by hand and made noise to ward off grasshoppers. Chemicals were also used early on. The crushed petals of the pyrethrum (a type of chrysanthemum), sulfur, and arsenic were used in the Middle East, Rome, and China, respectively. The Chinese also used natural predators such as ants to eat undesirable insects.

All attempts at pest control were pretty much individual affairs until the 1840s, when a North American fungus called powdery mildew invaded Britain, and the epidemic was controlled with large-scale applications of sulfur. The Colorado beetle in the western United States was the next target: by 1877 western settlers had learned to protect their potato crop by using water-insoluble chemicals such as paris green. Other pesticides such as derria, quassia, and tar oil followed, but nineteenth-century pesticides were weak. They had to be supplemented by introducing natural predators, or, in some cases, by grafting threatened plants onto more resistant rootstock.

By World War II, only about 30 pesticides existed. Research during the war yielded DDT (dichloro-diphenyl-trichloro-ethane), which had been synthesized in 1874 but wasn't recognized as an insecticide until 1942. Other strong pesticides soon followed, such as chlordane in 1945 and endrin in 1951. Poison gas research in Germany yielded the organophosphorus compounds, the best known of which is parathion. These new pesticides were very strong. Further research yielded hundreds of organophosphorus compounds, the most noteworthy being malathion, which was recently used in California against the medfly.

Until the 1800s, when people began to spray personal gardens using fairly large machines, pesticides were generally applied by hand. Airplanes were not used until the 1920s, and slow, well-controlled, low-level flights were not implemented until the 1950s. The first aerial spraying of synthetic pesticides used large amounts of inert materials, 4000 liters per hectare (a hectare equals 2.47 acres). This quantity was rapidly reduced to 100 to 200 liters/hectare, and by the 1970s the amount had been reduced (in some cases) to .3 liters per hectare of the ingredient itself (for example, malathion) applied directly to the fields.

Today, some 900 active chemical pesticides are used to manufacture 40,000 commercial preparations. The Environmental Protection Agency (EPA) estimates that the use of pesticides doubled between 1960 and 1980. Currently, over 372 million kilograms a year are used in the United States, with over 1.8 billion kilograms a year used worldwide.

Raw Materials

A pesticide consists of an active ingredient coupled with inert ingredients. The active ingredient kills the pests, while the inert ingredients facilitate spraying and coating the target plant; they can also contribute other advantages that are not conferred by the active ingredient alone.

Active ingredients were once distilled from natural substances; now they are largely synthesized in a laboratory. Almost all are hydrocarbons derived from petroleum. Most pesticides contain other elements, the type and number of which depend on the pesticide desired. Chlorine, oxygen, sulfur, phosphorus, nitrogen, and bromine are most common. Inert ingredients can be many substances, dependent on the type of pesticide. Liquid pesticides have traditionally used kerosene or some other petroleum distillate as a carrier, though water has recently begun to replace kerosene. Emulsifiers (such as soap) are also added to distribute the active ingredient evenly throughout the solvent. A powder or dust pesticide will typically contain vegetable matter such as ground up nut shells or corn cobs, clays such as diatomite or attapulgite, or powdered minerals such as talc or calcium carbonate as a base. To cause the pesticide to adhere better to the plant or soil, a material such as cornstarch or flour may be added.

The Manufacturing

Manufacturing a pesticide involves at least three separate activities. The active ingredient is first synthesized in a chemical factory, then formulated in the same place or sent to a formulator, who prepares the liquid or powder form. The pesticide is then sent to the farmer or other certified applicator, who dilutes it before applying it to the fields.

Synthesizing the pesticide

  • 1 When a new pesticide is first developed, it is manufactured on a small scale in a laboratory. If the substance proves viable, production begins in the factory. Batch or continuous manufacturing insures a high volume, perhaps as much as 500 kilograms per cycle. Synthesizing a pesticide is a complex chemical procedure that requires trained chemists and a large, sophisticated laboratory. The basic procedure entails altering an organic molecule to form a pesticide. This may involve any of a number of specific reagents and catalysts and often must take place in a controlled climate (within a certain temperature range, for example). Once synthesized, the active ingredient is packaged and sent to a formulator. Liquid insecticides can be shipped in tank trucks or 200-liter drums. Transport of the active ingredient follows all regulations for hazardous materials transportation.

Formulating the pesticide

  • 2 A formulator accepts the active ingredient, measures out the proper amount, mixes it with carrier if it is to be a liquid pesticide or with inert powders or dry fertilizers if it is to be a dust pesticide, then bottles or packages it. Liquid pesticides are packaged in 200-liter drums if a large-scale farmer is the anticipated customer or 20-liter jugs for small-scale operations. Dry formulations can be packaged in 5 to 10 kilogram plastic or plastic-lined bags. An emulsified formulation is usually concentrated to render transport easier (the active ingredient typically makes up 50 percent of the emulsified concentrate), but granulated and dry pesticides are ready to use.

Diluting the pesticide

  • 3 The pesticide might be stored a short time before it is requested. When it is ready for transport, the estimated necessary amount is sent to the farmer, who dilutes the emulsified concentrate to create the amount of pesticide desired. In most instances, the final product consists of only .5 to 1 percent of the original active ingredient. The pesticide is now ready to be applied.

Applying the pesticide

  • 4 There are several ways to apply a pesticide. The method with which Americans are most familiar is crop dusting, though its use is generally limited to large, flat areas. A plane loaded with 2000-liter (or larger) tanks flies over a field and sprays out the pesticide from booms. Booms are long, horizontal rods from which several sprinklers spray down. Another method is to attach the tanks and booms to a tractor and spray closer to the ground. For small farmers, the most economical method of spraying is to use one or more workers with hand-held sprayers attached to small tanks. A hand pump can be carried on the shoulder; its tank capacity is only about 3 to 12 liters. Small tanks with a capacity of around 200 liters are also used. The pesticides are applied with a hand gun. A rough estimate of the amount applied is 150 to 300 liters per hectare.

Quality Control

Pesticides are by their very nature toxic substances; hence, a great deal of concern has centered on safety. The laws dealing with pesticide safety are very strict and will become even stricter in the future. Besides legal restrictions, pesticides are also subject to stringent quality control standards like any other manufactured product.

Most large pesticide manufacturers have highly developed quality control laboratories that test each pesticide for potency, emulsification, density, color, pH, particle size (if a dust), and suspension (if a liquid). If the company makes more than one pesticide, the product's identity must also be verified. A pesticide must be stable, easy to apply, and easy to store. Shelf-life must extend past one year. In accelerated tests, the pesticide is subjected to high temperatures for a short period, then checked for effectiveness. A typical pesticide is 95 percent pure. Labels must be easy to read and meet all regulations. The manufacturer keeps files for each raw material, active ingredient, formulation, and packaged item, and samples are stored for three years.

Today's pesticides, when used properly, are very safe. Farmers who apply their own pesticides must be trained by the U.S. Agricultural Extension Service and certified by the state department of agriculture before they can purchase pesticides. Commercial applicators must also undergo training and pass a written test.

When preparing a formulation for application, which in most cases means diluting it, the applicator should wear protective clothing as directed by the label. Often, this protective garb includes an apron or coveralls, a broad-brimmed hat, long-sleeved shirt, long socks, unlined neoprene or rubber gloves, long pants, and unlined neoprene or rubber boots worn over shoes. For some pesticides, applicators must also wear goggles and/or a respirator.

As an additional precaution, application equipment is calibrated before each use. To calibrate a sprayer, the applicator measures off a distance in the field, then sprays it with a neutral substance such as water. The amount of water used is then checked to see if it is appropriate. All equipment is also checked to see if spraying is even, and worn equipment is replaced promptly.


When they were introduced, pesticides were seen as a wonderful technology that would increase crop yields and reduce insect-borne diseases. The first sign that this was a hopeful myth was the discovery in the 1950s that pesticide volume must be increased to have the same effect it once had. With the publication of Silent Spring by Rachel Carson in 1962, an awareness of the danger of unrestricted pesticide use grew.

Pesticides kill the pests they are aiming for most of the time, yet often they also kill the pests' natural predators, thereby exacerbating the problem. In some cases, exterminating a pest merely allows another pest to take its place. After a period of pesticide use, the insects become resistant to the pesticide, and stronger or more pesticides must be used to control the population. There is evidence that pesticides are misused, that their effect in some cases is negligible, and that applicators are not aware of the proper use of pesticides. Coupled with these concerns is the worry over blanket spraying of residential areas and contaminated food.

DDT is the most widely noted case of a pesticide that caused damage far from the farm. High levels of DDT have been found in birds of prey, causing them to become endangered because of the effect it has on their eggs. DDT becomes more concentrated the higher it climbs in the food chain, and many people have voiced their concern about its possible presence in humans. In 1972, the Environmental Protection Agency (EPA) announced a ban on almost all uses of DDT.

Several dozen other pesticides have also been banned, or their use restricted by the EPA. Ironically, these pesticides are still being exported to assist developing countries, where it is estimated that three million acute cases of pesticide poisoning occur per year, along with 20,000 deaths directly related to the misuse of pesticides. Because many of these countries export produce to the United States, the possibility of American contamination is high.

Integrated pest management (IPM) was begun in the 1960s in response to the pesticides dilemma. The idea behind IPM was to use a variety of insect controls instead of relying solely on chemical insecticides. The methods include introducing natural predators, parasites, and bacterial, viral, and fungal insecticides to the fields. Workers may simply vacuum up the insects, or introduce certain plants to ward off pests that attack a particular crop. Farmers may plow at the most effective time, plow their crop residue under, or strip harvest. They may plant pest-resistant plants. Sexual attractant traps may pull pests away from crops. Sterilized males can be released into the field. Insects can be engineered to remain juvenile and never reproduce, molt too rapidly and therefore die rapidly, or become too confused to locate crop foods. Other possibilities are being tested at present. It is possible that in the future pesticide use will diminish as research leads to ways to combat pests with more knowledge and planning and less reliance on chemical intervention.

Where To Learn More


Carson, Rachel. Silent Spring. Houghton Mifflin Company, 1962.

Lee, Sally. Pesticides. Franklin Watts, 1991.

Ware, George W. Pesticides: Theory and Application. W.H. Freeman, 1983.


Gibbons, Ann. "Overkilling the Insect Enemy." Science. August 10, 1990, p. 621.

Holmes, Bob. "The Joy Ride Is Over." U.S. News and World Report. September 14,1992, pp. 73-74.

Reganold, John P., Robert I. Papendick, and James F. Parr. "Sustainable Agriculture." Scientific American. June, 1990, pp. 112-120.

Richmond, Suzan. "Making Sure It's Organic." Changing Times. October, 1990, p. 102.

Satchell, Michael. "A Vicious 'Circle of Poison."' U.S. News and World Report. June 10, 1991, pp. 31-32.

Rose Secrest

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Pesticides are chemicals that are used to kill insects, weeds, and other organisms to protect humans, crops, and livestock. There have been many substantial benefits of the use of pesticides. The most important of these have been: (1) an increased production of food and fibre because of the protection of crop plants from pathogens, competition from weeds, defoliation by insects, and parasitism by nematodes; (2) the prevention of spoilage of harvested, stored foods; and (3) the prevention of debilitating illnesses and the saving of human lives by the control of certain diseases.

Unfortunately, the considerable benefits of the use of pesticides are partly offset by some serious environmental damages. There have been rare but spectacular incidents of toxicity to humans, as occurred in 1984 at Bhopal, India , where more than 2,800 people were killed and more than 20,000 seriously injured by a large emission of poisonous methyl isocyanate vapor, a chemical used in the production of an agricultural insecticide.

A more pervasive problem is the widespread environmental contamination by persistent pesticides, including the presence of chemical residues in wildlife , in well water, in produce, and even in humans. Ecological damages have included the poisoning of wildlife and the disruption of ecological processes such as productivity and nutrient cycling. Many of the worst cases of environmental damage were associated with the use of relatively persistent chemicals such as DDT. Most modern pesticide use involves lesspersistent chemicals.

Pesticides can be classified according to their intended pest target:

  • fungicides protect crop plants and animals from fungal pathogens;
  • herbicides kill weedy plants, decreasing the competition for desired crop plants;
  • insecticides kill insect defoliators and vectors of deadly human diseases such as malaria , yellow fever, plague , and typhus;
  • acaricides kill mites, which are pests in agriculture, and ticks, which can carry encephalitis of humans and domestic animals;
  • molluscicides destroy snails and slugs, which can be pests of agriculture or, in waterbodies, the vector of human diseases such as schistosomiasis;
  • nematicides kill nematodes, which can be parasites of the roots of crop plants;
  • rodenticides control rats, mice, gophers, and other rodent pests of human habitation and agriculture;
  • avicides kill birds, which can depredate agricultural fields;
  • antibiotics treat bacterial infections of humans and domestic animals.

The most important use-categories of pesticides are in human health, agriculture, and forestry:

Human Health

In various parts of the world, species of insects and ticks play a critical role as vectors in the transmission of disease-causing pathogens of humans. The most important of these diseases and their vectors are: (1) malaria, caused by the protozoan Plasmodium and spread to humans by an Anopheles -mosquito vector; (2) yellow fever and related viral diseases such as encephalitis, also spread by mosquitoes; (3) trypanosomiasis or sleeping sickness, caused by the protozoans Trypanosoma spp. and spread by the tsetse fly Glossina spp.; (4) plague or black death, caused by the bacterium Pasteurella pestis and transmitted to people by the flea Xenopsylla cheops, a parasite of rats; and (5) typhoid fever, caused by the bacterium Rickettsia prowazeki and transmitted to humans by the body louse Pediculus humanus.

The incidence of all of these diseases can be reduced by the judicious use of pesticides to control the abundance of their vectors. For example, there are many cases where the local abundance of mosquito vectors has been reduced by the application of insecticide to their aquatic breeding habitat , or by the application of a persistent insecticide to walls and ceilings of houses, which serve as a resting place for these insects. The use of insecticides to reduce the abundance of the mosquito vectors of malaria has been especially successful, although in many areas this disease is now reemerging because of the evolution of tolerance by mosquitoes to insecticides.


Modern, technological agriculture employs pesticides for the control of weeds, arthropods, and plant diseases, all of which cause large losses of crops. In agriculture, arthropod pests are regarded as competitors with humans for a common food resource. Sometimes, defoliation can result in a total loss of the economically harvestable agricultural yield, as in the case of acute infestations of locusts. More commonly, defoliation causes a reduction in crop yields. In some cases, insects may cause only trivial damage in terms of the quantity of biomass that they consume, but by causing cosmetic damage they can greatly reduce the economic value of the crop. For example, codling moth (Carpocapsa pomonella ) larvae do not consume much of the apple that they infest, but they cause great esthetic damage by their presence and can render produce unsalable.

In agriculture, weeds are considered to be any plants that interfere with the productivity of crop plants by competing for light, water, and nutrients. To reduce the effects of weeds on agricultural productivity, fields may be sprayed with a herbicide that is toxic to the weeds but not to the crop plant. Because there are several herbicides that are toxic to dicotyledonous weeds but not to members of the grass family, fields of maize, wheat, barley, rice, and other grasscrops are often treated with those herbicides to reduce weed populations.

There are also many diseases of agricultural plants that can be controlled by the use of pesticides. Examples of important fungal diseases of crop plants that can be managed with appropriate fungicides include: (1) late blight of potato, (2) apple scab, and (3) Pythium -caused seed-rot, dampingoff, and root-rot of many agricultural species.


In forestry, the most important uses of pesticides are for the control of defoliation by epidemic insects and the reduction of weeds. If left uncontrolled, these pest problems could result in large decreases in the yield of merchantable timber. In the case of some insect infestations, particularly spruce budworm (Choristoneura fumiferana ) and gypsy moth (Lymantria dispar ), repeated defoliation can cause the death of trees over a large area of forest. Most herbicide use in forestry is for the release of desired conifer species from the effects of competition with angiosperm herbs and shrubs. In most places, the quantity of pesticide used in forestry is much smaller than that used in agriculture.

Pesticides can also be classified according to their similarity of chemical structure. The most important of these are:

  • Inorganic pesticides, including compounds of arsenic , copper , lead , and mercury . Some prominent inorganic pesticides include Bordeaux mixture, a complex pesticide with several copper-based active ingredients, used as a fungicide for fruit and vegetable crops; and various arsenicals used as non-selective herbicides and soil sterilants and sometimes as insecticides.
  • Organic pesticides, which are a chemically diverse group of chemicals. Some are produced naturally by certain plants, but the great majority of organic pesticides have been synthesized by chemists. Some prominent classes of organic pesticides are:
  • Biological pesticides are bacteria, fungi , or viruses that are toxic to pests. One of the most widely used biological insecticide is a preparation manufactured from spores of the bacterium Bacillus thuringiensis , or B.t. Because this insecticide has a relatively specific activity against leaf-eating lepidopteran pests and a few other insects such as blackflies and mosquitoes, its non-target effects are small.

The intended ecological effect of a pesticide application is to control a pest species, usually by reducing its abundance to an economically acceptable level. In a few situations, this objective can be attained without important non-target damage. However, whenever a pesticide is broadcast-sprayed over a field or forest, a wide variety of on-site, non-target organisms are affected. In addition, some of the sprayed pesticide invariably drifts away from the intended site of deposition, and it deposits onto non-target organisms and ecosystems. The ecological importance of any damage caused to non-target, pesticide-sensitive organisms partly depends on their role in maintaining the integrity of their ecosystem . From human perspective, however, the importance of a non-target pesticide effect is also influenced by specific economic and esthetic considerations.

Some of the best known examples of ecological damage caused by pesticide use concern effects of DDT and other chlorinated hydrocarbons on predatory birds, marine mammals, and other wildlife. These chemicals accumulate to large concentrations in predatory birds, affecting their reproduction and sometimes killing adults. There have been high-profile, local and/or regional collapses of populations of peregrine falcon (Falco peregrinus ), bald eagle (Haliaeetus leucocephalus ), and other raptors, along with brown pelican (Pelecanus occidentalis ), western grebe (Aechmorphorus occidentalis ), and other waterbirds. It was the detrimental effects on birds and other wildlife, coupled with the discovery of a pervasive presence of various chlorinated hydrocarbons in human tissues, that led to the banning of DDT in most industrialized countries in the early 1970s. These same chemicals are, however, still manufactured and used in some tropical countries.

Some of the pesticides that replaced DDT and its relatives also cause damage to wildlife. For example, the commonly used agricultural insecticide carbofuran has killed thousands of waterfowl and other birds that feed in treated fields. Similarly, broadcast-spraying of the insecticides phosphamidon and fenitrothion to kill spruce budworm in infested forests in New Brunswick, Canada, has killed untold numbers of birds of many species.

These and other environmental effects of pesticide use are highly regrettable consequences of the broadcast-spraying of these toxic chemicals in order to cope with pest management problems. So far, similarly effective alternatives to most uses of pesticides have not been discovered, although this is a vigorously active field of research. Researchers are in the process of discovering pest-specific methods of control that cause little non-target damage and in developing methods of integrated pest management . So far, however, not all pest problems can be dealt with in these ways, and there will be continued reliance on pesticides to prevent human and domestic-animal diseases and to protect agricultural and forestry crops from weeds, diseases, and depredations caused by economically important pests.

See also Agent Orange; Agricultural chemicals; Agricultural pollution; Algicide; Chlordane; Cholinesterase inhibitor; Diazinon; Environmental health; Federal Insecticide, Fungicide and Rodenticide Act (1947); Kepone; Methylmercury seed dressings; National Coalition Against the Misuse of Pesticides; Organochloride; Persistent compound; Pesticide Action Network; Pesticide residue; 2,4-D; 2,4,5-T

[Bill Freedman Ph.D. ]



Baker, S., and C. Wilkinson, eds. The Effect of Pesticides on Human Health. Princeton, NJ: Princeton Scientific Publishing, 1990.

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

Hayes, W. J., and E. R. Laws, eds. Handbook of Pesticide Toxicology. San Diego: Academic Press, 1991.

McEwen, F. L., and G. R. Stephenson. The Use and Significance of Pesticides in the Environment. New York: Wiley, 1979.

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Pesticides are chemicals that kill pests, and are categorized by the types of pests they kill. For example, insecticides kill insects, herbicides kill weeds, bactericides kill bacteria, fungicides kill fungi, and algicides kill algae.

Approximately 90 percent of all pesticides used worldwide are used in agriculture, food storage, or shipping. Because of a growing world population, there is pressure to increase and preserve the food supply by using pesticides and other agricultural chemicals.

History of Pesticides

Throughout history, various types of pests, such as insects, weeds, bacteria, rodents, and other biological organisms, have bothered humans or threatened human health. People have been using pesticides for thousands of years to try to control these pests. The Sumerians used sulfur to control insects and mites 5,000 years ago. The Chinese used mercury and arsenic compounds to control body lice and other pests. The Greeks and Romans used oil, ash, sulfur, and other materials to protect themselves, their livestock, and their crops from various pests. And people in various cultures have used smoke, salt, spices, and insect-repelling plants to preserve food and keep pests away.

Classes of Pesticides

Although the use of pesticides is not new, the types of substances people have used as pesticides have changed over time. The earliest pesticides were inorganic substances such as sulfur, mercury, lead, arsenic, and ash. Some of these inorganic pesticides are still used today. For example, sulfur is still used as a fungicide, copper is used as an algicide, lead and arsenic were used as insecticides until World War II, and chromium, copper, and arsenic have been used as wood preservatives to prevent microorganisms from causing wood decay. Even though many of these substances are effective pesticides, the use of some of these materials has been banned or restricted because of health and environmental concerns. Lead and arsenic are no longer used as insecticides, the use of mercury as a fungicide has been restricted, and the U.S. Environmental Protection Agency (EPA) is phasing out the use of arsenic as a wood preservative.

The modern era of chemical pest control began around the time of World War II, when the synthetic organic chemical industry began to develop. The first synthetic organic pesticides were organochlorine compounds, such as dichlorodiphenyltrichloroethane (DDT). Commercial production of DDT began in 1943. At that time, DDT was considered to be a wonderful invention. It was cheap to produce, very toxic to insects, and much less toxic to mammals. DDT and other organochlorine insecticides were used for many years to control mosquitoes and as a broad-spectrum insecticide against insect pests that damaged food and crops. Unfortunately, scientists learned later that many organochlorine insecticides were persistent in the environment (they did not degrade readily) and were bioaccumulating in birds, humans, and other animals. In 1962 Rachel Carson wrote the book Silent Spring, in which she reported that DDT was causing eggshell thinning in bird eggs and thus was leading to the near extinction of bird species such as peregrine falcons and bald eagles. Today most of the organochlorine pesticides have been banned in the United States by the EPA because of the tendency of these compounds to persist in the environment and bioaccumulate in animals.

Other classes of insecticides include the organophosphates, carbamates, pyrethroids, and biopesticides. These other classes of pesticides are not as persistent in the environment as the organochlorine pesticides. The organophosphate and carbamate pesticides affect the nervous system by disrupting the enzyme that regulates acetylcholine , a neurotransmitter. However, carbamate pesticides are less toxic to humans because their interactions with important enzymes are reversible. As a group, the organophosphate and carbamate pesticides are probably the most widely used insecticides, although many are being restricted by the EPA because of their toxicity.

Pyrethroid pesticides were developed as synthetic versions of the naturally occurring pesticide pyrethrin, which is found in chrysanthemums. Most pyrethroids are safer than the organochlorines, organophosphates, and carbamates, although some synthetic pyrethroids are toxic to the nervous system. Pyrethroids have been modified to increase their stability in the environment, and many different pyrethroids are being used today.

RACHEL CARSON (19071964)

The role of chemistry became irreversibly intertwined with the environment in 1962 when the term "ecosystem" was introduced in Rachel Carson's Silent Spring. One of four works written by Carson, it targeted the now banned pesticide dichlorodiphenyl-trichloroethane (DDT), spawning a movement that resulted in the formation of the U.S. Environmental Protection Agency (EPA).

Valerie Borek

Biopesticides are substances that are derived from such natural materials as animals, plants, bacteria, and certain minerals. For example, canola oil and baking soda have pesticidal applications and are considered biopesticides. Biopesticides fall into three major classes, including microbial pesticides, plant-incorporated protectants, and biochemical pesticides. Microbial pesticides contain microorganisms, such as bacteria, fungi, and viruses, as their active ingredient. The most widely used microbial pesticides are strains of Bacillus thuringiensis, or Bt. Plant-incorporated protectants are pesticidal substances that plants produce from genetic material that has been added to the plant. Biochemical pesticides are naturally occurring substances that control pests by nontoxic mechanisms. Conventional pesticides, by contrast, are generally synthetic materials that directly kill or inactivate the pest. Biochemical pesticides include substances, such as insect sex pheromones, that interfere with mating, as well as various scented plant extracts that attract insect pests to traps. Because it is sometimes difficult to determine whether a substance meets the criteria for classification as a biochemical pesticide, the EPA has established a special committee to make such decisions.

Pesticide Residues

Pesticide residues are the materials that remain on plants and food when a crop is treated with a pesticide. The U.S. government establishes safe residue levels, called tolerances or maximum residue levels, for each food commodity. However, the presence of pesticide residues in food has been a public concern. There has also been a concern about pesticide residues in water, air, and soil. In response to this concern, the U.S. Congress passed the Food Quality Protection Act in 1996, which has had an impact on safety standards for pesticides.

Approaches to pest management have changed significantly since the 1950s and will continue to change as scientists learn more about the toxicity and environmental behavior of pesticides. Scientists will continue to develop newer approaches to insect pest management that are considered to be safer than the use of broad-spectrum pesticides. The most effective strategy for controlling pests may be to combine methods in an approach known as integrated pest management (IPM), which emphasizes preventing pest damage. In IPM information about pests and available pest control methods is used to manage pest damage by the most economical means and causing the least possible hazard to people, property, and the environment. Methods for pest management will continue to evolve as scientists conduct research and develop new information.

see also Herbicides; Insecticides.

Cynthia Atterholt


Cardé, R. (1990). "Principles of Mating Disruption." In Behavior-Modifying Chemicals for Insect Management: Applications of Pheromones and Other Attractants, ed. Richard L. Ridgway, Robert M. Silverstein, and May N. Inscoe. New York: Marcel Dekker.

Copping, L., and Hewitt, H. G. (1998). Chemistry and Mode of Action of Crop Protection Agents. Cambridge, U.K.: The Royal Society of Chemistry.

Cunningham, W., and Saigo, B. (2001). "Pest Control." In Environmental Science: A Global Concern, 6th edition. New York: McGraw-Hill.

Ecobichon, D. (1991). "Toxic Effects of Pesticides." In Casarett and Doull's Toxicology: The Basic Science of Poisons, 4th edition, ed. M. Amdur, J. Doull, and C. Klaassen. Elmsford, NY: Pergamon Press.

Farrell, K.; Flint, M.; Lyons, J.; Madden, J.; Schroth, M.; Weinhold, A.; White, J.; Zalom, F.; and Jaley, M. (1992). Beyond Pesticides: Biological Approaches to Pest Management in California: Executive Summary. Oakland: University of California.

Kydonieus, A., and Beroza, M. (1982). "Pheromones and Their Use." In Insect Suppression with Controlled Release Pheromone Systems, Vol. 1, ed. A. Kydonieus and M. Beroza. Boca Raton, FL: CRC Press.

Leonhardt, Barbara, and Beroza, Morton, eds. (1982). Insect Pheromone Technology: Chemistry and Applications. Washington, DC: American Chemical Society.

Lewis, D., and Cowsar, D. (1977). "Principles of Controlled Release Pesticides." In Controlled Release Pesticides: A Symposium, ed. Herbert Scher. Washington, DC: American Chemical Society.

Rice, R., and Kirsch, P. (1990). "Mating Disruption of Oriental Fruit Moth in the United States." In Behavior-Modifying Chemicals for Insect Management: Applications of Pheromones and Other Attractants, ed. Richard L. Ridgway, Robert M. Silverstein, and May N. Inscoe. New York: Marcel Dekker.

Scher, H., ed. (1977). Controlled Release Pesticides. Washington, DC: American Chemical Society.

Weatherston, I. (1990). "Principles of Design of Controlled-Release Formulations." In Behavior-Modifying Chemicals for Insect Management: Applications of Pheromones and Other Attractants, ed. Richard L. Ridgway, Robert M. Silverstein, and May N. Inscoe. New York: Marcel Dekker.

Wheeler, W. (2002). "Role of Research and Regulation in 50 Years of Pest Management in Agriculture." Journal of Agricultural and Food Chemistry 50:41514155.

Internet Resources

U.S. Environmental Protection Agency. "Pesticides." Available from <>.

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PESTICIDES. A pesticide is any agent used to kill or control a pest. Pests include insects, weeds, and diseases, such as fungi. In addition, mice, rats, birds, and algae may become pests at some time. When pests damage plants or property, people often use pesticides to control them. The term "pesticide" can apply to insecticides, herbicides, fungicides, antimicrobials, growth regulators, defoliants, and desiccants, most of which are applied to food or food plants before or after harvest. Common pesticides are encountered every dayin pet flea collars, kitchen disinfectants, cockroach baits, swimming pool chemicals, and mosquito repellents. Pesticide products contain both active and inert ingredients, and both must be specified on the label.

Pesticide Controversy

Modern farmers use pesticides to help them to grow almost all of the world's food. In general, pesticides have been a quick, effective, and inexpensive method of control for pests that attack most of the world's food crops. Pesticides are credited with helping to save millions of lives by controlling diseases, such as malaria and yellow fever, which are spread by insects. However, most pesticides present some risk of harm to humans, animals, or the environment because they are designed to kill living organisms.

Sulfur, herbal extracts, tobacco, soaps, oil, arsenic, pyrethrum, and lime have been used as pesticides for many centuries, but the widespread use of synthetic pesticides is a relatively recent phenomenon. Dichlorodiphenyltrichloroethane, or DDT, is probably the best known early pesticide. DDT was created in 1873, but it was not until the late 1930s that Swiss researcher Paul Müller discovered that the compound was effective in killing insects. Müller won the Nobel Prize in Physiology and Medicine in 1948 for his work. DDT was an inexpensive and effective solution to many insect problems, and it virtually eliminated malaria from parts of the world. After World War II, DDT became a common agricultural pesticide. In the 1950s, the United States was producing 220 million pounds of DDT per year.

Insect resistance to the substance developed quickly. DDT residues were found in human milk and fatty tissues, and in wildlife food chains. In 1962 writer and ecologist Rachel Carson wrote Silent Spring to warn the public about the long-term effects of misusing pesticides. Carson challenged the practices of agricultural scientists and the government, and called for a change in the way humankind viewed the natural world. Carson testified before Congress in 1963, calling for new policies to protect human health and the environment. While no longer used in the United States, DDT use continues in other parts of the world. Many tropical countries still use DDT to control malaria.

All pesticides (natural and synthetic) have the potential to cause harm during their manufacture or refinement, at the time of application to crops, as residues that persist on food, and in the disruption of the natural balance that exists between pests and their natural enemies. For example, traces of the natural insecticide "rotenone" may be found on vegetables after cooking. Atrazine, a weed-killer commonly used on corn and soybeans, suburban lawns, and utility rights-of-way, has contaminated groundwater where those crops are grown. Insecticides like DDE and dieldrin, which are related to DDT, were banned in the United States in the 1970s, but still show up in the U.S. food supply. Persistent residues of these chemicals travel long distances in global air and water currents. These insecticides are still produced and used in many countries. Recent studies have linked pesticides with acute poisonings, cancer, brain damage, reproductive harm, and many childhood illnesses and learning problems, leading concerned citizens to feel that pesticides should be banned.

Organic Agriculture

Some agricultural experts predict that the quality and quantity of our food supply would be lessened if pesticides were eliminated. However, practitioners of organic agriculture (organic farmers use no synthetic agricultural chemicals and instead rely on management practices such as crop rotation, disease-resistant varieties, and natural enemies to control crop pests) claim that food quality and yield are equally productive under organic management. Fortunately for conventional and organic farmers, the number of safer, reduced-risk options for pest control is increasing. For example, there were approximately seven hundred new, biological pesticide products registered by 1999. Biological pesticides are certain types of pesticides derived from such natural materials as animals, plants, bacteria, and minerals.

Garlic, mint, and baking soda all have pesticide-like properties and are considered biological pesticides. Biological pesticides include the common cabbage worm killer Bacillus thuringiensis, which produces a protein that helps to kill specific worm pests. Some of the new reduced-risk pesticides, while synthesized in a laboratory, are considered safer because they do not kill beneficial insects (such as lady beetles and lacewings), or they break down quickly to inactive products. In 1977 U.S. president Jimmy Carter issued a Presidential Decree that mandated the use of integrated pest management (IPM)a comprehensive approach to pest control that uses a combination of less toxic means to reduce the status of pests to tolerant levels, while maintaining a quality environment. Together, the new reduced-risk pesticides and IPM practices have helped to lessen the amount of pesticides that are used on food and other crops. Levels of pesticide residues on IPM produce have been reported as higher than those of organically grown food, but lower than those in conventionally grown produce.

Pesticides and Their Regulation

In the United States, pesticides are regulated by the Environmental Protection Agency (EPA). EPA regulates the sale, distribution, and use of pesticides and has the authority to suspend or cancel the registration of a pesticide if information shows that continued use would pose unreasonable risks. In 1996 the Food Quality Protection Act (FQPA) was signed into law, giving EPA more effective power. Among its many benefits, the FQPA established a new health-based safety standard for pesticide residues in food; included special provisions for infants and children; required periodic tolerance reevaluations; incorporated provisions for endocrine testing; and allowed for enhanced enforcement of pesticide residue standards.

Scientists predict that, in the future, pesticides will continue to play a role in pest management of food crops, partly because reduced-risk pesticides have become less harmful to the environment, and less toxic to people and wildlife. Societal concerns, scientific advances, and regulatory pressures continue to drive some of the more hazardous pesticides from the marketplace. In addition, consumer interest in safe and healthy food will create more demand for organically grown products.

See also Herbicides ; Organic Agriculture ; Organic Farming and Gardening ; Food Safety ; Toxins, Unnatural, and Food Safety .


Cruising chemistry. An introduction to the chemistry of the world around you. "DDT: An Introduction." University of California, San Diego. Available at

Entomology at Rutgers. Agricultural Entomology and Pest Management course. Entomology 370350Spring 2001, Dr. George Hamilton. Available at

"The Future Role of Pesticides in U.S. Agriculture." 2000. Committee on the Future Role of Pesticides in U.S. Agriculture, Board on Agriculture and Natural Resources and Board on Environmental Studies and Toxicology, Commission on Life. Available at

Lear, Linda. The Rachel Carson Website. Available at

Natural Resources Defense Council. Available at

Paul Hermann MüllerBiography. Nobel e-Museum. The Nobel Foundation. The Official Web Site of The Nobel Foundation. Available at

Pesticide Action Network Pesticide Database. Available at

Pesticide Action Network Toxicity Ratings. Available at

Pesticide Data Program. USDA Agricultural Marketing Service Science and Technology Programs. Progress Report 2001. Available at

U.S. EPA Office of Pesticide Programs. Biopesticides. Available at

U.S. EPA Office of Pesticide Programs. Highlights of the Food Quality Protection Act of 1996. Available at

U.S. EPA Office of Pesticide Programs. What the Pesticide Residue Limits Are on Food. Available at

Patricia S. Michalak

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Pesticides are natural or human-made substances used to kill pest species such as rodents and insects. It is not surprising that many of these substances are highly toxic not only to the pests, but to other biological organisms as well. Pesticides are used in forests, agricultural regions, parks, residential areas, and within the home.

The bulk of pesticide use is related to agricultural pest control. In fact, pesticide application increased dramatically when intensive agricultural methods began to be used near the start of the twentieth century. Although pesticides clearly help to increase agricultural production, they also harm humans and other animal species. In addition, they contaminate the environment, often persisting in water, air, and soil for long periods of time. The World Health Organization reports over one million human pesticide poisonings every year, including twenty thousand that result in death.

These numbers do not include the slower and more subtle effects that exposure to pesticides can have on human health. Many pesticides, for example, are carcinogenic, or cancer-causing. Finally, because pest species always evolve resistance to pesticides over time, ever-increasing amounts or different types of pesticides are constantly required to maintain the same effect.

Some pesticides are inorganic, containing naturally toxic compounds such as lead, arsenic, or mercury. Because these chemicals cannot be broken down, they accumulate in the environment. Natural pesticides include substances produced by plants such as tobacco and certain conifer trees. These are used by the plant species that produce them to ward off herbivores. The majority of pesticides, however, are human-made organic chemicals that function by affecting some essential physiological function of pest species.

One of the best-known pesticides is dichloro-diphenyl-trichloroethane, commonly known as DDT. When DDT was first invented in 1939, by Swiss chemist Paul Muller, it was hailed as a major breakthrough in pesticide development. In fact, Muller received a Nobel Prize for the achievement. DDT found its first use in World War II, when it was sprayed in malarial areas to kill disease-carrying insects to safeguard U.S. troops.

After the war, DDT was widely used in the United States for agricultural control, and like many pesticides seemed highly effective at first. DDT was praised particularly for being highly toxic to insects while comparatively harmless for other species. DDT also had the advantages of being inexpensive to produce and easy to spray. By the 1950s, however, there was evidence that insect pests were evolving resistance to DDT. There were also hints that DDT might not be so harmless after all.

Rachel Carson's monumental book, Silent Spring (1962), was critical in bringing public attention to the serious side effects of DDT use for all living species. The title of the book refers to the absence of birdsong, a result of countless massive bird deaths throughout the country that Carson traced to DDT spraying. Studies of the impact of DDT have shown that the chemical breaks down very slowly, often lingering in the environment for decades after application. DDT is taken up by organisms through diet, and then accumulates in the fatty tissues. This effect is magnified higher up the food chain because any time a predator eats a prey item, the predator takes in all the DDT stored in the tissues of that prey, and then stores it in its own body.

This process is called bio-accumulation . Bio-accumulation explains why birds high in the food chain, such as eagles, owls, and other birds of prey, are particularly vulnerable to DDT poisoning. DDT affects the endocrine systems of birds, throwing off the hormonal control of reproduction. Therefore, large amounts of bio-accumulated DDT cause the delay or cessation of egg laying. When eggs are produced, they are characterized by extremely thin eggshells that break easily during incubation. Although birds appear to be particularly vulnerable to DDT, numerous other species are affected as well.

Carson also showed that there were causal links between pesticides, genetic mutations , and diseases such as cancer. Concerns regarding the tremendous health risks posed by DDT contributed to its being banned in the United States in 1972. Since then, many once-threatened species are now returning. Silent Spring is often credited not only with the ban of DDT, but with initiating awareness that toxic substances can be extremely harmful not only to the environment but to all the species that live within it, including humans. Silent Spring was crucial to the beginnings of environmentalism, as well as to the creation of the Environmental Protection Agency (EPA) in 1970.

Numerous pesticides are still in use now, including many that are even more toxic than DDT. Some of these break down more easily, however, and therefore do not remain in the environment for as long a period. Nonetheless, as awareness of some of the damaging cumulative effects of pesticides has increased, the popularity of and demand for organic foods has also increased.

In addition to toxicity, another problem with pesticide application is that pests inevitably evolve resistance. Pesticide resistance is a striking example of how efficiently natural selection can operate. In many cases, alleles that offer resistance to particular pesticides already exist in the population at very low frequencies. The application of pesticides selects strongly for these resistant alleles and causes them to spread quickly throughout the population. A classic example of the evolution of pesticide resistance is that of rats and warfarin. Warfarin is a pesticide that interferes with vitamin K and prevents blood coagulation, resulting in internal bleeding and death. Resistance to warfarin is conferred by a single gene, which spreads quickly through the rat population upon large-scale application of warfarin.

Because of the many harmful side effects of pesticide use, scientists have worked to develop alternative means for pest control. These include mechanical strategies such as screens or traps, the development of pestresistant plants, crop cycling, and biological control , which aims to control pest populations by releasing large numbers of predators or parasites of a pest. In general, thorough information on the natural history of pest species, such as its life cycle requirements and natural enemies, helps to provide insight into the sort of strategies that may be effective in controlling it.

see also Carson, Rachel; DDT; Silent Spring.

Jennifer Yeh


Carson, Rachel. Silent Spring. Boston: Houghton Mifflin, 1962.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Internet Resources

Office of Pesticide Programs. United States Environment Protection Agency. <>.

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A pesticide is a chemical that is used to kill insects, weeds, and other organisms to protect humans, crops, and livestock.

A broad-spectrum pesticide that kills all living organisms is called a biocide. Fumigants, such as ethylene dibromide or dibromochloropropane, used to protect stored grain or sterilize soil fall into this category. More typically, however, a pesticide has a narrower range. Having the selective ability to kill just one or a few species is preferable to killing the majority of species, some of which are beneficial. There are several narrow spectrum pesticides. Herbicides kill plants; insecticides kill insects; fungicides kill fungi; acaricides kill mites, ticks, and spiders; nematicides kill nematodes (microscopic roundworms); rodenticides kill rodents; and avicides kill birds.

Pesticides can also be grouped by their method of application (fumigation, for example, is dispersal as a gaseous vapor) or by their mode of action (an ovicide kills the eggs of pests).

There are thousands of kinds of natural pesticides.

Some pesticides are naturally produced. As one example, plants have been engaged for millions of years in chemical warfare with predators, most of which are insects. They have evolved a wide variety of complex protective mechanisms, many of which are toxic chemicals. Humans have probably known for a very long time that natural products such as nicotine from tobacco, turpentine from pines, pyrethrum from chrysanthemum species, and quinine from cinchona bark can provide protection from pests and parasites. Our diet contains a large number of such chemicals but ordinarily we have mechanisms to detoxify or excrete them so that they are not a problem.

Many pesticides are synthetic. Indeed, the modern era of chemical pest control began in 1934 with the discovery of the insecticidal properties of DDT (dichloro-diphenyl-trichloroethane) by Swiss chemist Paul Müller (18991965). It became extremely important during World War II in areas where tropical diseases and parasites posed greater threats to soldiers than did enemy bullets. DDT seemed like a wonderful discovery. It is cheap, stable, easily applied, and highly toxic to insects while being relatively nontoxic to mammals. It seemed like the magic bullet that would provide better living through chemistry. In 1948, Müller received a Nobel Prize for his discovery. It was quickly discovered, however, that this magic bullet was not always benevolent. Within a short time, many beneficial organisms were exterminated by DDT, while the pests it was created to control had developed resistance and had rebounded to higher levels than ever. Furthermore, persistent chlorinated hydrocarbons such as this tend to be taken up by living organisms and concentrated through food chains until they reach toxic levels in the top carnivores such as birds of prey or game fish. Species such as peregrine falcons, brown pelicans, osprey, and bald eagles disappeared from much of their range in the eastern United States before DDT and similar persistent pesticides were banned. Rachel Carsons Silent Spring, possibly the most influential book in all of American environmental history, presents the argument against excessive, widespread pesticide use.

In spite of continuing worries about the dangers of pesticides, there was still a heavy dependence on them. The Environmental Protection Agency (EPA) reports that around 500,000 metric tons of pesticides are used in the United States every year. We rely on them for disease control, agricultural production, preservation of buildings and materials, elimination of biting and troublesome organisms, forest protection, and a host of other purposes. Pesticide advocates claim that without modern pesticides, we would lose as much as half of our harvest to pests and that the world would suffer widespread and calamitous famines. Pesticide opponents argue that we could use cultural practices and natural pest predators or repellents to accomplish many of these same goals more safely and more cheaply than we now do with toxic synthetic chemicals.

Several movements aimed at reducing pesticide use have gained adherents in the United States and around the world. Integrated pest management (IPM) is a flexible, ecologically-based, pest-control strategy that uses a combination of techniques applied at specific times and aimed at specific crops and pests. It does not shun pesticides entirely but uses them judiciously when, where, and only in the minimum amount needed. It also employs biological controls (natural predators, resistant crop species) and practices such as mechanical cultivation to reduce pest populations. Many consumers choose to buy organic foods grown without synthetic pesticides or fertilizers as a way of reducing their own personal exposure and to encourage growers to adopt environmentally sound production methods.

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A pesticide is a chemical that is used to kill insects , weeds, and other organisms to protect humans, crops , and livestock . A broad-spectrum pesticide that kills all living organisms is called a biocide. Fumigants, such as ethylene dibromide or dibromochloropropane, used to protect stored grain or sterilize soil fall into this category. Generally, however, we prefer narrower spectrum agents that attack a specific type of pest: herbicides kill plants; insecticides kill insects; fungicides kill fungi ; acaricides kill mites , ticks, and spiders; nematicides kill nematodes (microscopic roundworms ); rodenticides kill rodents ; and avicides kill birds . Pesticides can also be grouped by their method of application (fumigation, for example, is dispersal as a gaseous vapor) or by their mode of action (an ovicide kills the eggs of pests ).

There are thousands of kinds of natural pesticides. Plants have been engaged for millions of years in chemical warfare with predators, most of which are insects. They have evolved a wide variety of complex protective mechanisms, many of which are toxic chemicals. Humans have probably known for a very long time that natural products such as nicotine from tobacco, turpentine from pines , pyrethrum from chrysanthemum species , and quinine from cinchona bark can provide protection from pests and parasites . Our diet contains a large number of such chemicals but ordinarily we have mechanisms to detoxify or excrete them so that they are not a problem.

The modern era of chemical pest control began in 1934 with the discovery of the insecticidal properties of DDT (dichloro-diphenyl-trichloroethane) by Swiss chemist Paul Müller. It became extremely important during World War II in areas where tropical diseases and parasites posed greater threats to soldiers than did enemy bullets. DDT seemed like a wonderful discovery. It is cheap, stable, easily applied, and highly toxic to insects while being relatively nontoxic to mammals . It seemed like the magic bullet that would provide "better living through chemistry." In 1948, Müller received a Nobel Prize for his discovery. It was quickly discovered, however, that this magic bullet was not always benevolent. Within a short time, many beneficial organisms were exterminated by DDT, while the pests it was created to control had developed resistance and had rebounded to higher levels than ever. Furthermore, persistent chlorinated hydrocarbons such as this tend to be taken up by living organisms and concentrated through food chains until they reach toxic levels in the top carnivores such as birds of prey or game fish . Species such as peregrine falcons , brown pelicans , osprey, and bald eagles disappeared from much of their range in the Eastern United States before DDT and similar persistent pesticides were banned. Rachel Carson's Silent Spring, possibly the most influential book in all of American environmental history, presents the argument against excessive, widespread pesticide use.

In spite of continuing worries about the dangers of pesticides, we still depend heavily on them. The Environmental Protection Agency reports (EPA) that around 500,000 metric tons of pesticides are used in the United States every year. We rely on them for disease control, agricultural production, preservation of buildings and materials, elimination of biting and troublesome organisms, forest protection, and a host of other purposes. Herbicides account for about 59%, insecticides 22%, fungicides 11%, and all other types together about 8% of our total use. Pesticide advocates claim that without modern pesticides, we would lose as much as half of our harvest to pests and that the world would suffer widespread and calamitous famines. Pesticide opponents argue that we could use cultural practices and natural pest predators or repellents to accomplish many of these same goals more safely and more cheaply than we now do with toxic synthetic chemicals.

Several movements aimed at reducing pesticide use have gained adherents in the United States and around the world. Integrated pest management (IPM) is a flexible, ecologically-based, pest-control strategy that uses a combination of techniques applied at specific times and aimed at specific crops and pests. It does not shun pesticides entirely but uses them judiciously when, where, and only in the minimum amount needed. It also employs biological controls (natural predators, resistant crop species) and practices such as mechanical cultivation to reduce pest populations. Many consumers choose to buy organic foods grown without synthetic pesticides or fertilizers as a way of reducing their own personal exposure and to encourage growers to adopt environmentally sound production methods.

In addition to buying organic products, there are a number of things that individuals can do to reduce their exposure to dangerous pesticides. Plant ground cover that competes successfully with weeds. Install or repair screens on doors and windows to keep out insects. Wash house and garden plants with soapy water to get rid of pests. Plant pest-repelling species such as marigolds, garlic, basil, or peppermint around your sensitive garden crops. Put out a cup of stale beer to control slugs . Learn to accept slightly blemished fruits and vegetables . Trim away bad parts or give up a part of your crop rather than saturate your environment with toxic chemicals.

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Pesticides are a broad class of chemicals and biological agents that are specifically designed and applied to kill a pest. Specific types of pesticides target specific types of pests: insecticides kill insects, fungicides kill fungi and bacteria, herbicides kill weeds and other unwanted plant vegetation, molluscacides kill mollusks, acaricides kill spiders, and so on. Pesticide use dates back to ancient times.

Pesticides are regulated in the United States at both the federal and state level. The primary legislation, one of the oldest environmental laws, is the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA, 1972), which is administered by the Environmental Protection Agency (EPA). Each state also has an agency responsible for carrying out FIFRA mandates. These agencies may be environmental or agricultural in nature, depending on the state. State laws can be more restrictive than the federal laws.

Pesticides are sometimes called "economic poisons." They are developed to kill something, and they are, therefore, inherently toxic. Pesticides that are less toxic are classified as "general use pesticides." These can be purchased by the average homeowner and applied without any special license or permits. More toxic compounds are called "restricted use pesticides" and their use requires a license. In some cases the restricted use materials have the same active ingredients as the general use materials, but at a higher concentration.

Anything that claims that it has pesticidal activity is, by law, a pesticide, and is subject to registration by the EPA and local state agencies. Household cleaners and bleach are legally pesticidesthe pesticide registration number can be found on the product container.

Within the broad classes of products that have similar types of action (e.g., weed killers, insect killers) there are further distinctions regarding the type of chemistry. For example, among insect killers, there are synthetic pyrethroids, organophosphates, and organochlorines. The most well known are the organochlorines, such as chlordane and DDT, which became popular after World War II, and were used in agriculture, and for home and commercial use, for decades. These compounds have low acute toxicity, but are persistent in the environment and have caused a series of long-term environmental health problems. They remain in soil and tissue for a very long time, and they have been shown to have a harmful impact on animal endocrine systems. Most organochlorines were phased out of use in the 1980s. They were replaced by organophosphate materials that are less persistent, but more acutely toxic. In the beginning of the 1990s these compounds, too, were beginning to be phased out through government actions, and voluntarily by the manufacturers.

Pesticides have entered the food system in many parts of the world. Though credited with an enormous increase in food and fiber production, indiscriminate use of these products has led to acute and long-term health problems for humans and animals. There are risks associated with the application of a pesticide into a system, while at the same time there are benefits for using these materials to reduce disease, increased food production, and lessen the risk of starvation.

Pesticides have been applied in many part of the world to control vector-borne diseases such as malaria, yellow fever, dengue, and others. The most prudent way to balance the benefits with the risks is an integrated approach to pesticide use, combining all control methodsphysical, biological, cultural, and chemical.

Mark G. Robson

(see also: Environmental Movement; Environmental Protection Agency; Fungicides; Toxicology )


Hayes, W., and Laws, E. (1991). Handbook of Pesticide Toxicology, Vol. 1. San Diego, CA: Academic Press.

Wallace, R., ed. (1998). Maxcy-Rosenau-Last Public Health and Preventive Medicine. Stamford, CT: Appleton and Lange.

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pesticide, biological, physical, or chemical agent used to kill plants or animals that are harmful to people; in practice, the term pesticide is often applied only to chemical agents. Various pesticides are known as insecticides, nematicides, fungicides, herbicides, and rodenticides, i.e., agents primarily effective against insects, nematodes (or roundworms), fungi, weeds, and rodents, respectively.

Pesticides can be derived from plants (e.g., pyrethrin, neem) or minerals, or they can be chemically manufactured (e.g., DDT, 2,4-D). Natural predators and other biological methods are also used. Among the biological agents, parasites and predators feed on pests, pathogens sicken them, and pheromones interfere with insect mating. There are also genetically engineered pesticides, such as the toxin-producing Bacillus thuringiensis strain used against moth larvae.

Chemical pesticides are usually contact, stomach, or fumigant poisons. Contact poisons may have immediate or delayed effects after physical contact with a pest. Fumigants, which may initially have the form of a solid, liquid, or gas, kill pests while in a gaseous state.

Some insecticides and fungicides are systemic, i.e., they are translocated by a plant from the area of application to other plant parts, where they affect only pests that feed on the crop. Nonselective pesticides can affect both the targeted pest and other organisms; selective pesticides affect only the target pest. Persistent pesticides are those that remain in the environment for a long time.

Since the publication of Rachel Carson's Silent Spring in the 1960s, there has been concern regarding the effects of chemical pesticides on humans and on the environment. In the environment, the biological concentration of chemical pesticides (the amount retained in an organism through direct contact or consumption of affected plants or animals) tends to increase the higher the animal is in the food chain. DDT, for example, severely reduced the rate of reproduction in many fish and birds.

Chemical pesticides now undergo exhaustive and expensive trials prior to government registration and release. The carcinogenicity of some pesticide components, however, is a vigorously debated topic. Government testing often uses massive amounts of such substances on laboratory animals, creating what some critics feel is an exaggerated assessment of their danger. Humans are heavily exposed to pesticides usually as a result of acute exposure, such as accidental inhalation, on the job.

Potential dangers from pesticide use must be weighed against improved crop quality and yield and greatly improved human health around the world, as well as the availability of disease-preventing fresh fruits and vegetables that the use of pesticides has made possible. Nevertheless, many consumers are concerned about the effects of pesticide residues in foods, especially for infants, whose systems may not be able to convert toxic chemicals into harmless substances as readily as adult systems can. In addition, concerns have been raised for farm workers in developing countries that lack the protective safeguards required in the United States; their health is threatened by the continued use of pesticides that are known health hazards. Efforts are being made to reduce chemical pesticide use in favor of Integrated Pest Management (IPM), biological controls, and plant breeding for inherent pest resistance.

See R. Carson, Silent Spring (1962); P. Hurst et al., The Pesticides Handbook (1991); G. J. Marco et al., ed., Regulation of Agrochemicals (1991).