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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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