pest, any organism judged as a threat to human beings or to their interests. When early man hunted animals and foraged for food, he shared the natural resources with other organisms in the community. As human culture developed and population rose, people made ever-increasing demands on these resources. One result of changing the environment has been a great increase in the number of species that are now recognized as competitors of humans. These competitors are usually referred to as pests. The definition of pest is, of course, subjective. An ecologist would not necessarily consider several leaf-eating caterpillars on a plant as pests, whereas a gardener who cultivated the plant might very well do so. And only one bat, rat, or mouse is enough to qualify as a household pest.
Natural communities have always contained organisms that were economically significant; locusts, for example, have plagued humans throughout history, and grain from Stone Age locations has been found infected with bunt and ergot diseases. Most species that became pests, however, did so because of environmental modification, occasionally from natural causes but usually from human activities.
In order to appreciate some of the methods devised to combat pests, one should consider how advancing technology has increased the number of harmful insect species. The change from natural vegetation to large areas of single-crop (monoculture) agriculture has three consequences. First, given a more uniform food source, some plant-eating species increase to large populations. Second, the uniform plant cover is easily invaded by attacking pests. Third, the introduction of new crops over large areas results in the transfer of previously harmless insects from scattered native plants to the new and abundant sources of food. Cultural practices such as fertilization, irrigation, and the use of modern harvesting equipment (which often leaves large amounts of plant litter in the field) enhance still further the ability of pest species to increase rapidly. In addition, the elimination of species that compete with or prey upon pests—an unintended effect in some pest control programs—has also exacerbated certain pest problems. Also, the ease with which people and goods can be transported around the world has resulted in the introduction of exotic pests in many places.
Pests are found throughout the animal kingdom. Microorganisms such as fungi, bacteria, and viruses are here considered with the pests even though they are usually thought of as agents of disease. Most animal pests are invertebrates, among them protozoa, flatworms, nematodes, snails, slugs, insects, and mites. Among the vertebrates, rabbits, elk, deer, and many kinds of rodents are sometimes injurious to crops.
Insects are also serious pests, especially those involved with the transmission of disease. Each year millions of lives, particularly in the tropics, are threatened by insect-borne diseases. Malaria and yellow fever are transmitted by mosquitoes, plague by fleas, typhus by the human louse, sleeping sickness by tsetse flies, Chagas’ disease by blood-sucking bugs, and leishmaniasis by sandflies. Other diseases may be spread by insects accidentally as a result of their habits. The emerald ash borer, a pest responsible for the deaths of hundreds of millions of ash trees in North America, arrived in the United States in the 1990s, probably embedded in shipping materials from eastern Asia.
Pest control began to receive attention in the 18th century, and it has become increasingly important since then. As control programs were developed, they tended to fall primarily into two somewhat mutually exclusive categories: chemical and biological. Physical or mechanical methods, including sticky barriers, heat killing (for storage pests), and flooding (for ground pests), were also developed. Because of their limited utility and short-term effectiveness, they have been superseded largely by chemical and biological methods.
The chemical, or pesticide, approach probably began with the use of poisonous plant compounds—ground tobacco was used in France to kill aphids about 1763. Other natural products such as nicotine, rotenone, petroleum, kerosene, creosote, and turpentine were used in the 19th century. Inorganic compounds such as Paris green, lime sulfur, Bordeaux mixture, hydrogen cyanide, and lead arsenate were also introduced in the 1800s.
With the appearance of the synthetic organic compounds during World War II, a dramatic change occurred in pest control. Some organic compounds, such as the dinitrophenols, had been used earlier, but the discovery of the insecticidal properties of DDT (dichloro-diphenyl-trichloroethane) and of BHC (benzene hexachloride) made the concept of pest-free crops possible. Research in the 1930s on plant hormones and related compounds led to the development of the selective herbicide 2,4-D (2,4-dichlorophenoxyacetic acid), and this became commercially available about the same time as DDT. Following the appearance of these new synthetic organic compounds, a whole new series of pesticides—insecticides, fungicides, herbicides, and plant growth regulators—was introduced.
Although the ancient Chinese used predacious ants to control foliage-feeding insects, the control of pests by biological means appears to have been initiated in the Western world with the importation of the Indian Mynah bird into Mauritius in 1762 to control the red locust. Control of bedbugs by predatory agents was recommended in 1776. A number of other projects in biological control were carried out in the 1800s, but the modern era in this phase of pest control began in 1888 with the importation into California of the vedalia beetle (Rodolia cardinalis) to control the cottony-cushion scale (Icerya purchasi). This beetle, imported from Australia, saved the citrus-fruit industry.
The use of plants resistant to insects was suggested about 1788 as a means of controlling the Hessian fly, a pest of wheat. The classic example of this approach was the control of phylloxera, aphid-like insects that attacked the root stocks of the European wine grape and almost ruined the European wine industry. The solution lay in grafting the European wine grape onto resistant American stocks. The effort, about 1900, to control the lantana shrub in Hawaii by introducing an insect appears to have been the first attempt to control weeds biologically. The use of microorganisms to destroy insect pests began in the late 1800s and early 1900s.
With the appearance of the synthetic organic insecticides, however, emphasis was placed on chemical control, and biological means fell into neglect. The extensive use of powerful chemical agents soon resulted in a number of serious ecological problems. Consequently, current pest control practice minimizes the use of pesticides and combines them with biological methods in an approach called integrated control. The integrated control of an insect that attacks crops may, for example, involve the breeding of pest-resistant crop varieties; the development of crop culture methods that inhibit pest proliferation; the release of organisms that are predators or parasites of the pest species; the placement of traps baited with the pest’s own sex attractants (pheromones); the disruption of the pest’s reproduction by the release of sterilized pests; and, often as a last resort, the application of chemical insecticides. Other important tools of modern pest control or importation and quarantine regulations, which are designed to prevent the introduction of exotic pests.