The study of insects that have a direct influence on humanity. Though this includes beneficial as well as harmful species, most attention is devoted to the latter and how they become pests and are controlled. The emphasis on managing harmful insects reflects the immediacy and seriousness of pest problems, particularly the destruction of food and the transmission of disease. These are highly visible problems, whereas the benefits gained from useful insects are not so clearly understood, nor so well documented economically.
Central to the definition of a pest is determination of the economic threshold. Any insect population, when introduced into a favorable environment, increases numbers until reaching an environmental carrying capacity. In pest insects, there exists a density above which the insect population interferes with human health, comfort, convenience, or profits. When this economic threshold is reached, a decision must be made to utilize some control measure to prevent further increase in numbers. Often, the presence of even a single insect is sufficient to warrant control measures, for instance, when that insect is a flea harboring the plague bacillus, or a mosquito capable of transmitting malaria. Also, consumer expectations in most markets are for insect-free produce, so that the economic threshold can be very low on items that people eat. However, economic thresholds may be higher for insects that damage only the inedible portions of crop plants such as the leaves of beans, tomatoes, and apple trees. In any case, knowledge of the amount of injury which is due to different densities of insects is an important prerequisite for efficient management.
Pest management
Management of insect pests begins with prevention. Many of the United States' most noxious insects have been imported from overseas: most domestic cockroaches, the gypsy moth, Japanese beetle, corn borer, housefly, cabbageworm, and codling moth are just a few. Some North American insects have spread elsewhere too—the Colorado potato beetle to Europe and the fall webworm to Japan. To stem the flow of insect invasions, the federal government's Animal and Plant Health Inspection Service maintains inspection facilities for the examination of all incoming shipments of plant or other material that may harbor pests.
Once a pest is established, its spread can sometimes be slowed by an efficient system of local quarantines, early detection, and local eradication. A widely used eradication method is the application of synthetic chemical insecticides. Insecticides were once regarded as a panacea for pest problems, but the development of resistant strains of major insect pests, together with the rising cost of materials and application, has led to recognition that insecticides are more efficiently utilized in a program integrating them with other techniques in a framework of total crop management. See also Insecticide.
Another method is biological control, in which insects have had their numbers checked by natural enemies. Economic entomologists have effectively reduced densities of several pests by releasing parasites or predators. Natural enemies that are mobile and relatively restricted in diet do the best job of biological control. See also Insect control, biological.
Crop rotation is a standard agronomic practice that often reduces damage due to insects. Rotation of alfalfa or soybeans with corn reduces populations of corn rootworms, wireworms, and white grubs. The physical disruption of autumnal plowing and disking destroys many insects that could overwinter in stubble or on the soil surface.
The cleanup of breeding and gathering sites is useful, especially in management of medically important insects, many of which have evolved resistance to the commoner insecticides. In general, the most successful programs of insect pest management rely on integrated control or the use of several methods in concert to control a complex of pests.
Beneficial insects
It has been estimated that the dollar value gained from a single insect, the honeybee, equals the loss from damage plus cost of control for all pests combined. Honeybees are managed for their honey and beeswax, but their most valued service is pollination of crop plants. Bees of many species are the chief pollinators, though wasps, flies, moths, butterflies, and beetles pollinate as well. See also Pollination.
Silk is produced by larvae of the silkworm, an insect so thoroughly domesticated that it cannot climb its food plant, mulberry, with its degenerated legs. The silkworm apparently no longer survives in the wild. Many uses of silk have been taken over by less expensive synthetic materials.
Other insects may be equally beneficial, but their value is not so easily calculated. Foremost among these are predatory insects of several orders. These predators may prevent other insects from ever reaching an economic threshold and thus becoming pests. Innumerable insect species are scavengers, quietly but efficiently breaking down the remains of dead plants and animals. A lack of scavenging insects would, however, result in a great increase of decomposing organic material lying about. Plant-eating insects have been set to beneficial use when their diets consist mainly of unwanted weeds.
Certain rare and showy butterflies and beetles are sought after so that they have considerable economic worth. Conservation of rare and endangered insects incurs some expense as well. Habitat management to conserve rare insects is a valid and growing concern of economic entomologists.
Finally, insects have rendered invaluable service to science, and thus to humanity, as easily reared experimental animals for investigation of basic principles of genetics, biochemistry, development, and behavior. See also Insecta.
