Useful and Harmful Insects
Although humans generally regard insects as harmful, most insects are actually harmless or helpful. In fact, less than 2 per cent of all species are harmful; but among them, they can cause major crop damage and spread serious diseases on a large scale.
Insects constitute more than half of the diet of fishes, birds, amphibians, reptiles, and small mammals. In some parts of the world insects are eaten by humans.
Insects pollinate many different types of plants. They improve the soil by bringing nutrient-rich soil from deeper layers to the surface. Many insect pests are destroyed by predatory or parasitic insects.
Several commercial products are obtained from insects. Among these are honey, beeswax, natural silk, and shellac. The tree galls made by certain insects contain substances that are used in medicines and dyes, and in tanning leather.
Insect pests are responsible for causing billions of dollars' worth of damage each year. More than 600 species of insects in North America are considered harmful pests. Insects not only harm or kill plants and animals, but they also spread diseases, infest stored foods and grains, and cause damage to homes and other buildings and to clothing and furniture.
Most insects that spread disease do so by biting their victims, passing the disease organisms—such as bacteria, fungi, and protozoans—in their saliva. Plants and animals, including humans, are subject to various viral diseases carried by insects. Insect-borne diseases transmitted to humans include malaria and encephalitis.
Insects are controlled by a variety of methods. Common backyard insects are killed by insecticides or by “bug zappers,” fluorescent tubes covered by wires carrying an electric current. The tubes emit ultraviolet light, attracting insects; the insects are killed when they come in contact with the wires. Agricultural pests are killed by insecticides or by such organisms as parasitic bacteria or roundworms, which are released into infested areas.