Scale Insects, a family of insects that feed on plants. The name refers to a secretion of waxy scales that serve as a protective covering. Scale insects attack almost any part of a plant and feed by sucking out the juices. They inhabit almost all parts of the world. Many species are serious agricultural pests. They range in size from 1/16 inch (1.5 mm) to one inch (25 mm) in length.

Scale insectsScale insects secrete waxy scales that serve as a protective covering.

Scale insects are classified by the type of scale they secrete. Armored scale insects secrete a hard, crusty covering. The scales differ in shape and color in each species. Adult males develop wings and legs and shed the protective covering before mating. Adult females have no legs, antennae, or wings. They remain under the protective covering for life. They lay 40 to 100 oval white eggs on the underside of the covering. The newly hatched young, or crawlers, attach themselves to the host plant with their tubular mouths. Among the most destructive species are the San Jose scale and the oyster shell scale. The San Jose scale is gray and about 1/16 inch (1.5 mm) in length. It attacks such fruit trees as apple, pear, and peach. The oyster shell scale is brownish and about 1/8 inch (3 mm) long. It attacks many kinds of trees and shrubs, such as lilac, ash, poplar, and maple.

Unarmored, or soft, scale insects secrete a small amount of powdery or mealy wax that adheres to the insect. The most destructive species are the black scale and citricola scale. Both feed on fruit trees. Mealybugs are pests of citrus trees and of ornamental and house plants. Scale insects are controlled by introducing natural predators, such as beetles and parasitic wasps.

A few species of scale insects are beneficial to humans. For example, the cochineal insect yields a dye and the lac insect is a source of shellac. The manna of the Bible is believed by some persons to have been the secretion of certain species of scale insects.

Scale insects belong to the superfamily Coccoidca of the order Hemiptera. The San Jose scale is Quadraspidiotus perniciosus; the oyster shell scale, Lepidosaphes ulmi. Both are of the family Diaspididae. The black scale is Saissetia oleae; the citricola scale, Coccus pseudomagnolarium. Both are of the family Coccidae.