Scale insects vary dramatically in appearance; some are very small organisms (1–2 mm) that grow beneath wax covers (some shaped like oyster shells, others like mussel shells), to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax. Adult female scales are almost always immobile (aside from mealy bugs) and permanently attached to the plant they have parasitized. They secrete a waxy coating for defense; this coating causes them to resemble reptilian scales or fish scales, hence their common name.
The first instars of most species of scale insects emerge from the egg with functional legs and are informally called “crawlers”. They immediately crawl around in search of a favorable spot to settle down and feed. In some species they delay settling down either until they are starving, or until they have been blown away by wind onto what presumably is another plant, where they may establish a colony separate from the parents.
Most scale insects are parasites of plants, feeding on sap drawn directly from the plant’s vascular system. A few species feed on fungal mats and fungi, e.g., some species in the genus Newsteadia in the family Ortheziidae. Scale insects feed on a wide variety of plants, though particular species commonly are specific to particular host plants or plant groups. For example, various kinds of cochineal are restricted to cactus hosts. Some scale insects species evolved symbiotically with some ant species.
Many scale species are serious crop pests. The waxy covering of many species of scale insects protects them effectively from contact insecticides, which are only effective against the first-instar nymph stage known as the crawler. Some types of scale insect are economically valuable for the substances they can yield under proper husbandry. Some, such as the cochineal, kermes, lac amernian, and Polish Cochineal have been used to produce red dyes for coloring foods and dyeing fabrics.