Scale insects vary dramatically in appearance; some are very small organisms (1–2 mm) that grow beneath wax covers, to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax. Adult female scales are almost always immobile 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.
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. However, scales often are controlled by use of horticultural oils, that suffocate them, systemic pesticides that poison the sap of the host plants, or by biological control agents such as tiny parasitoid wasps and Coccinellid beetles. Insecticidal soap may also be used against scales. It is commercially available or can be made of certain types of household soap.
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