Horticulture/Scale Insects

The scale insects are small insects of the order ../Hemiptera/, generally classified as the superfamily Coccoidea. There are over 7,000 species of scale insects, all of which are parasites of plants, feeding on sap drawn directly from the plant's vascular system.

Description
Scale insects vary dramatically in their appearance from very small organisms (1-2 mm) that occur under wax covers (some look like oyster shells), to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax. Adult female scales are 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, hence the name.

Ecology
Scale insects feed on a wide variety of plants, and many scale species are considered pests. Some types are economically valuable, such as the cochineal and lac scales. Scale insects' waxy covering makes them quite resistant to pesticides, which are only effective against the juvenile crawler stage. However, scale can be controlled with horticultural oil, which suffocates them, or through biological controls. Soapy water is also reported to be effective against infestations on houseplants. Female scale insects, unusually for Hemiptera, retain their larval form at sexual maturity (neoteny). Adult males have wings but never feed and die within a day or two. The specifics of their reproductive systems vary considerably within the group, including hermaphroditism and seven forms of parthenogenesis.

Host plants
Scales feed on a very wide variety of plants, though most of the particular species have fairly narrow host ranges. If the particular scale can be identified, part of the control strategy is to inspect the area for other host plants, since the scale insects do not travel very far.