Scale insect
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 Wax scales on a lemon tree branch
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Aclerdidae |
The scale insects are small insects of the order Hemiptera, generally classified as the superfamily Coccoidea. There are over 7,000 species of scale insect.
Scale insects are all parasites of plants, feeding on sap drawn directly from the plant's vascular system. 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 almost always immobile (aside from mealybugs) 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.
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 the immature external morphology at sexual maturity (neoteny). Adult males have wings but never feed and die within a day or two. Male scale insects are unusual in possessing only one pair of wings, thus making them resemble true flies (Diptera), though they lack the halteres (rudimentary hind wings) seen in flies, and have tail filaments, which do not occur in flies. The specifics of their reproductive systems vary considerably within the group, including hermaphroditism and seven forms of parthenogenesis.
