scale insect

 

common name for members of a highly modified group of insects belonging to several families of the superfamily Coccoidea. Scales possess antennae and are characterized by reduced legs. Only the males have wings; females are always wingless. Scales are popularly subdivided into three groups; the armored scales, the unarmored scales, and the mealybugs. The armored scales secrete a wax covering, the shape of which is characteristic for any given species. Under this coat, the insects develop and feed, sucking the sap of plants with their thin tubular mouthparts. The females never leave the protection of the scale after once forming it, but the adult males, which do not feed, develop a single pair of wings, leave the scale, and seek out the females, fertilizing them after the females are under the shell. Among the important armored scale pests of citrus, other fruits, and ornamentals are the San Jose scale, the oyster scale, the purple scale, and the California and Florida red scales. The unarmored scales (or soft scales) are similar to the armored scales except that only a small amount of wax is secreted, which adheres to the insect. Unarmored scale pests of citrus fruits include the black scale and citricola scale. Mealybugs appear as white cottony clusters on citrus, ornamentals, and greenhouse plants. Like other scale insects, newly hatched nymphs, called crawlers, have legs and actively seek out food. When they find a suitable spot, they settle down to feed. Some scales secrete a resinous covering, which is used in the commercial production of shellac, varnish, and paints (see lac). Control of scale insects has been largely by use of natural enemies, especially ladybird beetles and small parasitic wasps, which are natural predators of these pests. Scale insects have proved difficult to control by chemical means. Scale insects are classified in the phylum Arthropoda, class Insecta, order Homoptera, superfamily Coccoidea.

Scale insect
Wax scales on a lemon tree branch
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Insecta Order: Homoptera Suborder: Sternorrhyncha Superfamily: Coccoidea
Families
Aclerdidae Asterolecaniidae Beesoniidae Carayonemidae Cerococcidae Coccidae Conchaspididae Dactylopiidae Diaspididae Electrococcidae Eriococcidae Grimaldiellidae Halimococcidae Inkaidae Jersicoccidae Kermesidae Kerriidae Kukaspididae Labiococcidae Lecanodiaspididae Margarodidae Micrococcidae Ortheziidae Phenacoleachiidae Phoenicococcidae Pseudococcidae Putoidae Stictococcidae

The scale insects are small insects of the order Homoptera, generally classified as the superfamily Coccoidea. There are about 8,000 species of scale insects.

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 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 or fish 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, Polish cochineal and lac scales. Scale insects' waxy covering makes them quite resistant to pesticides, which are only effective against the juvenile crawler stage. However, scales are often controlled with horticultural oils, which suffocates them, or through biological control. Soapy water is also reported to be effective against infestations on houseplants.

Female scale insects, unusually for Homoptera, 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.

A female cottony cushion scale, (Icerya purchasi Maskell) scale with young crawlers

Verbascum thapsus showing scale damage.

External links

• ScaleNet homepage
• Cottony Cushion Scale: the pest that launched a pest control revolution with photos
• Diaspididae of the World
• Pests & Diseases of Roses

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