Scale insects, mealybugs, or coccids are a diverse group of species of insects in the superfamily Coccoidea, order Homoptera. The females of scale insects are wingless, and are also often legless and virtually immobile. For protection, female scale insects are covered by a scale-like, waxy material. Like other homopterans, scale insects are herbivores with piercing mouth parts that are used to suck juices from plant tissues. Male scale insects generally have a pair of wings and can fly, although some species are wingless. Male scale insects only have vestigial mouth parts and do not feed, dying soon after mating.
Like other homopterans, scale insects have an incomplete metamorphosis with three stages: egg, nymph, and adult. The first nymphal stage is known as a "crawler," because it has legs and actively moves about. After the next molt, however, the legs are lost in most species, and the scale insect becomes sessile, secreting a scale-like, waxy covering for protection.
Some species of scale insects are economically important as agricultural pests, because of the severe damage that they cause to some crop plants. This injury is usually associated with mechanical damage to foliage caused by piercing by the feeding apparatus of the scale insect. Damage is also caused by the withdrawal of large quantities of carbohydrates and other nutrients with the sap.
The cottony cushion scale (Icerya purchasi) is an important pest of citrus crops in the southern United States, where it has been introduced from its native Australia. This species is much less of a pest than it used to be, because it has been relatively well controlled by several predators that were later discovered in their native habitat and subsequently introduced to the United States, namely, the vedalia lady beetle (Vedalia cardinalis) and a parasitic fly (Cryptochetum iceryae). This case is commonly cited as one of the great successes of non-pesticidal, biological control of a serious insect pest.
The California red scale (Aonidiella aurantii) is another important agricultural pest of western citrus trees. The San Jose scale (Quadraspidiotus perniciosus) was introduced to North America from Asia in the 1880s, and is a serious pest of many species of orchard and ornamental trees and shrubs. Various species of mealybugs are also important pests, for example, the citrus mealy-bug (Planococcus citri), and the greenhouse mealybug (Pseudococcus longispinus).
Females of the Indian lac insect (Laccifer lacca) of southern and southeastern Asia produce large quantities of a waxy substance that is collected, refined, and used to prepare a varnish and shellac. Males of the Chinese wax scale (Ericerus pela) secrete relatively large amounts of a white wax, which can be collected for use in making candles. The Indian wax scale (Ceroplastes ceriferus) produces a wax that is collected for use in traditional medicine.
The tamarisk manna scale (Trabutina mannipara) occurs in the Middle East, where it feeds on tamarisk trees (Tamarix spp.). The females of this insect excrete large quantities of honeydew, which in arid regions can accumulate abundantly on foliage, drying into a sweet, sugar-rich material known as manna. Manna is featured in the Old Testament of the Bible, in which it is portrayed as a miraculous food delivered from the heavens, sustaining the Israelites as they wandered in the wilderness after their exodus from Egypt (Exodus 16: 14-36).