Scale insects are strange. Adult females resemble imature stages: they are soft-bodied and lack wings, compound eyes and obvious segmentation. The last three developmental stages (pre-pupa, pupa, adult) in male scale insects do not feed. Adult males usually live for less than a day; they are listeless and slow. They may be dipterous or wingless, have compound eyes, or a variable number of simple eyes aranged in a line around the head, or dorsal and ventral pairs. Many scale insect species have done away with males completely, reproducing asexually. Scales have the greatest diversity of sperm structure and sex determining chromosome systems of any known group of organisms. One group has a placenta-like structure in the female that is used to feed first-instar males. Another is hermaphroditic - the only known example among insects. Most scale insects produce a waxy secretion that either coats the body or protects it beneath a domicilelike structure (called a scale cover). Secretions vary from a thin translucent sheet to a thick, wet mass, to a powdery bloom.
Scale insects are serious plant pests and because they are small and cryptic, they frequently are not detected until they have caused significant damage. They are difficult to detect in quarantine inspections, especially at low levels. They are most important as agricultural pest of perennial plants and can cause serious damage to nut and fruit trees, woody ornamentals, forest vegetation, greenhouse plants, and house plants. Damage is usually caused by removal of plant sap, but also may be caused by plant pathogens, toxins and the production of large quantities of honeydew with resultant growth of sooty mold fungi that cover leaf surfaces and reduce photosynthesis.
Scales also can be beneficial. They have been used as sources of dyes (cochineal scales, gall-like scales, giant scale, and lac scale), of shellac and lacquerlike substances (lac scales and giant scales), of candle wax (soft scales), of the manna of the Israelites (mealybugs), of pearls for necklaces (ground pearls or giant scales), and even chewing gum (ornate pit scale). Cochineal scales and mealybugs have been used in the control of noxious weeds.
Distribution and Ecology
Scale insects occur in nearly all available botanical habitats from the tundra to the tropics. They are found on nearly all parts of the host including the leaves, branches, trunks, fruits, and roots. They sometimes occur under bark and may cause various kinds of plant deformities including chlorotic spots, pits, and galls.