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Scale insects are the superfamily Coccoidea of the insect order Homoptera. About 5,000 species of them have been described. The scale insects are minute to small, highly specialized, and generally spend their entire life near the spot where they hatched. All of their forms that possess legs have one-segmented or twosegmented tarsi, bearing a single claw. Females are always wingless, but males may be winged (with one pair of wings) or wingless. Adult males have no functional mouthparts. First instars (crawlers) have functional legs and are very mobile, but later instars may be legless and sedentary. Immature stages and adult females feed on plant juices by inserting their long stylet mouthparts into a host plant and sucking out the juices.
Worldwide there are 21 families of scale insects. Margarodidae and Ortheziidae are considered "most primitive" because the females have abdominal spiracles and the males often have faceted eyes. The other families are considered "most advanced" because the adult females never have abdominal spiracles and the adult males have simple eyes. The 3 largest families are Diaspididae (armored scales), Pseudococcidae (mealybugs), and Coccidae (softscales).Generally scale insects are considered pests destructive to agricultural crops. However, some species can furnish useful products well known in commerce, such as shellac (from Laccifer lacca), cochineal red dye from (Dactylopius coccus), crimson lake, and other dyes. Some species produce wax of marketable value, such as Chinese wax which is used for candles. Jewelry is fashioned from some of the hard shelled margarodids. Dactylopius spp. have been used as biological control agents for Opuntia cacti in Australia, Ceylon, India, Hawaii, and Mauritius. More than half of the successful biological control projects in the world have been against scale insects. Even with these impressive figures scale insects are still horrendous pests of agriculture. It has been estimated that if they were left uncontrolled by natural means and chemicals their potential damage could reach several billions of dollars in the U.S. alone.
Scale insects attack all parts of plants from roots to buds, flowers, and fruits. Some are subterranean in habit. Some produce honeydew in great quantities, whereas others, such as Diaspididae (armored scales) do not produce the excretion. Some species cause galls to form on their hosts, others cause leaf rolling, surface pitting or other tissue changes. Other species cause disease-like symptoms by injecting irritating salivary products into the host plant. The dispersal of coccids on a plant or plants nearby is by the active first instar or crawler stage. Over longer distances some crawlers may be spread by wind, but generally the movement of infested plant material in commerce has been responsible for dispersal over long distances.
Coccoidea (scale insects)
Margarodidae (ground pearls)
Diaspididae (armored scales)
Coccidae (soft scales)
and 16 other families