Scales are sucking insects closely related to aphids, mealybugs and whiteflies. Like their counterparts, their mouthparts are fused into a slender tube, or stylet, that is used to pierce the plant surface. After hatching, the young scales wander over the plant searching for a spot in which to settle and begin producing their distinctive shell coverings. As adults, they are sedentary; only a few species have the ability to move.
Scales primarily attack indoor, tropical and subtropical plants, but may also be found on a number of popular flowering plants grown out of doors in temperate regions: canna, geranium, peony, rose, tuberous begonia and wax begonia; therefore, be particularly careful if moving some of these plants indoors in the fall. The most common symptom of a scale infestation is that the leaves of the affected plant turn yellow and may drop; rose leaves may wilt and turn dark. In addition to leaf drop, scales may cause reduced growth and stunted plants. Some species also produce a shiny or sticky material (honeydew) that will cover the leaves of infested plants.
Scales belong to 2 common families: armored scales and soft scales. Use a magnifying glass or hand lens with a pen knife to distinguish between the two main types of scales (although the methods used to control both types are the same). Armored scales are small, 1/8" long, and do not produce the copious amounts of sticky honeydew that characterize their "soft" cousins. Another distinguishing feature is that armored scales remain attached to host plants when their shells are lifted up and pried away with a sharp knife. In addition to the problems described above, armored scales carry viruses that can cause plant disease, even when present in small numbers. Contrary to what one might logically presuppose, the shells of the soft scale are, in fact, as hard or harder than those of armored scales. At maturity, their shells are generally larger and darker as well. The most common soft scales have semicircular profiles in the adult stage. They also secrete varying amounts of honeydew, thereby attracting ant species that feed this substance to their young. They can be easily distinguished from their armored cousins because their shells do not lift off their bodies when pried away with a knife.
Infestations caught early on can be controlled by scraping insects off plant surfaces with a dull knife or your fingernail. Alternately, treat light infestations with a cotton swab dipped in rubbing alcohol, making sure the alcohol contacts the scale insects. Spray more seriously infested plants with a mixture of 1 cup isopropyl alcohol, 1 tablespoon insecticidal soap, and 1 quart of water; repeat applications every 3 days for 2 weeks. Since the young scales are unprotected by the waxy covering that renders mature scales difficult to kill, sprays timed to coincide with the crawler stage are most effective. You will need a hand lens to detect the tiny young scales; they will resemble minute, animated pancakes.