This spring and summer at the Extension Center we have seen many cases of scale insects on landscape plants, especially hollies. Scale insects are a peculiar group and look different from the typical insects we encounter day to day. They look so different, many people fail to recognize them as an insect until plant damage is severe. Small, immobile, with no visible legs or antennae, they resemble individual fish scales pressed tightly against the plant on which they are feeding. There are over l50 different kinds of scales in North Carolina and many are common and serious pests of trees, shrubs, and indoor plants.
There are two general types of scales: armored scales (e.g., oystershell scale, gloomy scale, euonymus scale and pine needle scale) and soft scales (e.g., cottony camellia scale, magnolia scale and tuliptree scale). Armored scales are so named because they secrete a protective cover over their bodies. Most species overwinter as eggs beneath the female cover. In spring, eggs hatch into tiny mobile crawlers, which migrate to new feeding sites. After a few days, crawlers settle, insert their mouthparts, and begin feeding. Soon they secrete a protective cover and lose their legs. Large populations can build up unnoticed before plants begin to show visible symptoms. Generally, soft scales are larger and more convex than armored scales. Many resemble miniature tortoise shells.
Soft Scales usually cover themselves with wax, but they lack the detachable protective cover for which armored scales are named. Most soft scales overwinter as immature, fertilized females. In spring they resume feeding, mature, and lay eggs. These hatch into tiny crawlers. After locating suitable feeding sites, crawlers settle and begin feeding. Some species lose their legs once they’ve settled, but others retain them and are able to crawl short distances to find suitable overwintering sites in the fall. Except for soft scales, which infest indoor plants, most have only a single generation per year at our latitude.
Damage: Plants heavily infested with armored scale often look water stressed. Leaves turn yellow and drop, twigs and limbs may die and the bark cracks and gums. Armored scales can kill plants and must be controlled when their populations rise.
Soft scales also reduce plant vigor, but usually not enough to kill. The main problem this type of scale causes is that unlike armored scales, they produce large amounts of honeydew (like aphids) that can cover leaves and fruit and act as a growth medium for black, sooty mold. Honeydew also attracts ants, flies, wasps and bees, whose populations around infested plants can become a nuisance.
Control: Dormant oils are effective on the overwintering stage of most species, but they can only be applied in early spring before buds start to swell. Adult scales are protected from insecticides by waxy coverings. Control measures, therefore, must be aimed at the unprotected immatures (crawlers) or the overwintering stage. If during the growing season you notice light leaf discoloration, minor production of honeydew and branch dieback from scales, apply an oil or insecticidal soap spray. Oils and insecticidal soaps work by smothering the insect, both adults and crawlers. In addition, such alternative pesticides are less harmful to natural enemies than conventional insecticides.
Sometimes a conventional, broad spectrum insecticidal treatment is required. These will be most effective when applied during the crawler stage. Good control requires accurate identification of the pest species so that hatching dates of crawlers can be determined. Once the pest is identified and proper timing known, any one of several common insecticides can be used. Consult your local Extension Agent for current insecticide recommendations.