Scale insects feed on plant sap. They have long, threadlike mouthparts (stylets) which are six to eight times longer than the insect itself. Scale feeding slowly reduces plant vigor; heavily infested plants grow poorly and may suffer dieback of twigs and branches. An infested host is occasionally so weakened that it dies. Scales often secrete a sticky honeydew which is attractive to wasps and ants and which supports the growth of black sooty molds.
Scale insects are generally controlled by natural enemies, including tiny parasitic wasps and predators such as ladybugs. It is very common for ladybugs to move onto a plant with a growing scale infestation; before deciding upon treatment, look for adult and immature ladybugs on your plants. These pests are often controlled with horticultural spray oils, biological control or pest resistant plants. Dormant oil treatments can be used against almost all scale problems and are generally applied in very early spring, before bud break. Summer oils can also be very effective against most scales, but as with dormant oils, some plants are sensitive to these treatments. Check labels to make sure your plant is not harmed by the oil treatment you are considering. Most other insecticides, including insecticidal soaps, can be used only against the mobile crawler stage of scales since adult scales are protected from insecticides by a waxy covering. These treatments are very effective, but must be carefully timed as crawlers are only active for a limited period.