Scale insects are some of the most destructive pests of shade trees and ornamentals, but few are serious forest pests. All scale insects pierce plant tissues and obtain nutrients by the ingestion of large amounts of plant sap. Localized injury may occur around feeding sites and serious damage or death may occur in heavily infested trees. All adult scales produce a waxy or shell-like covering. Many scale insects are often very inconspicuous (some scale coverings act as camouflage) making diagnosis of an infestation difficult. Others may produce an obvious waxy coating that is easily visible. There are hundreds of species of scale insects that feed on North Carolina trees and shrubs. However, each scale species usually infests only one (or a few) host species. Therefore, many scales are named for the specific host species on which they feed.
Scale insects: Order Hemiptera, Suborder Sternorryncha, Superfamily Coccoidea Many species of conifers and hardwoods. Scale insects are usually very host specific.
Symptoms vary widely with the scale species, host, and host tissue attacked. The most common symptoms observed may include foliage spotting, speckling, chlorosis, curling, and/or wilting; as well as galls, distorted growth patterns, bark swelling, twig dieback, branch dieback, decline, and mortality.
Adult females are sedentary, wingless, and may lack distinctive divisions between the head, thorax, and abdomen. They are covered with a hard scale or waxy secretion, and can range from 1/50 inch to 1⁄4 inch long. Scale coverings can be flattened and shield-like, spherical, or anywhere in between. Wax coatings (frequently white) may simply be a thin transparent film, but some species produce powdery bloom-like secretions. Adult males usually have wings, lack mouthparts, and are very active flying insects but are rarely observed. Nymphs are nearly microscopic and only mobile for a few days to a couple of weeks. After their first molt, they lose their legs, become sedentary, and begin to form a scale or wax covering. The best way to detect crawlers is to hold a white sheet of paper under an infested branch. Shaking the branch will cause the crawlers to fall onto the paper, where they may be visible as small moving dark specks.
In addition, like many other sap-sucking insects, some scales produce large amounts of honeydew (waste and excess plant sap that could not be processed by the digestive system) that drips down onto lower surfaces. Specialized fungi known as “sooty molds” grow on the honeydew, turning those surfaces dark gray or black. Other insects, such as ants and wasps, may also invade the area to feed on fresh honeydew.
Life cycles vary by scale species. Usually there are 1-4 generations per year in North Carolina. Most scales overwinter as late-stage nymphs. In the spring after maturation is complete, the eggs inside the adult female’s body mature within one to several weeks after fertilization. When the eggs hatch, the first stage nymphs (known as “crawlers”) search for feeding sites, or may spread to neighboring trees on the wind or by animal vectors such as birds and small mammals. Once a feeding site is located, the crawlers molt and become sedentary. Their long piercing-sucking mouthparts may penetrate deep into the plant to reach nutrient-rich sap. Feeding sites may be leaves, buds, twigs, or main stems depending on the scale species. It may take 2-8 weeks for nymphs to transform into fully mature adults. Populations can grow exponentially, resulting in heavy infestations in short periods of time, and are frequently cyclical.
Moderate. Most scales pose no serious threats to tree health. However, scales can be a serious nuisance on landscape trees and ornamentals. Gloomy scale can cause serious dieback or even death in many maple species. The tuliptree scale (Fig. 1) is a serious pest of yellow poplar that can cause branch dieback or death. The beech bark scale releases a potent toxin and carries a pathogen that threatens the survival of American beech. Lecanium scales are a very common pest of hardwoods, but rarely require control measures.
Chemical control options are available for high-value trees. Treatments are usually ineffective against adults. Therefore, applications of insecticides or horticultural oils must target crawlers when they are active. Close monitoring of crawler activity and repeated chemical applications are usually necessary for successful control.
Life stages may overlap significantly. Crawlers are most active in the spring and fall.
Thanks to the NC Forest Service for this information.