Scale insects can be serious pests of conifers and deciduous trees alike. They get their name from the protective waxy shell covering their bodies. Scale insects are subdivided into two main groups: the soft scales, which secrete a thin attached waxy layer to their bodies; and the armored scales, which use shed skin to form a hard, shell-like covering.
Adult females range in color from gray to brown to white and vary in length from 1/16 inch to over 3/8 inch depending on the species. Male scale insects are tiny and gnat-like in appearance. First stage nymphs are referred to as crawlers. They are tiny, six-legged, yellowish and highly mobile. Crawlers migrate to new feeding sites on the tree.
Although different species can be quite different in appearance, most share the common features of being closely attached to the tree on which they feed, are covered with a waxy coating, and do not move around much. Most do not even look like insects. After all, as adults they don’t possess the antennae, legs or wings of typical insects. However, scale insects can be devastating plant feeders, and for that reason, tree care workers and arborists should be concerned.
Damage by scales
If it weren’t for the damage that scales cause to trees and shrubs, they could be looked upon merely as a marvelous part of nature’s wondrous pageantry. However, that’s just not the case. Scale insects have the capacity to extract plant sap, a little at a time, slowly reducing the vigor of a tree. When heavily infested, they cause stems and branches to die.
Damage from scales is twofold: the aforementioned loss of tissue in the tree canopy, as well as the weakness that they cause. As many of us know from watching specials on PBS or the Discovery Channel, the weak specimens in nature tend to be the ones that attract other predators. In a similar manner, trees stressed from scale insects become more attractive to borers, spider mites and other insect pests as a result of the stress caused by their feeding.
The method in which scale insects feed is quite different from leaf chewers such as sawflies. Scales are classified as one of the “piercing-sucking” insects. They insert long, threadlike mouthparts known as stylets into plant tissues and extract cell sap. As a result of this feeding, scales secrete a sweet, sticky substance called honeydew, which drops on the surfaces of understory plants (or cars and picnic tables). The honeydew is attractive to other insects, especially ants and wasps. Once deposited below, the honeydew supports the growth of black sooty mold fungi that is unsightly and interferes with photosynthesis.
Inspecting for scales
Because of their cryptic nature and potential to injure trees, it’s important to include scouting for scales as part of a regular property inspection. Honing in on the leaves, needles, twigs and branches, visually inspect for raised elongate, oval or round structures generally ranging in size from 1/16 to 1/8 inch.
If scale insects are suspected, use a pocketknife to gently scratch the stem over the suspected scale. This is a useful technique, because scales can be confused with lenticels, which are naturally occurring parts of tree stems. Using your knife will help distinguish between the two, as a scale will usually lift off easily, while a lenticel will remain on the stem.
Some species of scale are easier to spot than others. For example, pine needle scales are chalky white in color. When cast against a green needle, they are quite obvious. Other scale insects, such as oystershell scales, match the bark color of their host. When attached to the stem of a lilac, they are nearly invisible. These scales can become so numerous they can appear to be a normal feature of the plant.
In addition to your pocketknife, a magnifying glass or 10x hand lens is a useful inspection tool. The lens magnifies small structures and allows you to accurately identify the scale insect. It can also be useful for showing clients the pests in their landscape up close. Inspection for scales should be conducted on a regular basis, especially if they have been observed on previous visits.
Scale insect control
Chemical controls are most effective when directed against the crawler stage, so products should be applied just as eggs are beginning to hatch.
To detect crawlers, examine needles, twigs and branches with your hand lens, or shake infested branches over a piece of black paper and look for tiny, yellow, moving dots. Since hatching may continue over a two to four-week period, applications should be repeated at weekly intervals until crawlers are no longer observed. Thorough insecticide coverage is essential for effective scale control.
Common scale insects
Pine needle scale — Needles are the feeding sites for pine needle scale, which can also infest spruce and fir. Mature scales are slender, chalky white and about 1/16 inch in length. There are two generations per season in most locations.
Eggs overwinter under the old female scales. When crawlers emerge in the spring, they are vulnerable to insecticidal sprays. Well-timed applications in late spring and midsummer with horticultural oil, insecticidal soap, acephate, bifenthrin or permethrin should provide excellent control. Because horticultural oil applications can disfigure spruce and fir trees by removing the desirable blue color on the needles, always use caution and heed label warnings.
Oystershell scale — Oystershell scale attack a variety of plants but are common on maple, sycamore, apple, crabapple, dogwood and lilac, and occasionally infesting river birch and aspen. Mature females are about 3/16 inch long and may be brown or gray. Their waxy covering is wider at one end, and they are similar in shape to an oyster shell, hence the common name. Scales attach to twigs and branches and grow a waxy covering that camouflages them as they feed on plant sap.
When oystershell scale infestations are heavy, a good start is to prune and destroy the infested branches. Follow branch removal with insecticide spray containing bifenthrin or acephate. To be effective, insecticide applications should be applied to the crawlers soon after egg hatch, usually in mid-spring. Monitor plants in mid to late summer for a second generation of crawlers. Read and follow label directions, as acephate is not recommended for some maples.
Cottony maple scale — Large, conspicuous, white egg sacs are produced on the twigs and small branches of host plants. Winter is spent as a flat, oval, immature female about the color of the host plant twigs. Overwintering females mature in the spring and deposit eggs in late May and June. Eggs are laid in a white, cottony mass that extends from the female scale. During summer, immature scales feed on leaves, but migrate to twigs as fall approaches.
Cottony maple scale is most common on silver maple, but is also found on other maples, boxelder, linden, black locust, red mulberry, white ash, apple, beech, cherry, dogwood, elm, hickory, holly, honeylocust, peach, plum, sycamore, willow and others. There is one generation per year, with crawlers active in midsummer. As many maples are sensitive to oils — both dormant and summer — check pesticide labels carefully. As with all scale insects, the primary control efforts should be aimed at crawlers.
San Jose scale — Mature San Jose scales are round, slightly raised and gray to black in color. They are about 1/16 inch in diameter. When viewed with a hand lens, a conspicuous, dark gray, concentric ring is visible.
San Jose scale has a broad host range and is widely distributed throughout the U.S., making it one of the most destructive scale species. There are often three generations a year. When scouting for San Jose scale, be aware that broods often overlap, which makes identification of life stages and treatment timing for crawlers difficult. Both dormant oils and summer oils are effective.
Kermes scale — Kermes scale is a soft scale that feeds mostly on oaks. Heavy infestations cause “flagging,” where leaves on branch tips turn off-color, droop and often drop off. Mature scales are easy to detect because of their size and are often found in clusters. They are rounded, buckshot-sized and brown in color. They can appear similar to buds or other plant growth.
Timing of egg hatch and the presence of crawlers varies depending on the species. Kermes scale found on bur oak hatch in early summer, whereas the species found on pin oak hatch in early fall. At egg hatch, crawlers can be controlled with insecticides including acephate, bifenthrin, permethrin and insecticidal soaps. Imidacloprid can be applied as a soil drench in late fall or early spring to control scales the following season. Overwintered scales may be treated with dormant oils in late winter or early spring.
Lecanium scale — The lecanium scale is sometimes referred to as the terrapin scale because of its turtle-like appearance. This scale insect occurs on numerous tree species including oak, maple, ash, redbud and linden. Infestations on terminal branches will sometimes kill the new growth.
Mature female scales are convex, oval, usually brown in color, and about 3/l6 inch in diameter. Partially grown females overwinter attached to twigs and branches. In the spring they complete development and females give birth to living young. These crawlers migrate to the leaves and settle on the midribs or larger veins. The lecanium scale has only one generation per year. Insecticides should be applied when crawlers first appear in the spring.