Scale insects are small, immobile insects with no visible legs or antennae, pressed tightly against the plant on which they are feeding. Many are common and serious pests of trees, shrubs and indoor plants.

Damage:

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.

Control:

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. 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.

Armored Scales

(Homoptera: Diaspididae)

Scale insects can be roughly divided into two groups: armored scales and soft scales. Armored scales secrete a protective cover over their bodies. Most species of armored scales overwinter as eggs beneath the female cover. In spring, these eggs hatch into tiny mobile crawlers which migrate to new feeding sites. The crawlers settle after a few days, insert their mouthparts in the plant, and begin to feed. Soon they secrete a protective cover and lose their legs. Large populations can build up before plants begin to show visible symptoms. Our most common armored scale pests are described and illustrated below.

San Jose Scale (Quadraspidiotus perniciousus)

Mature scales are circular, slightly convex and smoky black. They are about 2 mm (l/l6 inch) long. A conspicuous, dark gray, concentric ring is visible under a magnifying glass. San Jose scale is perhaps the most widely distributed and most destructive scale insect pest of fruit trees, shade trees and ornamental shrubs in the United States. Over 60 host plants are known, including apple, pear, peach, cherry, ash, poplar, lilac, elm, willow, pyracantha and cotoneaster. There are at least three generations a year; broods often overlap, making treatment of crawlers difficult. Both dormant oils and summer oils are effective.

Oystershell Scale (Lepidosaphes ulmi)

These scales are shaped like the shell of an oyster. They are chestnut to dark brown, sometimes with lighter transverse bands. Infested twigs are often completely encrusted with scales. This is a common and destructive pest of over l20 different species of fruit trees, shade trees, and woody ornamental shrubs. Hosts include apple, lilac, dogwood, boxwood, birch, elm, sycamore, viburnum and many others. Eggs hatch in late May. Use a dormant spray in April or treat crawlers between late May and mid-June.

Pine Needle Scale (Chionaspsis pinifoliae)

Mature scales are pure white and shaped like oyster shells. Pine needle scales are common and serious pests of ornamental pines and various spruces. Less preferred hosts include hemlock and fir. Ornamental plants, Christmas tree plantations and nursery stock are more frequently infested than forest trees. In heavy infestations, needles may be completely whitened by a continuous layer of scales. Use dormant or summer oils against adults or treat crawlers when lilacs are in bloom.

Euonymus Scale (Unaspis euonymi)

Females are pear-shaped and blackish-brown. Males are elongate and white. This is a common and serious pest of evergreen euonymus, often causing defoliation and dieback. Pachysandra and bittersweet are also suitable hosts. There are two generations per year. Crawlers are active in early June and mid-July. Avoid use of Euonymus japonica, a highly susceptible species. Watch for a small black ladybug with 2 red spots which often provides good control of this scale.

Juniper Scale (Carulaspis juniperi)

Females are round and dirty-white with yellow centers. Under a magnifying glass they resemble miniature fried eggs. Males are also white, but smaller and narrower. Hosts include junipers, arborvitae, incense cedar and cypress. Crawlers are active mid-June to early July.

Hemlock Scales (Abgrallaspis spp.)

We have two species of hemlock scale--one is round and the other oblong. Both are brown and found on the undersides of needles of hemlock and occasionally on other conifers. These scales are usually effectively controlled with biological controls, but dormant or summer oils may be used when needed.

Soft Scales

(Homoptera: Coccidae):

In general, 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; others retain legs and are able to crawl short distances to find suitable overwintering sites in the fall. Except for those soft scales which infest indoor plants, most have only a single generation per year at our latitude. Our most common soft scale pests are described and illustrated below:

Magnolia Scale (Neolecanium cornuparvum)

Our largest scale insect, this species reaches 12 mm (1/2 inch) in length. Color ranges from dark brown to pink-orange and older scales are covered with a white waxy powder. This scale produces large amounts of honeydew and sooty mold. Treat with dormant oil in April or treat crawlers in October with two treatments seven to ten days apart.

Fletcher Scale (Parthenolecanium fletcheri)

Mature scales are dark brown, shiny and very convex. Arborvitae and yew are the most frequently attacked hosts, but pachysandra and Eastern red cedar are also susceptible. Honeydew excreted by the scales supports unsightly, sooty molds. There is one generation per year with crawlers active in late June through early July. There are several effective parasites which are best conserved by using dormant oil treatment in late April.

Cottony Maple Scale (Pulvinaria innumerabilis)

Large, conspicuous, white egg sacs are produced on the twigs and small branches of host plants. During summer, immature scales feed on leaves, but they migrate to twigs as fall approaches. Honeydew excreted by the scales supports unsightly, sooty mold growth. Cottony maple scale is most common on silver maple, but 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 late June through July. Many maples are sensitive to oils--both dormant and summer. Check labels carefully or treat crawlers.

Oak Kermes Scale (Kermes galliformis)

Mature scales are tan, globular and hard. They are easily mistaken for galls or buds. Oak is the only host. This species is not particularly injurious to host trees, but, if necessary, treat crawlers between late May and mid-July, or use a dormant oil in April.

Adapted from the Virginia Cooperative Extension and the University of Massachusetts Extension, 1999