Scale insects appear as little bumps on the twigs, branches, or needles of trees and shrubs in our yards. Once the crawling stage is over, usually late spring to early summer, the scale become fixed on the branches. You won’t notice any movement so you might not even be aware that they are alive. Scale insects are sap-feeding insects with a shell-like waxy covering that conceals their bodies. Nearly all trees are susceptible and although a few insects don’t pose a problem, in high numbers they can weaken and, in rare cases, even kill trees.

Depending on the species, scale insects may be found on plant stems, twigs, trunks, foliage, or fruit. Most scale insects are small and inconspicuous. The size of scale insects ranges from 1/8 to ½ inch. Colors can vary widely including white, brown, black, gray and red. Size, shape, and texture also vary with the species. Common scale insects we see in high numbers right now in southeast Idaho are:

• Cottony Cushion Scale in Maple trees
• Oystershell Scale in Aspen trees
• European Elm Scale on Camperdown Elm trees

Scale insects are generally divided into two categories:

• Soft scales produce a soft, thin, cottony, powdery or waxy layer over themselves that cannot be separated from the insect body.
• Armored scales have a hard, shield-like cover composed of shed skins and wax that conceals the body but is not attached to the body of the insect.

Scale insects feed by sucking sap from trees and shrubs through piercing-sucking mouth parts. Sap feeding by scale insects may cause yellowing or wilting of leaves, stunting or unhealthy appearance of the plants, and eventually death of all or part of the plant when infestations are heavy. Weakened plants may lose vigor and become more susceptible to injury caused by drought, severe winters, attack by other insects (such as borers), or infection by diseases.

Cottony Cushion Scale

The body of the female cottony cushion scale is orangish brown, but its most distinguishing feature is the elongated, fluted white cottony egg sac that is attached to its body. The egg sac contains 600 to 800 red eggs and may become two to three times as long as the body of the female; the resulting length of the female plus the egg sac can be almost 1/2 inch.

Life History

This soft scale overwinters as a second instar nymph on the bark of host twigs and branches. The female completes development in June and lays egg masses through late summer. They hatch into crawlers (first instar nymphs) from mid-June through mid-July, and migrate to the underside of host plant foliage where they insert their piercing-sucking mouthparts. They feed by withdrawing sap from vascular cells of the plant. This pest spends the remainder of the summer feeding on leaves. Male scale mature in late summer, mate with the female and then die. Just before leaf drop in the fall, nymphs move back to host plant twigs and branches to overwinter.

Eggs hatch into crawlers in a few days during warm weather. The crawlers are red with black legs and antennae. They settle along leaf veins and begin to produce the white cottony secretion they are known for. In order to increase in size, scale shed their outer skin (molt) and grows a new, larger covering. Each time the scale molts, it leaves behind its white, cottony molting skin. Immature scale looks reddish for a short period of time before they begin producing more cottony secretions.

Damage

Like other types of scale, cottony cushion scale decreases the vitality of its host by sucking phloem sap from the leaves, twigs, branches, and trunk. Feeding can result in defoliation and dieback of twigs and small branches when infestations are extremely heavy. Heavy populations can severely reduce the yield of citrus trees. Like soft scales, cottony cushion scale excretes honeydew, which is usually accompanied by blackish sooty mold growth and ants.

Oyster Scale

The oystershell scale is one of the most common armored scale insects that cause injury to shade trees and shrubs. The waxy cover of mature specimens is about 2.5 mm long, grayish brown, and noticeably convex, resembling miniature oyster shells. This armored scale develops on the bark of host plants. Tiny white eggs are found beneath the waxy cover of the female. Eggs hatch into a life stage called a crawler. The crawler stage of this scale insect is pale yellow and less than one millimeter long. Adult males have one pair of wings. When observed closely, adult males are often misidentified as parasitoids as they walk over infested twigs.

Life History

This species overwinters as eggs beneath the protective waxy covering of females. The literature reports that one female may lay 20-100 eggs. These hatch in late May through early June into first instar nymphs called crawlers. This life stage wanders over the bark for a short time and then settles down to feed. They continue to feed and reach maturity in late summer or early fall. Females have three developmental life stages after the egg and males have five. When mature, males emerge, mate with the female, and then die. Males are active from late June through early July.

Damage

Plants are injured by this scale insect when it removes plant fluid from non-vascular cells with its piercing-sucking mouthparts. Eventually, branches become encrusted with this armored scale. Twig or branch dieback is common when an infestation of this insect occurs. Occasionally, a tree or shrub will die as the result of a severe infestation if it is not effectively managed.

European Elm Scale

European Elm Scale is an insect that attacks a variety of elm trees by feeding on the sap. These insects feed on the trees and produce a large amount of honeydew (a sticky secretion) which eventually covers the leaves and bark in a black mold.

Life History

European elm scale spends the winter as second instar nymphs, packed into cracks on twigs and smaller branches. They are oval in general form and pale grey due to the light waxy cover of the body. In spring, they resume development and the females swell greatly, becoming darker with a distinct waxy fringe. During late April and May, male scale may also begin emerging from small white cocoons and mate with the females. However, males are not always produced and this species can reproduce asexually.

Eggs hatch within the body of the female and crawlers emerge over a period of several weeks, peaking between mid-June and mid-July. They move to leaves and settle on the leaf underside. The dark yellow nymphs are almost always found tucked next to main leaf veins. In late summer they migrate back to the twigs where they overwinter.

Damage

Some scale species, when abundant, weaken a plant and cause it to grow slowly. Infested plants appear drought stressed, leaves turn yellow and may drop prematurely, and plant parts that remain heavily infested may die. The dead brownish leaves may remain on scale-killed branches, giving plants a scorched appearance. If the scale produces honeydew, this sticky excrement, sooty mold, and the ants attracted to honeydew can annoy people even when scales are not harming the plant.

Contact Us Today