One of the more common and damaging tree pests in the Front Range is a tiny insect known as oystershell scale. It gets its name from the protective covering of the mature scales, which resemble a miniature oyster’s shell. They feed by inserting siphon-like feeding tubes into the stems of trees and suck the plant juices. Feeding by large numbers of scales can cause the smooth bark of young trees to become rough, dry and cracked. This may lead to attacks by fungi, which can cause branch dieback. Heavy infestations on the main branches can lead to tree death.

Oystershell scales infest the bark of many species of trees and can be present in large numbers. Over 130 species of woody plants are known hosts. Among the more common hosts are ash, dogwood, elm, lilac, poplar and willow. In Fort Collins heavy infestations have been spotted on the smooth bark of quaking aspens.

Oystershell scale has been known in Europe since 1758. It may have been accidentally introduced into North America and today can be found throughout the United States. In most of Colorado, oystershell scale has one generation a year. The scales spend winter in the egg stage underneath the protective covering of the mother scale. Eggs hatch in late May or early June at low elevations and in mid-June at higher elevations. The newly hatched scales are known as “crawlers.” They are tiny, less than the size of a pinhead and light reddish brown in color. The crawler stage moves around the bark for several days and then settles down to feed and begins to produce its characteristic waxy covering. Once they settle down to feed, the female scales remain in the same place for the rest of their lives. They feed and reach maturity in late summer or early fall. In eastern North America, both male and females scales can be found; males have a pair of wings. In the west, only female scales are produced. Eggs are produced in late summer and fall.

Infestations of this pesky insect are fairly easy to spot. Look for large numbers of gray to brown, slightly banded, oyster-like scales about 1/8" in length on the main stems or branches of smooth barked trees. Patches of rough or dark colored bark on trees that normally have smooth, light colored bark, such as aspen, is another indicator that scale infestations may be present.

This scale’s protective covering makes it especially difficult to control. If infestations are restricted to branches, the most effective control is to simply prune the affected branches. Localized infestations on main stems can be removed with a stiff brush or a plastic scrubbing pad. However, this must be done carefully or the bark can be damaged. Horticultural oils, which smother the scales, insecticidal soaps or chemical insecticides, such as acephate, carbaryl or malathion can be applied and are most effective during the crawler stage, before they have selected a feeding site and begun construction of the oystershell-like protective cover. Treatments should be repeated at about 10 day intervals to kill late hatching crawlers. The use of the insecticide acephate (Orthene) on aspen, poplar or crabapple may cause injury to the plant.

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

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This page updated:  June 23, 2006