Common name: Oystershell scale
Scientific name: Lepidosaphes ulmi
Hosts: Oystershell scale can feed on a wide range of trees and shrubs. It is commonly found on apple (Malus), ash (Fraxinus), dogwood (Cornus), lilac (Syringa), maple (Acer), pear (Pyrus), poplar (Populus) and willow (Salix).
There are two races of this scale, the brown race and the gray-banded race. They can vary in their host range.
Oystershell scale is an introduced species, but is commonly found throughout North America.
Large populations of this pest can kill entire branches or small trees or shrubs.
Scale insects are unique and look quite different from other insects. In their juvenile growth stage, they are referred to as crawlers. As crawlers, they are highly mobile, six-legged, have no protective cover, and are very small (<1/32 inch). At maturity, scale insects are immobile, have a covering over their body, and are large enough to see without a magnifying glass.
Scale insects fall into two categories, soft scales and armored scales. Oystershell scale is an armored scale. The armored scales are usually smaller than the soft scales. They secrete a hard, protective cover over their bodies. They usually overwinter as eggs beneath the female cover.
Damage
All scale insects have sucking mouthparts and feed on sap from the tree. Armored scale, like oystershell scale, burst plant cells and feed on the contents. Scale can remove large quantities of sap, stressing the host tree. Plants can usually tolerate small populations of scale. The extensive feeding by a larger population often leads to yellowing of leaves and dieback of twigs. Over time, an untreated population of scale may lead to the decline of the plant. Armored scale inject their waste back into the plant’s tissues, so no honeydew is produced.
Oystershell scale causes damage to the host by using needle-like mouth parts to feed on the sap or other plant fluids. Signs of infestation are found primarily on the twigs and branches where the scales attach. Damage includes yellow leaves and stunted growth. Heavy infestation may kill entire branches and lead to overall decline of the host plant. Small trees and shrubs may be killed. An infested plant will show oyster-shaped scales attached to twigs, branches, or possibly the trunk. With an extremely heavy infestation, the bark of an infested branch may be completely covered.
Life Cycle
Oystershell scale may have one or two generations per year in the Midwest. The gray-banded race has one generation, while the brown race has two. This species of scale overwinters as eggs under the female cover. The eggs hatch in spring about the time when Vanhoutte spirea (Spiraea x vanhouttei) is flowering. The crawlers of the gray-banded race emerge when the spirea is in full flower, and the brown race crawlers emerge a little later when the spirea has finished flowering. The crawlers of both races are light colored. They migrate around the leaves and stems and finally settle down on the stems. At this point, the crawlers lose their legs and antennae, secrete their waxy protective shell, and spend most of their lives there feeding via their needle-like mouth parts.
Adult females of the gray-banded race have a cover that is grayish and slightly banded, with a distinct oyster shell shape. They mature in summer and produce their overwintering eggs. The adult females of the brown race have a cover that is more brown in color, but which also has the distinctive oyster shell shape. They mature earlier and then produce a second generation of crawlers in mid-summer when Queen Anne’s lace is flowering. The second generation of adult females produce the overwintering egg stage. Adult males are winged, but a male population is not always produced.
Management
Cultural Management
In cases of less serious infestations of oystershell scale, infested parts of a plant can be removed. Pruning small infestations may solve the problem, while pruning larger infestations can make chemical treatment simpler and more effective. If heavily infested and severely damaged, the plant may need to be completely removed. Be sure to destroy removed plant material to prevent spread.
Biological Management
Oystershell scale does have some natural enemies (lady beetles and parasitic wasps), but they are seldom effective in managing this pest.
Chemical Management
As with any form of management, the level of infestation and life stage of the pest should be monitored, but this is especially important when dealing with chemical treatment because effectiveness relies heavily on the appropriate timing of application when the product targets a certain life stage.
Oystershell scale crawlers can be treated directly with insecticides. This is the most vulnerable stage of their life, when they lack their protective armor. Monitoring is important so treatments can occur when the crawlers emerge in late spring and again in mid to late summer. Horticultural oil sprays are another option that target overwintering scales, but these are less effective than crawler sprays. Horticultural oils and insecticidal soaps are the least toxic chemical options, so they may be best when trying to conserve and protect beneficial natural enemies. Some products may need to be applied several times, so always read the label to ensure optimal effectiveness.
Some, but not all systemic insecticides, used in the fall or spring, may be used against this species of scale. They are absorbed internally by the host plant to target the scales, reducing the risk of harm to beneficial insects or the applicator, as compared with foliar sprays.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.