Oystershell Scale

Oystershell Scale is one of many scale insects. Scale insects do not look like most insects, since you cannot usually see their legs or other body parts.

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.

Lilac, ash, cotoneaster, willow and many other deciduous trees and shrubs.

Early evidences of feeding are small holes in the leaf produced by young larvae feeding on the expanding foliage. Older larvae consume the entire leaf, except the midribs and major veins.

Life Cycle:The most familiar stage of the oystershell scale is the covering of the full-grown female scale that overwinters attached to the bark. The mother scale is about 1/8-inch long, brown or gray, slightly banded, and the general shape of an oyster shell. The overall appearance of the scale often is similar to that of the underlying bark and these insects are easily overlooked. Old scales can stay attached to the tree for several years before falling off.

The oystershell scale overwinters in Colorado only in the egg stage. Eggs are underneath the old scale covering of the mother. At lower elevations, eggs typically hatch in late May or early June. At higher elevations, egg hatch may be delayed into mid-June. Eggs from all the scale insects do not hatch at the same time, and egg hatch may last a couple of weeks. Eggs of oystershell scale with two generations per year are reported to hatch earlier than one-generation scales. In most areas of the state, there is only one generation of the insect per year. Where two-generation races exist, second generation egg hatch occurs in July and August.

The newly hatched scale insects are called crawlers. The crawlers are pale and smaller than a pinhead. This is the only mobile stage in the life history of the oystershell scale.After a few hours, the crawlers find a suitable location, usually on a shaded area of the tree. They insert their mouthparts into the plant, begin to feed and soon molt. They remain in this location for the rest of their lives. Within a week they are covered with a waxy scale that protects them from most insecticides.

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.

Control: Oystershell scales are most susceptible to insecticides during their crawler stage – after the eggs have hatched, but before the nymphs have settled down and secreted the protective waxy coating. Materials should be applied when crawlers are first noticed and reapplied in about 10 days to provide control of late-hatching nymphs. The crawlers can be detected by wrapping a piece of double-sided sticky tape around an infested twig around the middle of May.

The tape should be checked every other day for the presence of crawlers. Replace the tape whenever it becomes wet or covered with dust and continue the monitoring process until egg hatch has been detected. Another method of detection is to hold a piece of white paper or a white pan under the branches and tap the branches with a stick. Any crawlers can be easily seen on the white surface. The color of the crawlers may be shades of white, yellow, orange, or purple. Heavily infested or sickly branches can be pruned from the tree. In some cases, heavy accumulations of scale can be removed from the bark with a stiff scrub brush or a plastic scrub pad.

Insecticides frequently recommended for oystershell scale control which can be used by homeowners include formulations of acephate (Isotox, Orthene), calcium polysulfide Oystershell Scale (Orthorix), carbaryl (Sevin), malathion, and various oils. Products for commercial use include certain restricted-use-products such as azinphos-methyl (Guthion, Sniper) and chlorpyrifos (Dursban), Superior-type oils (dormant oils), light-to-medium summer oils, and canola oil are registered for scale control on some ornamentals, although they are generally not as effective on oystershell scale as are crawler sprays. Oils should not be used on certain plants including maple, beech, walnut, and Japanese flowering cherry. Check the product labels for these and other restrictions before use.

Before using any insecticide, refer to the specific product label for a list of registered plants, application rates, application timings, techniques,restrictions, and cautions. Not all products are registered on all plants and some products will actually damage some species of plants. Check the label carefully for a list of plants that may suffer a phytotoxic reaction if the product is used. Most of the insecticides above are potentially toxic to honeybees. Do not treat when the plant is in bloom or when bees are foraging. Natural enemies of oystershell scales may provide some degree of control.

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