Lepidosaphes ulmi
See:
Pest description and crop damage The mature scale is approximately 0.125 inch long, hard-shelled, brownish or gray, and usually elongated like an oyster or mussel shell. Oystershell scale are found on trunks, branches, and twigs of many broadleaf deciduous plants. They occur less frequently on the leaves and other plant parts. Scale infestations often are limited initially to isolated colonies on single branches or twigs. Scale are closely related to aphid, mealybugs, and whiteflies and, like these insects, also have piercing-sucking mouthparts. Large populations of scale can devitalize plants and retard growth. Infested plants may show off-color foliage. Severe infestations can kill twigs.
Biology and life history Oystershell scale overwinter as eggs beneath the shell of the female scale. The young scale (called "crawlers") move to feeding sites in late spring or early summer. Young scale also can be dispersed from plant to plant by wind, rain, irrigation, or by the movement of people and machinery.
Management-chemical control
See:
Chemical Control of Nursery Pests
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 370).