Lepidosaphes ulmi
Pest description and damage The mature scale is approximately 0.125 inch in length, hard-shelled, brownish or gray in color, and usually elongated and slightly curved like an oyster or mussel shell. Oystershell scale are on trunks, branches, and twigs. They occur less frequently on the leaves and fruit. Scale infestations often are limited initially to isolated colonies on single branches or twigs. Newly emerged crawlers look like tiny bright white pimples on the bark.
For biology, life history, monitoring and management
See:
Ash (Fraxinus)-Oystershell scale
Management-chemical control
See Table 1 in: