If an infestation is detected, make plans to apply a dormant oil for control by March 1. Be sure the temperature is 40 degrees or above before spraying. Scale insects are easily overlooked because they are small and immobile most of their lives and they do not resemble most other insects. Many of them resemble small shells that are oval or circular, but some have more unusual shapes like oyster shells. Coloring varies, but can include white, tan, and brown.
Plants that should be inspected for scales include apples, pears, other fruit trees, bush fruits, lilac, crabapple, oak, ash, elm, lilac, maple, linden, arborvitae, juniper, pine, spruce and yew. Manhattan euonymus, a broadleaf evergreen, is especially noted for having scale problems.
Plants are not harmed if only a few scales are present. But scale population can increase dramatically during the growing season. Heavy scale infestations can damage fruit crops, destroy branches and kill entire plants. (Ward Upham)