In Vermont, oystershell scale has been the greatest problem on lilac. In forest stands, it occasionally heavily infests ash reproduction and beech trees of all ages. The oystershell scale is an armored scale. The scale or waxy coating of the oystershell scale closely resembles one half of an oystershell. All but the youngest scale nymphs are similar to the mature scale except in size.
Male scales resemble the female but are very rare.
Egg laying continues for about a month. After egg laying is completed, the shrivelled female scale dies and the eggs overwinter. The eggs hatch in mid- to late May, about the time the common lilac is in bloom. The young scales, called crawlers, are the only motile stage. These pale-yellow insects are rarely visible to the naked eye.