Oystershell scale, Lepidosaphes ulmi, eggs are hatching throughout portions of Illinois. At this stage, the young crawlers are susceptible to insecticide applications. As scales mature, they are more difficult to kill because they form a protective covering. Oystershell scale has a wide range of hosts, which include lilac, birch, dog-wood, ash, elm, poplar, hemlock, walnut, willow, privet, and maple. There are two races of oystershell scale, the brown and gray banded. These two races differ based on their plant preferences.
Oystershell scale is small (2 to 3 millimeters long), gray or brown, and shaped like oyster shells. The scale overwinters as eggs underneath the female covering. The eggs hatch into young, creamy-white to brown crawlers, which are active from May through June. Once the crawlers locate a place to settle, they use their piercing–sucking mouthparts to remove plant fluids, which causes leaf yellowing, plant stunting, and possible death. Branches or twigs that are encrusted with oystershell scale may die. In some situations, the scale may not kill a tree but stress it enough to increase susceptibility to wood-boring insects.
Proper implementation of cultural practices such as irrigation, fertility, and mulching reduces stress and thus allows plants to tolerate low to moderate infestations. However, when scale populations are high,
insecticides are generally required to prevent plant damage. Insecticides that are recommended for managing oystershell scale include acephate (Orthene), carbaryl (Sevin), chlorpyrifos (Dursban), malathion, insecticidal soap, and summer oil. These insecticides should be applied when the crawlers are most active. Repeat applications 10 to 12 days later.
The brown-race crawlers of oystershell scale on dogwood and lilac are generally sprayed when Van-houtte spirea is in full to late bloom. The gray-banded race crawlers on ash, lilac, and maple are sprayed when Vanhoutte spirea has completed blooming. The brown race has hatched in southern Illinois but had not yet hatched in central Illinois as of May 2. Hatch in northern Illinois will occur even later.
Oystershell scale is susceptible to natural enemies such as parasitoids and predators. However, natural enemies generally appear too late to prevent injury. In addition, natural enemies are usually present only when scale populations are high.