Blooming of Vanhoutte spirea (Spiraea x vanhouttei) means that oystershell scale, Lepidoasaphes 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 control because they form a protective covering. Oystershell scale has a wide host range, including ash, birch, dogwood, elm, hemlock, maple, poplar, privet, walnut, and willow. There are two races of oystershell scale, the brown and gray banded. The two races differ, based on their plant preferences.
Oystershell scale is small, about 2 to 3 millimeters long, gray or brown, and shaped like oyster shells (hence the name). The scale overwinters as eggs be neath the female covering. The eggs hatch into young, creamy white to brown crawlers that are active from May to June. The crawlers locate a place to settle and then use their piercing-sucking mouthparts to remove plant fluids, which causes leaf yellowing, plant stunting, and possibly death. Branches or twigs encrusted with oystershell scale may die. In certain cases, 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 watering, fertilizing, and mulching reduces stress and thus allows plants to tolerate low to moderate infestations. However, when scale populations are high, then insecticides are generally required to prevent damage. Insecticides recommended for managing thisscale include acephate (Orthene), carbaryl (Sevin), malathion, insecticidal soap, and horticultural (summer) oil. These insecticides should be applied when the crawlers are most active. Visually inspect branches for scale crawlers or use double-sided sticky tape wrapped around selected branches or twigs infested with scales. When the crawlers emerge and move around, they get stuck on the tape. Examining it regularly helps to determine when the scales are in the stage most susceptible to spray applications. Repeat applications may be needed 10 to 12 days later.
The brown-race crawlers of oystershell scale on dogwood and lilac are generally the first to hatch from eggs and are sprayed when Vanhoutte spirea is in full to late bloom. The gray-banded-race crawlers on ash, lilac, and maple hatch from eggs later and are sprayed when Vanhoutte spirea has completed blooming.
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.