The blooming of Vanhoutte spirea (Spiraea x vanhouttei) indicates that it is time to take action against the notorious oystershell scale, Lepidoasaphes ulmi, because the eggs will be hatching throughout portions of Illinois into young crawlers that are extremely susceptible to insecticide applications. However, as the scales mature later in the season, they are more difficult to control because they form an impenetrable 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--brown and gray banded. The two races differ primarily in their plant host preferences.
Oystershell scale is about 2 to 3 millimeters in length, gray or brown, and shaped like oyster shells (hence the common name). Oystershell scale overwinters as eggs located beneath the female covering. Eggs hatch into young, creamy white to brown crawlers that are active from May through 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 totally encrusted with oystershell scale eventually die. In certain instances, the scale may not kill a tree or shrub but may stress it enough to increase susceptibility to wood-boring insects.
Proper implementation of cultural practices--including irrigation, fertilization, and mulching--go a long way in reducing stress and thus allowing plants to tolerate low to moderate infestations of oystershell scale. However, when scale populations are excessive, then the use of insecticides is warranted to prevent permanent plant damage. Insecticides recommended for managing oystershell scale include acephate (Orthene), carbaryl (Sevin), malathion, insecticidal soap, and horticultural (=summer) oil. All these insecticides should be applied when the crawlers are most active, which increases their overall effectiveness in controlling oystershell scale populations. Visually inspect branches for scale crawlers or use double-sided sticky tape wrapped around selected branches or twigs infested with scales. When crawlers emerge from underneath the covering of the dead female and move around, they get stuck on the tape. Examining the tape regularly can help to determine when the scales are in the stage most susceptible to insecticide spray applications. Repeat applications may be needed 10 to 12 days later, as the eggs don’t all hatch at the same time.
The brown-race crawlers of oystershell scale on plants such as dogwood and lilac are generally the first to hatch from eggs and typically are sprayed with an insecticide when Vanhoutte spirea is in full to late bloom. The gray-banded-race crawlers on plants including ash, lilac, and maple tend to hatch from eggs later and are sprayed with an insecticide when Vanhoutte spirea has completed blooming. Vanhoutte spirea is in full to late bloom in central southern Illinois north of I-70 and is completing bloom south of I-64.
Oystershell scale is susceptible to a number of parasitoids and predators (=natural enemies). However, the natural enemies generally appear too late during the season to prevent injury. Additionally, the natural enemies are usually present only when oystershell scale populations are too high.