By Whitney Cranshaw, Colorado State University Cooperative Extension specialist, entomology
The most damaging of the scale insects, this pest affects ash, aspen, lilac and cotoneaster, along with dozens of other plants. Its tiny size and similarity to normal tree bark allow large populations of oystershell scale to develop before being detected. And its hard waxy covering shields the scale from normal insecticides, making it difficult to treat.
Carbaryl, insecticidal soaps and spray oils can be effective if applied when eggs are hatching in mid-to-late May, or later, when cool conditions prevail. Check for emerging populations of scale by shaking branches over a sheet of dark paper. If scale is present, the small, pale-colored crawlers will show up on the paper.
Oystershell scale can be treated by spraying dormant oils after leaves drop in the fall, or before new leaves emerge in the spring. At these times, you also can scrape tree branches with a plastic, kitchen scubbing pad to remove eggs or you can prune heavily infested branches.
For more information, see CSU Fact Sheet 5.513.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010