Cottony maple scale occurs most commonly on silver maple, but it can also feed on other maple species, boxelder, basswood, birch, elm, and linden. The insect spends winter in an immature stage on twigs and branches, maturing in late May or early June. Mature females are easily recognizable by the distinctive cottony white sac that contains about 1,000 eggs.
Eggs of this scale hatch during June and crawlers move to lower surfaces of leaves where they settle to feed on sap for the rest of summer. Just before leaf drop, these insects return to twigs and branches for winter. There is one generation each year. Natural enemies usually keep populations in check, but there are occasional outbreaks.
Managing Cottony Maple Scale
Control is seldom needed on healthy, established trees unless there is an excessive build-up of honeydew. However, infestations on newly transplanted trees in the landscape can cause premature leaf drop, stunted growth, and, in some cases, branch or tree death. In this situation, control is advisable.
Scale control is challenging and may require action over several seasons. Here are some points to consider:
- Proper timing of foliar sprays is a key to success. The spray must target newly hatched crawlers during June. Once settled on the plant, scale insects develop a protective covering that insecticides do not penetrate easily. The extended period of egg hatch may require two or more applications.
- Thorough spray coverage is essential. Depending on the type of insecticide used, control may be achieved by direct contact with the insects and/or picked up as they crawl over treated surfaces.
Many insecticides are labeled for control of scale infestations on trees and shrubs. Biorational choices include ultra-fine horticultural oils, insecticidal soap, and neem (azadirachtin). These insecticides will have minimal impact on beneficial species that help to regulate scale infestations, but they provide no residual protection. Conventional insecticides include those with one of the following active ingredients: bifenthrin, cyfluthrin, permethrin, or malathion. These products will provide residual control of active crawlers, but they have a significant impact on the lady beetles and tiny wasps that attack scales.
A dormant oil spray can be effective in controlling overwintering scales. Check the label for instructions and restrictions.
Scale infestations are often associated with stressed trees. Evaluate infested trees to identify and correct factors that may be contributing to stress. Prune heavily infested branches if practical.
(Note: This article includes information from the Purdue University fact sheet Scale Insects in Shade Trees and Shrubs, E-29-W)
By Lee Townsend, Extension Entomologist