Cottony maple scale is most common on silver maples, but it can feed on other maple species, boxelder, basswood, birch, elm, and linden. Arborist Dave Leonard recently brought samples to the on black gum. The insect spends the 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 (Figure 1).
Eggs of this scale hatch during June, and crawlers move to lower leaf surfaces where they settle for the rest of the summer to feed on sap. Just before leaf drop, the 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.
Control is seldom needed on healthy, established trees unless there is an excessive rain 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 case, 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 key to success. Sprays must target newly hatched crawlers that are present during June. Once scale insects settled on plants, they 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 tree and shrub insecticides containing bifenthrin, cyfluthrin, permethrin, or malathion will provide residual control of active crawlers. However, they can have a significant impact on pollinators, as well as 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 often are associated with stressed trees. Evaluate infested trees to identify and correct factors that may be contributing to stress.
- Prune heavily infested branches if practical.
This article includes information from Purdue University fact sheet, Scale Insects in Shade Trees and Shrubs (E-29-W)
By Lee Townsend, Extension Entomologist