Cotton on Your Trees?

An interesting pest is infesting trees in the area, covering them with tiny puffs of cotton. It is cottony maple scale. This insect creates egg sacks that look like ¼- to ½-inch long balls of cotton on the trees. When there are heavy infestations, whole branches may be covered in white. The scale can occur on silver or red maple, honeylocust, black locust, euonymus, oak, boxelder, dogwood, hackberry, sycamore, elm, linden, willow and poplar.

Photo by UNCE Master Gardener Volunteer Jocelyn Helling.
Photo by UNCE Master Gardener Volunteer Jocelyn Helling.

Usually these insects are a mere curiosity and don’t cause problems for trees. Healthy trees can tolerate a small to moderate infestation, but stressed or weak trees can suffer. Twig dieback can occur. Severe infestations can kill major limbs and possibly entire trees. In addition to possibly damaging trees, the insects are nuisances because they produce a sticky honeydew that drips on everything. The honeydew these sap-eating insects excrete is excess water and sugar, so it attracts bees and wasps.

Mature cottony maple scales are small, flat, oval, brown insects without obvious legs, antennae or wings. They may be ¼ inch to 3/8 inch in diameter. The females begin producing egg sacks by late May and early June, which is when we notice them. Each of the egg sacks may contain up to 1,500 eggs, but only small portions of these ever hatch into microscopic-sized nymphs in summer.

Cottony maple scale has numerous predators, such as lady beetles (ladybugs), which normally keep populations in check. Applying an insecticide can actually make an infestation last longer, because chemicals can kill the parasites and predators that destroy the scales. The scales that survive the insecticides develop resistance to the chemicals and become harder to control.

The main management strategy is to keep trees healthy with sufficient water, appropriate fertilization and timely pruning so they can resist the pests and survive an infestation. However, if the population is heavy and stressing a tree, or if a tree is dripping too much sap, wash the tree off with a strong jet of water to dislodge the critters and remove the honeydew. If you can’t wait for the predators and parasites to take care of the scales, and water isn’t enough, use insecticidal soaps or horticulture oils following label directions. Some maples are sensitive to oil sprays, so do check the label for sensitive plants before spraying.

Ohio State University Extension has an informative fact sheet for more information on cottony maple scale