A group of woolly alder aphids on an alder branch. / Photo Credit: Linda Williams, Wisconsin DNR

By Linda Williams, DNR Forest Health Specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Have you seen white lumps of fluff on alder stems? Woolly alder aphids congregate together for the winter. These aphids produce white filaments from their bodies to protect them from predators. When they congregate, they can look like a big clump of snow stuck to alder branches.

The aphids are also called maple blight aphids because they fly to maple trees and feed on the sap throughout the summer months. When aphids are flying between hosts, they show wings and soft fluff, making them look like floating bits of cotton.

An adult woolly alder aphid (bottom right) and an immature aphid (top left), showing the difference in wool production. / Photo Credit: Linda Williams, Wisconsin DNR

Woolly alder aphids require both hosts, alder and maple, to complete their life cycle.

Other woolly aphids live on other hosts, such as beech blight aphids on beech or the woolly apple aphid, which use apple and elm trees as hosts.

Woolly aphids often feed in groups and may be found on leaves, twigs, branches and bark. As with all aphids, they suck the sap from the plant and secrete a sweet waste referred to as honeydew. As honeydew drips onto things below the aphids, a black sooty mold will begin to grow on it that can discolor vehicles, lawn furniture and other things beneath the trees the aphids are feeding on.

Aphids are soft and tasty, and many predators and parasites feed on them, generally keeping their population in check. Insecticides are not usually needed. Woolly aphids can be washed off branches with a strong spray of water, and sooty mold should be washed off periodically.