Woolly Alder Aphid

A peculiar phenomenon occasionally reported around Iowa is the unmistakable sight of small cottony white fuzz-balls flying through the air under their own power. If you are deft enough to gently catch one of the apparitions you see a plump bluish-black body and transparent wings pulling the cottony tuft through the air.

The alternate common name for woolly alder aphid is the maple blight aphid because of the dense, white, woolly masses it produces on the leaves and twigs of its primary host, silver maple (and occasionally red maple). The aphids on the trees are wingless, plump, gray, and concealed beneath their own dense, white, waxy strands. These feed on sap from the maple trees from the time of bud-break until late June. Then winged adults, some with abdomens covered in white fluffy wax, are produced in the colonies. These winged migrants readily fly when disturbed and create the illusion of tiny masses of cotton floating through the air. These aphids are leaving the maple trees and flying to alders where they will establish new colonies on the secondary host. Woolly alder aphids require both alder and maple trees to complete their life cycle.

Infested maple trees should be easily recognized in May and June by the presence of large, fuzzy, white colonies of aphids on the foliage or twigs. Although their presence may cause alarm, these aphids apparently cause no permanent damage. Some infested leaves may droop or fold downward and eventually shrivel and drop prematurely. This does not reduce the vigor of healthy trees. Alders are similarly not harmed by aphid colonies on the branches and stems. Honeydew and sooty mold have been a problem on cars and buildings underneath or adjacent to heavily infested trees in other parts of the country but not in Iowa. Control is not warranted.

Woolly aphids are an important resource for natural biological controls such as lacewings, lady beetles, hover flies, and parasitic wasps. Tolerance of aphid presence is one way to encourage beneficial insects.

Flying adults are a wonderment. They are intriguing, not harmful. When adults are migrating the feeding and honeydew production on the maples has been accomplished and no control is needed. Relax and enjoy the fascination of Nature.