Woolly aphids are being found on river birch in northern Illinois. Although there is no accepted common name for Hamamelistes spinosus, it could be called the spiny witch-hazel gall aphid or woolly birch aphid for its appearance on the hosts that it attacks.
On birch, this insect overwinters on the bark as a hibernating female, sometimes called a “pupa.” In the spring at birch bud break, the female moves to the leaves, where it gives birth to young on the leaf undersides. The feeding of these young aphids causes corrugations, elongate wide ridges between the major veins on the upper sides of the leaves. These form troughs on the underside that are soon filled with whitish, fuzzy aphids. Heavily affected leaves are curled and twisted, and some will turn brown. As the aphids become adults, they fly to witch-hazel to lay eggs on the twigs by the end of June.
On witch-hazel, eggs overwinter on the twigs, hatching in the spring into stem mothers. Feeding by the stem mother aphid on the buds causes the witch-hazel to form a gall around the aphid. This green to reddish, spiny gall is about 3/4-inch long and hollow. The stem mother produces many reddish aphids inside the gall that then escape through an exit hole at the base and fly to birch.
The galls on witch-hazel do not damage the plant enough to warrant treatment. On birch, the aphids tend to attack just a few leaves on a branch or two that can be pruned off without the use of insecticides. They can be controlled on heavily attacked birches with acephate (Orthene), imidaloprid (Merit), or insecticidal soap.