Jim Chatfield and Joe Boggs reported that the unusual leaf damage caused by the SPINY WITCHHAZEL GALL APHID (Hamamelistes spinosus) is beginning to appear on river birch in central and southwest Ohio, respectively. The aphid produces raised ribs or "corrugations" on the upper leaf surface that match deep furrows between the veins where the aphids live on the lower leaf surface. The aphid has a complex life cycle that involves two hosts: witchhazel (Hamamelis spp.) and birch (Betula spp.). Winter is spent either as eggs on witchhazel bark or as immature female aphids under birch bark.
The aphid is sometimes called the "river birch aphid" owing to its affinity for B. nigra. On birch, the females move to newly expanding leaves in the spring where they feed, mature, and give birth to a new crop of aphids. Aphid numbers expand quickly with each succeeding generation contributing to an ever-expanding aphid population. The feeding damage on birch causes the expanding leaves to pucker and bulge length-wise producing the characteristic leaf corrugations. The aphids cover themselves in a waxy, white, flocculent material and live on the underside of the leaves within the corrugations. The affected leaves will usually turn yellow and may prematurely fall off of the tree.
Eventually, the aphids on birch produce winged females that fly to witchhazel. They lay eggs on the bark that will hatch into "stem mothers" the following spring. The stem mothers feed on newly expanding buds and inject chemicals that cause the buds to form a hollow, spiny, globular gall around their progeny. The winged aphids arising from the witchhazel galls fly back to birch.
Damage to both plant hosts is usually not severe enough to warrant treatment, particularly on witchhazel where the galls have little impact on plant health. Frequently, numerous predators will destroy aphid populations on the birch leaves. However, if heavy infestations on birch occur on highly visible plants, aphid populations can be reduced with a fall soil drench application of imidacloprid, or a spring topical application of acephate or insecticidal soap.