Woolly Alder Aphids, Honeydew & Black Sooty Mold

Once leaves start to fall, one often observes white, fuzzy patches along the branches of Speckled Alder (Alnus incana). These fuzzy patches consist of colonies of aphids feeding on the sap of the shrub. In order to get enough nitrogen, they must drink volumes of sap, much of which is exuded from their abdomens as a sweet liquid called honeydew. The honeydew accumulates and hardens onto the branches as well as the ground beneath the shrub. Yesterday’s Mystery Photo was the honeydew of Woolly Alder Aphids (Paraprociphilus tessellates) which has been colonized by a fungus known as black sooty mold, a fairly common phenomenon.

Woolly Alder Aphids produce white wax, or “wool,” filaments from their abdominal glands. Clustered together, these aphids look like a white mold. If disturbed, the individual aphids pulse their abdomens in unison – apparently an effective defense mechanism.

Woolly Alder Aphids, also known as Maple Blight Aphids, have two host plants at two different stages of their lives. In the fall they lay their eggs on Silver Maple trees. The eggs hatch in the spring and the aphids feed on the maple leaves. During the summer a winged generation flies from maple leaves to alder shrubs and establishes colonies. In the fall, some of these aphids fly to Silver Maples and lay eggs, while some overwinter in the leaf litter beneath alders.

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