Beech trees infested with beech blight aphids (Grylloprociphilus imbricator) frequently experience a simultaneous fungal infestation of mildew-like sooty mold (Scorias spongosia). Although these two pests can infest beech trees anywhere, they are most common in the eastern United States. Beech branches covered with swarms of the woolly, white insects above mounds of black fungus on the ground are an unpleasant sight. Damage to the trees, however, is usually minor.
Pale blue beech blight aphids flock to the twigs, young branches and foliage of American beech trees (Fagus grandifolia) in the summer and fall. Heavily infested branches look white with snow, says University of Massachusetts Extension entomologist Robert D. Childs. The aphids often perform the defensive measure of raising their back ends and swaying in unison to ward off danger. This maneuver is responsible for their nickname of "Boogie-Woogie" aphids.
Like other leaf-sap sucking insects, beech blight aphids excrete a sugary substance called honeydew. This waste product remains after the insects have digested the sap’s nutrients. Honeydew attracts ants to garden plants infested with aphids, notes Childs. On those plants, the ants will mingle with the aphid colonies to get to the food. Beech blight aphids, however, produce so much honeydew that large amounts of it fall onto the plants and ground beneath the trees. The ants access it there.
Mildew and Aphid Excretions
Scorias spondiosa belongs to the Capnodiales family of sooty molds. These molds feed on insect honeydew. They appear in black layers wherever the honeydew falls, say the University of Wisconsin's Tom Volk and Hannah Reynolds. Beech blight aphids excrete massive amounts of honeydew, resulting in severe infestations of this mildew-like substance. The mold feeds on the honeydew, not on the plant tissue beneath it.
Effects of Infestation
The effects of beech blight aphids on beech trees may range from smaller or malformed leaves to death of the trees small limbs. In addition to damaging the trees' appearance, sooty mold on leaves may slow the trees' growth by reducing photosynthesis. The mold, however, may also be a more effective air cleaner than bare beech leaves. Leaves covered with the fungus adsorb more atmospheric heavy metals and hydrocarbons, note Volk and Reynolds.
Treatment and Prevention
The most effective treatment for existing aphid infestations is an application of a pyrethroid-based insecticide, horticultural oil spray or insecticidal soap. Prevent aphid infestations with a systemic application of Imidacloprid applied to the soil around beech trees in the spring. The soil temperature must be at 50 degrees Fahrenheit or higher and dried out from spring rains or melting snow, cautions Childs. Eliminating the aphids also eliminates the mold.