Beech, Fagus spp., is one of the most beautiful specimen trees that is less susceptible to insect and mite pests compared to other trees. However, there is one insect, the woolly beach aphid, Phyllaphis fagi, that may be a problem when populations are excessive. This aphid has only a single host—beech. The woolly beech aphid gets its name because the body is covered with waxy wool-like filaments. These aphids are gregarious and tend to congregate primarily on the undersides of leaves. Often large numbers of the molting or cast “skins” will be attached to leaf hairs, which gives the leaf a whitish appearance. The woolly beech aphid has piercing-sucking mouthparts, which are used to remove plant fluids. However, woolly beech aphid is not considered an economic pest because beech trees, especially large specimen types, can sustain large populations without suffering any injury. Large populations of woolly beech aphid can, however, produce tremendous amounts of honeydew, a clear, sticky liquid that may attract wasps, ants, or yellowjackets. In addition, the honeydew serves as an excellent growing medium for black sooty mold fungi. Black sooty mold fungi can detract from the aesthetic appearance of a beech tree and most importantly can reduce the production of food via photosynthesis by blocking the entry of light.
For large beech trees, control is typically not warranted. A hard spray of water will dislodge aphids from the tree without harming any natural enemies. Insecticides that will provide control include acephate (Orthene), imidacloprid (Merit), and insecticidal soap.