One of the great things about beech, Fagus spp., is that the tree is less susceptible to insect and mite pests than many other specimen trees. However, there is one insect, the woolly beech aphid, Phyllaphis fagi , that may be a problem when populations are excessive. This aphid has only a single host: beech.
Growers report seeing them on several cultivated varieties including Fagus sylvatica ‘Asplenifolia,’ ‘Fastigiata,’ ‘Roseo-Marginata’ and ‘Purpurea Pendula.’ The sap-sucking insects coat the upper leaf surface with honeydew, making the foliage sticky and opening the door for black, sooty mold to develop.
THE FACTS ABOUT WOOLLY BEECH APHID Colony on beech leafWhitney Cranshaw, Colorado State University, Bugwood.org
APPEARANCE The woolly beech aphid gets its name because its body is covered with waxy wool-like filaments. These aphids tend to congregate in groups, 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.
LIFE CYCLE Woolly beech aphid overwinters as eggs that are laid around buds and in bark crevices in autumn. The eggs hatch in spring a few weeks after new foliage has appeared. For most of spring and summer, the aphids are wingless forms that reproduce by producing live young. In mid-summer, winged forms develop that fly off in search of new host plants. These winged woolly beech aphids can be mistaken for glasshouse whitefly or woolly aphid, a pest that does not occur on beech. As many as 10 generations were documented in nursery production during one growing season.
Damage, rolling leavesLouis-Michel Nageleisen, Département de la Santé des Forêts, Bugwood.org
DAMAGE The woolly beech aphid has piercing-sucking mouthparts, which are used to remove plant fluids. Beech trees, especially large specimen types, can sustain large populations of aphids without suffering any injury. However, large populations can 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.
CONTROL Monitor for the eggs near buds and the aphids on new growth and leaf undersides. Control with pesticides is usually unnecessary and only feasible on beech trees and hedges that are small enough to be sprayed thoroughly. If infestations are severe, affected plants can be sprayed in April-May. It is not worthwhile spraying later in the summer, as by then any damage has already occurred and the infestation will be past its peak and in decline. Organic sprays, such as natural pyrethrum or plant oils can give good control but have a very short persistence and so may require reapplication.
Sources: Oregon State University IPM, University of Illinois Extension, Royal Horticultural Society
