There are several species of aphids attacking apples, and among them, woolly apple aphid is the most difficult to manage. Unlike other aphids, this species attacks trees above and below ground, but it is the damage you don’t see that can cause most damage to the health of trees. Mature trees usually suffer little damage from root infestations, but root infestations are very damaging to young trees. Control of these aphids is very difficult when they attack roots. Yellowish foliage is a sign that woolly apple aphid may be infesting roots.
Woolly apple aphid differs from other apple aphids in appearance, life cycle, and type of damage inflicted. A colony appears as a cottony mass, generally clustered in wounds and pruning scars on trunks and branches of trees (Figure 1). Colonies form on new wood or at wound sites on trunks, limbs, and twigs where bark is thin. Pruning, hail damage, and egg-laying wounds by the periodical cicada can create the ideal wound sites for attack by this pest. Aphids may occur on water sprouts in the center of the tree as populations grow. Woolly apple aphid feeds with piercing-sucking mouthparts; wood will begin to swell and form galls at these feeding sites. As the number of aphids on aboveground portions of trees increase, many work their way down to roots and trunks below ground surface.
Root systems of nursery stock can be damaged by woolly apple aphid, and severe root infestations can stunt or kill young trees. Infested trees often have short fibrous roots, which predisposes them to being easily uprooted. Swollen galls also form on roots (Figure 2); galls increase in size from year to year and are sites where fungi can attack. Aphid feeding on root systems also disrupts the nutrient balance of root tissue, which can affect growth of other parts of trees. Trees can have above-ground infestations of woolly apple aphid but no root infestations. Rootstocks vary in susceptibility to woolly apple aphid and susceptible rootstocks will form galls around the infestation sites. M111 or M106 rootstocks have resistance if woolly apple aphid is a serious problem.
Monitoring & Management
When monitoring for woolly apple aphid, examine pruning scars on several trees. Carefully brush away the white waxy secretions that surround colonies to determine if live aphids are present. Predators, such as lady beetles, hover fly larvae, and lacewing larvae can completely destroy colonies, but waxy residue will remain. Treatments for woolly apple aphid are recommended when 10% of the pruning scars are infested with live colonies.
Ric Bessin, Extension Entomologist.