While I have not received reports yet of woolly apple aphid activity on apple trees, there were several commercial orchards last year with serious infestations. In some years, this can be attributed to wounds caused by periodical cicada, but it has been several years since these have emerged. This may be due to the increasing use of pyrethoids insecticides to manage other pests. Pyrethroid insecticides (IRAC group 3A) are highly toxic to natural enemies, and it is those natural enemies that keep woolly apple aphid numbers low.
Figure 1. Woolly apple aphid colonies form where the bark is thin, often on new growth or near wounds to the bark. (Photo: Ric Bessin, UKY)
Woolly apple aphid is a serious pest of apples, particularly young trees. Mature trees usually suffer little damage from root infestations, but root infestations relative to young trees. Above- ground colonies form at wound sites on trunks, limbs, and twigs, where they feed through tender bark (Figure 1). Besides cicada egg-laying wounds, pruning and hail damage can create the wound sites for attack by this pest. As populations grow, aphids are commonly found on water sprouts in the center of the tree. The tree will begin to swell and form galls at the feeding sites. Even after they are controlled, these swollen nodes that remain are signs of previous infestations.
The woolly apple aphid differs from other apple aphids in appearance, life cycle, and the type of damage inflicted. A colony appears as a cottony mass generally clustered in wounds and pruning scars on the trunk and branches of the tree. The aphids themselves are purplish in color surrounded by white, cottony secretions. Woolly apple aphid is a sucking insect pest that weakens the tree by feeding on limbs and roots and produces honeydew. Long strands of white wax are produced that help to protect the colony of purple aphids from predators and pesticide sprays. As the number of aphids on the above ground portion of the tree increase, many work their way down to the roots below ground surface. It is the feeding on the roots that produces the greatest damage. Control of these aphids is very difficult when they attack the roots.
Figure 2. Yellowed leaves can be a sign of wooly apple aphid infestations. (Photo: Ric Bessin, UKY)
During the summer, repeated woolly apple aphids generations of wingless individuals are produced. Yellowish foliage is a sign that woolly apple aphid may be infesting roots (Figure 2). The root systems of nursery stock can be damaged, 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; galls increase in size from year to year and are sites where fungi can attack. Aphid feeding on the root systems also disrupts the nutrient balance of root tissue, which can affect growth of other parts of the tree. Rootstocks vary in susceptibility to woolly apple aphid and susceptible rootstocks will form galls around the infestation sites. The M111 or M106 rootstock series has some resistance, but the G41, G213, G214, G22, G202, G210, G890, G969 have even higher levels of resistance where they have been a problem. Rootstocks appearing more susceptible to woolly apple aphid infestation include B9, M9, and M26.
It is relatively easy to find where the colonies have formed. When monitoring for woolly apple aphid, examine four pruning scars on each of 5 scaffold limbs per tree. Carefully examine woolly apple aphid colonies to determine if live aphids are present. Predators, such as lady beetles, syrphid fly larvae (Figure 3), and lacewing larvae can completely destroy the colony, but the waxy residue will remain. When examining colonies, blow hard on the branch to remove the waxy filaments to reveal live aphids. Treatments for woolly apple aphid are recommended when 10% of the pruning/egg laying scars are infested with live colonies. There are few insecticides specifically labeled for control of woolly apple aphid. Admire, Beleaf, Closer, Diazinon (commercial use only), Movento, PQZ, and Versys are recommended for control of above-ground infestations.
Figure 3.Natural enemies, like this syrphid fly larva, play a key role in keeping wooly apple aphid numbers low. (Photo: Ric Bessin, UKY)
By Ric Bessin, Extension Entomologist
Kentucky Pest News 