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The woolly apple aphid, Eriosoma lanigerum, is one of several species of aphids that can infest apple trees in Utah. Woolly apple aphid is an occasional pest and is not usually found in most orchards in most years. It often found backyard trees. Woolly apple aphid is native to eastern North America, and feeds mainly on apple. It is also found on elm, pear, quince, hawthorn, mountain ash, and cotoneaster. Woolly apple aphid is an indirect pest that weakens the tree by its feeding on bark and roots. This sap the strength of the tree and prevents wounds from healing. Woolly apple aphid can also attack the fruit cores of some cultivars. It is a nuisance pest during harvest when its waxy covering brushes onto the clothing of pickers. Colonies of woolly apple aphid form at wound sites on trunks, limbs, and twigs, where they feed on tender bark. They usually are first seen on pruning wounds. As populations grow, aphids move to the axils of leaves on water sprouts or on terminal shoots. Swollen galls form on stems where aphids have fed. Foliage turns yellowish on infested branches. Occasionally woolly apple aphid damage contributes to the development of black sooty mold. As aphids feed, they excrete excess sap as honeydew. Honeydew on leaves and fruit encourages the growth of black sooty mold. Sooty mold on leaves can affect photosynthesis and may reduce fruit yield, while sooty mold on fruit can lower fruit quality and marketability. In addition to feeding on small branches and wounds, woolly apple aphid may be found year-round on roots where they often go unnoticed. Mature trees usually suffer little damage. Yellowish foliage is a sign that woolly apple aphid may be infesting roots. 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. The underground form of woolly apple aphid is more damaging than the aboveground form. Trees can have aboveground infestations of woolly apple aphid but no root infestations. Woolly apple aphid gets its name from its fuzzy appearance. The above ground aphids produce and surround themselves with long white waxy strings, while the underground form has a bluish-white covering of shorter rod-like wax particles. Woolly apple aphids usually overwinter as nymphs on apple tree roots several feet beneath the soil surface. Nymphs and adults also survive above ground in sheltered crevices of the bark during mild winters. In areas with many elm trees, woolly apple aphid overwinters in the egg stage in the cracks and crevices of elm bark. In the spring, wingless females give birth to live nymphs. The first-stage nymphs are called crawlers because they are the most active of the four nymph stages. Crawlers move from roots to aboveground parts of the tree. Wind, birds, or other insects carry the aphids from tree to tree. Natural controls include small parasitic wasps. They lay their eggs in aphids. The wasp egg hatches within the aphid, and the young wasp larva consumes the aphid. Parasitized aphids turn brown or black. The wasps are susceptible to insecticides and usually cannot survive in orchards where insecticides are used. Other natural enemies of apple aphids include predators such as hover fly larvae, lacewing larvae, lady beetle larvae, and lady beetle adults. These predators feed on many different aphid species in addition to other insect pests. Cool, wet spring favors aphid development because these conditions are unfavorable for the aphid's natural enemies. The reason we see so many wooly apple aphids in the fall is that the predators have become less active. Resistant varieties are used to prevent underground infestations. The Malling-Merton (MM) rootstock series provide resistance to woolly apple aphid. Some apple varieties such as Northern Spy are resistant to this pest. Removing suckers at the base of trees creates conditions that discourage development of woolly apple aphid populations in early spring. Summer pruning of water sprouts also contributes to woolly apple aphid suppression. An insecticide can be applied if woolly apple aphid is detected at damaging levels on aboveground parts of trees. Insecticides are most effective if applied when the aphid is in the active crawler stage and is just moving up into the tree. This may occur in late spring or mid-summer. Because of the aphids' waxy covering, high volume application is needed to get thorough spray coverage. A second application may be needed two weeks after the first if aphids continue to be detected. Home gardeners can use diazinon or insecticidal soap. Insecticides cannot control woolly apple aphid infestations on rootstocks. Larry A. Sagers Regional Horticulturist Utah State University Thanksgiving Point Office