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This aphid, native to North America, was identified in the United States in 1842. It is now distributed throughout the apple growing regions of the world where its importance as a pest varies. The woolly apple aphid may occur on the above-ground portions or roots of the apple tree. Aphid forms inhabiting above-ground parts of the apple tree are most common in mid- to late summer and are usually detected in colonies on old pruning wounds and scars or at the base of buds on actively growing terminals and watersprouts. The aphids are covered with long white waxy filaments which give the colony a woolly appearance.

The original primary (or overwintering) host of the woolly apple aphid is American elm. In areas where this species of elm occurs, elm is the overwintering host, and apple is the summer (or alternate) host. On elm, the aphids overwinter in the egg stage. However, this aphid has adapted to live and reproduce on apple year round in most fruit growing areas of the world (where the American elm does not occur), including the western United States. There is evidence that sexual reproduction occurs on apple in New Zealand, but the importance of this has not been established. Other alternate hosts include hawthorn, mountain ash and cotoneaster.

Egg: The egg stage is not known to occur in Pacific Northwest orchards.

Nymph: Shortly after birth, the nymph is salmon colored, and lacks the woolly coating. This stage is known as the crawler. The waxy filaments begin to form after the aphid settles to feed.

Adult: The adult is reddish brown to purple. The actual color, however, is usually concealed beneath a white, cotton-like substance secreted from the aphid's abdomen. This characteristic makes this aphid species easy to distinguish from other aphid species occurring on apple.

Woolly apple aphid overwinters as a nymph on the roots of apple. It can also overwinter as a young nymph on the above-ground part of the tree in protected areas on the trunk or main limbs. In severe winters above-ground colonies may be killed.

As temperatures warm in the spring, overwintering aphids produce live young that migrate up and down the tree. Nymphs on the roots move upward to provide a source of infestation if above-ground colonies do not survive the winter. Preferred feeding sites during the summer are leaf axils on terminal shoots. When populations are high, most leaves on a terminal will have a cottony mass at the base.

It is unclear whether winged adults are produced in the woolly apple aphid life cycle in the Pacific Northwest. British Columbia work indicates no winged adults are produced, but a 1966 USDA publication indicates the presence of winged adults. Research in Washington indicates that winged adults are present but relatively unimportant in the spread of this pest.

Galls, or swollen enlargements, form on the plant where aphid colonies feed on twigs or roots. These are not very noticeable after one year of feeding but increase in size as feeding continues in an area. Subterranean aphid colonies cause the most damage. Roots of infested trees have large, abnormal swellings. Continued feeding can kill roots and cause reduced growth or even death of young trees.

The Malling-Merton series of rootstocks (e.g., MM.106 and MM.111) were developed to be resistant to woolly apple aphid, as is the Merton 793 selection, which is commonly used in the southern hemisphere. This resistance is based on a scion cultivar, 'Northern Spy'. Recently, a new series of woolly apple aphid-resistant rootstocks is being developed by the Geneva rootstock breeding program. The resistance is based on Malus robusta, and apparently confers a higher level of resistance than the older Malling-Merton series.

Woolly apple aphids are attracted to sunken areas caused by the disease perennial canker. Galls caused by feeding of aphids are re-infection sites for the causal fungus of perennial canker, Cryptosporiopsis perennans.These galls are more sensitive to low temperatures than normal bark tissue and rupture at about 0°F or colder, providing an entry site for the fungus, continuing the perennial nature of the canker.

Honeydew produced by the woolly apple aphid can drip onto the fruit resulting in sooty mold and downgrading of fruit because of blackened or russeted areas. High populations of woolly apple aphid can create sticky and unpleasant working conditions for harvest crews. Woolly apple aphid can also infest the stem and calyx end of the apples; the presence of live (or even dead) aphids in packed apples is a potential quarantine issue. In some instances, especially varieties with an open calyx, aphids can also infest the apple core.

No specific monitoring procedures or treatment thresholds have been developed for woolly apple aphid. Generally, monitoring should begin in midsummer or perhaps earlier if the winter was mild. If many colonies are in the fruiting zone of the tree, treatment will probably be needed.

The parasite Aphelinus mali is generally given the most credit for biological control of woolly apple aphid. Although A. mali may play a role, research in Washington has shown that a complex of generalist predators including lady beetles, syrphid fly larvae, Deraeocoris brevis, and green lacewings is more important.

Predators can destroy woolly apple aphid colonies, leaving the woolly residue but no trace of its activities. Woolly apple aphids parasitized by A. mali are black, persist on the tree and may have an exit hole where the parasite emerged. The persistence of these mummies has led to the conclusion that most biological control was by A. mali. (For more information on specific natural enemies, see Predators or Parasitoids)

Woolly apple aphid is not a serious pest in the Pacific Northwest. Low winter temperatures reduce overwintering populations substantially in most years, and the parasite and predator complex attacking woolly apple aphid, along with insecticides used for other pests, usually suppress its population below economic injury levels during the growing season. Outbreaks have been blamed on pesticide applications that disrupted biological control (particularly after a mild winter) and/or poor pesticide suppression from sprays aimed at other pests. Low populations are likely to persist on all rootstocks including the Malling-Merton series, as their resistance to woolly apple aphid infestation is not total.