Adult -Several different forms of the adult woolly apple aphid exist. The globose, 2-mm-long stem mothers are yellowish or reddish with dark dorsal markings and are covered with bluish-white, waxy material that is longer caudally (Color Plate 4JJ). Other wingless females are rusty or reddish brown, occasionally slightly purple. These females are smaller and more elongate than the stem mothers. Winged females are reddish brown and do not possess as much wax as the wingless forms. The sexual forms, which are the smallest, are covered by a fine, powdery wax and lack mouthparts. The legs and heads of all the adult forms are darker than their bodies.
Egg -The dark, glistening egg is oval and about 0.3 mm long.
Nymph -The nymph is similar to the wingless adult but is smaller and does not have as much waxy material.
Host Plants -Elm is the primary host; but apple, hawthorn, mountain ash, pear, and quince are secondary hosts
Damage -Stem mothers feed at the base of leaf buds, causing the leaves to curl and thicken once the leaves begin to develop. This damage forms a rosette of deformed leaves. Woolly apple aphids are particularly drawn to open wounds or pruning scars. If the insects are feeding on branches or twigs, galls or knotty swellings are produced at the feeding sites. These galls may be 13 to 75 mm long depending on the severity of the infestation. Eventually, the bark splits as the tree attempts to cover the galls with new growth. Similar swellings are formed on the roots after the insects feed there.
Life History -Elm is the primary host of the woolly apple aphid. Apple is a secondary host, although the aphids are capable of living for several years on the roots of apple trees without migrating back to elm. Overwintering occurs on both hosts. On elm, the overwintering stage is the egg, which is deposited in crevices in the bark. On apple, the young nymph attached to the roots is the overwintering form.
In the spring, the eggs on elm hatch into wingless females called stem mothers. These aphids and succeeding generations give birth to live young without mating. The stem mothers' offspring are winged, and they migrate to apple. A single stem mother was recorded as producing 299 nymphs; however, the average number of offspring is much lower and varies according to the form of aphid. Wingless forms average about 30 young per female; winged forms, about 6; and those feeding on apple roots, about 85. The four nymphal instars develop in 8 to 20 days. Adults live about 25 days. As many as 18 generations can be produced in 1 year. Sometimes the next to the last generation on apple is winged and migrates to elm, where male and female aphids are produced. These aphids mate, and the female deposits one egg.
For specific chemical controls, see the current state extension service recommendations.
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