Woolly apple aphid (Eriosoma lanigerum)
Woolly elm aphid (Eriosoma americanum)
Pest description and damage Several species of aphids can become problems as foliar, twig or stem, or root feeding pests on elms. Some of the aphids have a summer alternate host where they infest roots, stems, lower trunk or leaves of Amelanchier, hawthorn, apple, pear, currant or other rosaceous plants. Some of the aphids form galls on elm while others distort tissue or cause leaves to pucker and roll tightly. Woolly elm aphids are tiny and dusted with a light gray wax. They cause leaves to roll under from the edge of the leaf toward the midrib. Their alternate summer host is serviceberry. Woolly apple aphids form clusters on galls on elm. On apple, they produce long, white, waxy filaments and form noticeable white patches on bark, water sprouts and the roots. They also form lumpy galls on Malus branches and roots; heavy root galling can kill small trees. Feeding damage on elm leaves is aesthetically displeasing, but usually does not cause significant tree damage. If abundant on elm, they produce galled and distorted leaves and honeydew and can compromise the vigor of small trees.
Biology and life history The Eriosoma species all winter over on elm, in bark crevices. In spring, they produce young which feed on the elm leaves, then become mature winged adults, which migrate to their alternate hosts. They spend summer on these alternate hosts until fall when winged adults are produced and migrate back to elm. Woolly apple aphid has adapted to overwinter on apple even in the absence of elms.
Pest monitoring Woolly elm bark aphids are best observed on bark before budbreak. Observe trees as they leaf out in spring to determine infestation levels, so that if pesticides are needed they can be applied before damage is significant. Once leaves are distorted, the damage cannot be undone.
The woolly apple aphid prefers to infest wounds, callus tissue or other damaged areas on trees. Keep trees in good health and watered during periods of drought.
Woolly aphids are regulated by predators and parasitoids. Syrphid fly larvae, lacewing larvae, and lady beetles are common predators on the aphids. Often by the time the aphids are noticed, predators are already on site and feeding. The wasp parasitoid Aphelinus mali also is reported to control woolly apple aphid.
See Table 1 in:
Chemical Control of Landscape Pests
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