Woolly Elm Aphid

Eriosoma americanum


American elm and Saskatoon

Appearance and Life Cycle

Description of this image follows
Infestation of a colony of woolly elm aphids on a root. Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org

The woolly elm aphid overwinters in the egg stage in cracks and crevices on the bark of American elms. In early May, just as the leaves on elm begin to unfold, aphids emerge from the eggs and begin feeding on the underside of the developing leaves. The feeding action of the aphid causes the leaves to curl providing a protected shelter for the aphids. In late May the aphids mature and begin giving birth to a generation of young that also feeds within the leaf curl. In mid to late June these aphids develop wings and leave the protection of the leaf curl in search of their secondary host, Saskatoons. Once the secondary host has been located, the winged aphids move to the underside of a leaf and each aphid gives birth to approximately 15 young. These young descend to the roots where they feed as a colony on the root. Through the rest of the summer the aphids multiply and produce large colonies on the roots. The soil around the colony takes on a blue-purple coloration due to the waxy secretions by the aphids. Starting in late August and continuing throughout September the young root aphids develop into winged aphids. They leave the soil and migrate back to American elms where they give birth to a minute wingless sexual stage. This is the only time in the life cycle of the woolly elm aphid that male forms are present. These aphids search for protected sites in the cracks and crevices of the bark, where they mate and the females each deposit a single overwintering egg.


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Elm leaf distorted by the woolly elm aphids. Photo credit: Steven Katovich, USDA Forest Service, Bugwood.org

The woolly elm aphid causes damage by removing sap from their host. On American elm, the aphid causes the leaves to swell and curl downward along the leaf edge. This results in numerous unsightly leaves that remain on the tree throughout the year, but it causes no permanent damage to the tree. As the aphid population increases within the curl, the aphid excrete a sweet, sticky substance called honeydew. This honeydew can be a nuisance especially when it covers walks, cars and other objects under the trees. On saskatoons, the root systems beyond the point of the aphid infestation is destroyed resulting in reduced vigor and fruit production of established plants. On saskatoon seedlings, severe aphid infestations can result in the death of young plants.


Several products are registered for the control of aphids on elm, but no products are registered for control of aphids on the roots of saskatoon. Eliminating elm trees or aphid populations on elms will not prevent aphid infestation of Saskatoon plants. Each leaf curl can contain hundreds of aphids which can travel long distances on wind currents.