Appearance and Life Cycle
There are two different species of aphids that feed on American elm; woolly elm aphid and woolly apple aphid. Both species overwinter in the egg stage on American elm. As the buds begin to break, the aphids emerge and begin feeding on the developing leaves. Once mature, the aphids produce a generation of young. The feeding action of these aphids cause the leaves to curl and develop abnormally. Within the damaged leaves the aphids produce two more generations. By late June, early July the aphids migrate to their secondary hosts. Several more generations of aphids are produced on the secondary hosts. In the fall the aphids migrate back to the American elm where overwintering eggs are produced.
Both species of aphids cause damage by sucking sap from the host. The feeding action of the woolly apple aphid causes a stunting and curling of leaves, forming a rosette-type deformation. By mid-June, the host may consist of many rosettes which appear extremely unsightly. The feeding action of the woolly elm aphid causes the leaves to swell and curl downward along the leafs edge. As the woolly elm aphids continue to feed and multiply they excrete an excessive amount of sweet, sticky substance called honeydew. The honeydew is extremely annoying especially when it covers walks, cars and other objects under the trees. Neither species of aphids cause permanent damage to the elms.
Satisfactory control of the elm aphids may be achieved by pruning out and destroying the aphid-infested leaves. Control of the aphids can be achieved by applying insecticidal soap, malathion or diazinon just before bud break and again ten days to two weeks later.