Jeffrey Hahn, Asst. Extension Entomologist
This June has seen a lot of activity by leafcurl ash aphids, a type of woolly aphid, not only in the Twin Cities but also in a number of other areas in Greater Minnesota. Like other aphids, leafcurl ash aphids use piercing - sucking mouthparts to feed on the sap in the leaves. This feeding causes leaves to become tightly curled, puckered, and distorted. To verify leafcurl ash aphids, unroll the leaves. The aphids are a light green and no more than 1/8 inch long. They produce a conspicuous white waxy material that covers the aphids as well as the leaves.
These aphids were even reported as nuisances when they would fall down on people in their yards (which is not conducive for graduation parties and other outdoor activities).
Leafcurl aphids feed on the new growth that expanded this spring. Colonies last until mid-summer. Winged forms are produced which migrate to the roots of ash where they remain for the rest of the year. Leafcurl ash aphids have always been in Minnesota but have been infrequently noticed or reported over the last 5 - 10 years.
Although the damaged leaves are conspicuous, when you look closely, just a small number of leaves within a tree are actually affected by leafcurl ash aphids. Even the leaves that are distorted can still photosynthesize so there is very little risk to the health of ash.
While insecticides, such as imidacloprid and dinotefuran are effective against these aphids, they are rarely warranted to protect trees and are not suggested. Even if you kill the aphids, the distorted leaves will remain for the rest of the season. While this can affect the trees' appearance that is of small consequence compared to other problems, especially the risk of emerald ash borer.