Leaf Curl Ash Aphids a.k.a. Woolly Ash Aphids

This spring we have been seeing a lot of samples in the office of ash trees with young leaves curling under and a funny white fluffy substance on them. This is the leaf curl aphid also known as the woolly ash aphid.

This tiny insect infests and feeds on the newly emerging leaves and young stems of ash trees in the early spring. The aphid is tiny, about 1/8 inch; pear shaped and usually white to green. The bodies are covered in a white, waxy filament that gives them a cottony or woolly appearance. In high humidity conditions, sooty mold can develop on the sugary droppings of the aphids giving the leaves a blackened appearance.

Host plants are primarily ash, but hawthorn, cotoneaster, beech, elm, crabapple, and other members of the Rosaceae family are often infested too. The aphid feeding causes the leaf to curl downward and further protects the insects on the underside of the leaves. The aphids overwinter as immatures on roots or as eggs on the bark of trees. In the fall, a winged generation of aphids will mate and lay eggs in the bark of nearby trees.

In general, no control is necessary as woolly aphids rarely cause damage to the tree, except for the unsightliness of the curling leaves. Native predators such as ladybugs and parasitic wasps usually keep populations in check. The aphids are protected under their waxy coating and difficult to manage with insecticides.

If the shrub or tree is very small, use a strong jet of water to dislodge the aphids from the underside of the leaves. You can also use a hose-end sprayer with a bit of Dawn dish soap in the water to spray down the undersides of the leaves and young stems. The dish soap clogs the aphid spiracles (breathing tubes) and suffocates them.

If you have any questions, please contact the MSU Extension Valley County office at 406-228-6241.