Over the past few weeks there have been numerous inquiries about whiteflies on trees. However, these insects are not whiteflies. They are actually a large infestation of Asian wooly aphids. Wooly aphids are small white insects that fall off hackberry trees and appear to float in the air. They suck the sap out of the leaves and secrete a sticky residue called honeydew. As a result, a black mold, known as sooty mold, covers the leaves, stems and possibly the bark of the tree. This black sticky substance covers the hackberry tree, and it can cover any plant or object underneath it. When I see black soot on certain plants that are not prone to any specific insect, I always ask, “what’s above this plant”? I will usually find a hackberry tree nearby. It does not have to be directly above, just nearby. This black soot gets on vehicles, lawn furniture and anything else in the fallout zone.
To control the problem, insecticides are not always necessary, but they may be justified when honeydew and sooty mold become intolerable. If the tree is small enough to spray, you can use spinosad. For larger hackberry trees, it may not be feasible to spray the leaves with an insecticide for control. It is possible to use a systemic insecticide such as Bayer Tree and Shrub Protect and Feed applied as a root drench around the base of the tree. Systemic chemicals take several weeks to move from the roots into the leaves. Systemic insecticides should be used in the spring shortly after the leaves emerge and again in late summer for longer control. I usually treat my neighbors tree in April since wooly aphids appear every year in June to July. That way I am sure the chemical is still at its optimal control strength.
Another option for control is to remove the host tree from the landscape. This will immediately eliminate the problem since these aphids only damage hackberry trees. The down side to systemic insecticides and tree removal is the hackberry is a wonderful tree for birds with its berries. It is also the host plant for the Snout butterfly. Most importantly, now you have the information needed to make the best decision for you, the tree and the environment.