Over the past few weeks, there have been numerous inquiries about “whiteflies” on trees. Reports indicated the trees turned black, and the “whiteflies” were all around the trees. However, these insects aren’t whiteflies. They are actually a large infestation of Asian woolly aphids.
Woolly aphids are small, white insects that fall off hackberry trees and appear to float in the air. Woolly aphids cause problems similar to whiteflies, scales, and other aphids. 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.
In the late 1990s, the woolly aphid was accidently introduced to the United States. Presently it can be found from Florida to Texas. Woolly aphids are frequently mistaken for whiteflies, which can be common on gardenias and other landscape plants. Woolly aphids only infest hackberry trees. If they are found on other plants or on the back porch furniture, they landed there by accident. These aphids are not harmful to people and do not bite or sting.
Hackberry trees are common native trees that grow 60 to 80 feet high with a trunk two to three feet in diameter. The Hackberry tree has leaves that are dark green on top and pale green underneath. The leaves are alternately arranged and have an oblong shape. The leaves are two to four inches long and one and one-fourth inches wide. The bark of hackberries is light gray and smooth, with prominent corky, somewhat warty, ridges on older trees. Hackberry trees are very prolific seed producers. Weedy hackberry saplings can spring up anywhere near the hackberry tree. Cut them down to help reduce the number of woolly aphids in the future.
While the number of woolly aphids produced annually can seem overwhelming, no long-term or serious damage to hackberry trees has been found. To control the problem, insecticides are not necessary, but they may be justified when honeydew and sooty mold become intolerable. If insecticides are warranted, several factors should be considered for control.
For larger hackberry trees, it may not be feasible to spray the leaves with an insecticide for control. Keep in mind that the tree will need to be treated annually because woolly aphids will return. It is possible to use a systemic insecticide, containing the active ingredient imidacloprid, applied as a root drench around the base of the tree. Systemic chemicals take several weeks to move from the roots into the leaves. Treatments made in late summer will not provide any benefit since the leaves will soon be falling for winter.
Systemic insecticides should be applied in the spring shortly after the leaves emerge and again in late summer for longer control. Eliminating the first generation of these aphids can dramatically decrease the severity of the infestation. Another option for control is to remove the host tree from the landscape.
This will immediately eliminate the problem since these aphids can only damage hackberry trees. However, tree removal may be very costly for larger trees and can eliminate valuable shade in your landscape.