The Southern Hackberry (Celtis laevigata) is a common tree in the Piedmont of North Carolina, often occurring along rivers and streams, and in suburban parks. In one large park near Falls Lake, and in the surrounding forests nearby, many Southern Hackberries are visibly discolored and ragged looking, as if someone had tossed a bucket of soot or ash over the foliage of the tree. The two trees pictured below illustrate this appearance.
Closer examination of the foliage shows most of the leaves covered with a sooty mold.
Examination of the undersides of the leaves reveals an extensive infestation of Hackberry Wooly Aphids (Shivaphis celti), an Asian species that may have been brought to the U.S. by the importation of Chinese Hackberries (Celtis sinensis). The Hackberry Wooly Aphid secretes lavish amounts of honeydew, so much that the leaves and twigs become coated with the sticky substance. The honeydew acts as a perfect substrate for the fungus – usually called sooty mold – which covers the leaves. The fungus does not kill the tree, but does cause some leaf drop. The honeydew, which is rich in carbohydrates, attracts ants ,which do not harm the aphids but instead benefit them by protecting against predators.
The photos below provide several more views of the leaves covered with sooty mold. Also seen are orange lady beetles that are present in small numbers on the affected trees.
The lady beetles are the Asian Multicolored Lady Beetle (Harmonia axyridis), a major predator of the wooly aphids. They are badly outnumbered on the affected trees and have very little impact on the course of the infestation. It is ironic that this prey and predator relationship that began in Asia continues in a similar setting on a distant continent where both species were introduced (the aphids by accident; the lady beetles both intentionally and accidentally).
The Southern Hackberries are also parasitized by Leaf Petiole Galls, caused by the Hackberry Petiole Gall Maker (Pachypsylla venusta). Petiole is the botanical term for the leaf stalk. These galls are very common on hackberries and do not kill the trees, even when present in large numbers, as they often are. Several examples are pictured below.
There are also small numbers of fruits on the trees. They can be distinguished from the galls by their smooth, symmetrical surface, reddish tint, and their position at the end of a stalk. The galls are always located between a leaf and a twig and are never at the end of a stalk.
The leaves on these hackberries, and those in the surrounding forests, are quite variable. Some have few to no serrations while others are serrated on each margin. Weakley’s Flora (May 2015) now recognizes the hackberries that have significant serrated leaf margins as Celtis smallii, and it is possible that these trees and others at Falls Lake may fall into that category.