If you see bumps like this on your hackberry tree, do not despair! We found these strange protrusions on a hackberry tree planted beside the Barton Centre on Kenilworth. One of our volunteers said that they were caused by bugs laying eggs and feeding on the leaves. Intrigued, we investigated a bit further.
Turns out our knowledgable volunteer was right. These galls are produced by hackberry gall psyllids – tiny grey bugs, about one tenth of an inch long. The adult psyllids lay their eggs on new springtime hackberry leaves, then the new psyllid feeds on the leaves. In response to this feeding, the tree increases production of plant growth hormone in that area, which creates the galls. These pockets form a nice protective pocket around the bugs so they can eat tree sap within the gall throughout the summer.
Although the galls can look unsightly and cause concern, they typically do not harm the host trees. Here are a few websites with more information: