Have you noticed strange button-like formations on the leaves of your hackberry trees?
These "buttons" are called galls.
Galls are abnormal growths of plant tissue induced by insects and other organisms. Gall-making parasites release growth-regulating chemicals as they feed, causing adjacent plant tissues to form a gall. The parasite then develops within the relative security of the gall. Several different groups of insects and one family of mites have developed the ability to induce plant galls. In addition, there are a few galls produced by nematodes, bacteria, fungi, and viruses.
Galls come in an endless variety of forms. Many are strikingly colored or curiously shaped. Each gall-making species causes a gall structurally different from all others. By noting the type of host plant and the structure of the gall, one can identify the gall-making organism without actually seeing it.
The most important groups of gall-producers are gall mites, aphids, adelgids, phylloxerans, psyllids, gall midges, and gall wasps.
Psyllids, commonly known as jumping plant lice, resemble miniature cicadas. Little is known about their biology. All species feed on plant juices, but only a few produce galls. Psyllids that feed on hackberry cause the Hackberry Button Gall, Hackberry Flask Gall, Hackberry Nipple Gall, Hackberry Star Gall and the Hackberry Melon Gall. In the Gulf states, several galls on bay are caused by psyllids.
Hackberrys also harbor a number of gall-forming midge species (Diptera: Cecidomyiidae) such as the species that produces the thorn gall, Celticecis spiniformis. Immature stages of these species, when carefully dissected out of galls, appear maggot or grub-like and have no legs or antennae as do psyllid immatures.
Controlling gall insects is difficult. Any treatment applied after galls are already present is useless because the galls will not go away even if the parasite is killed. Fortunately, the vast majority of galls are not particularly injurious and are of no economic significance. Most plants can support a large number of galls and continue to grow normally. Pruning out heavily galled portions of a plant is sometimes feasible and may help reduce populations of the gall insects, but for the most part they can be ignored.