If you look on the underside of the leaves of a Common Hackberry (Celtis occidentalis) tree at this time of year, you often find light-colored, raised bumps, commonly referred to as Hackberry Nipples Galls. The creatures responsible for these growths (through chemical interactions with the leaves) are a group of small insects called jumping lice, or psyllids, which resemble miniature (1/6 “ long) cicadas, with their large eyes and wings held roof-like over their backs.
Adult Hackberry psyllids emerge in September and October from the galls they have formed and seek shelter for the winter, often in the cracks and crevices of tree bark. Because they are attracted to lights and can often fit through the mesh of window screens, these insects also seek shelter in houses. Although considered a nuisance by some, Hackberry psyllids do not sting, nor do they carry disease. They pass the winter as adults and when they break dormancy in the spring, the psyllids exit houses, tree bark fissures, etc. and lay eggs on the emerging leaves of Hackberry trees. After the eggs hatch, the young psyllids start feeding, stimulating abnormal growth in the leaves, forming small pockets, or galls, surrounding the insects. The psyllids spend the rest of the summer sucking on tree sap safely within the protective galls before exiting in the fall. As a rule, these insects do not cause serious damage to their Hackberry tree hosts.