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.
Occasionally you see a wild Blackberry bush with leaves that have stunted growth and are often curled up. This malformation is due to the Blackberry Psyllid (Trioza tripunctata), also known as the Jumping Plant Louse. Closely related to aphids and scale insects, psyllids are plant-feeding bugs which typically have one specific host on which they feed and lay their eggs. Blackberry Psyllids, small, cicada-like insects that hold their wings tent-like over their body, feed only on Blackberry and in so doing, cause this leaf distortion.
Blackberry Psyllids have one generation per year. The adults mate and lay eggs (39-202) on Blackberry bushes in early summer. The nymphs, small and wingless, also feed on the sap of Blackberries. They are often found inside the curled leaves during the summer months where they secrete several types of waxy structures as they feed. In the fall the nymphs mature and overwinter as adults in conifers (pines, spruces and cedars) prior to returning to Blackberry bushes in the spring. If Blackberries are one mile or more from conifers, no psyllid damage will be found; those growing within one-eighth of a mile from conifers are at the greatest risk of psyllid damage.