Every summer the hackberry tree (Celtis occidentalis, Ulmaceae) in my yard becomes infested with hackberry nipplegall makers (Pachypsylla celtidismamma, Psyllidae). Adult insects lay their eggs on new hackberry leaves in the spring. The eggs hatch into nymphs, which start to feed on the leaves. The tree responds to the threat by growing a gall around the insect. Here’s a gall torn open to reveal the tiny insect nymph inside:
The nymphs feed inside these galls until early autumn, when they emerge and molt into adults. After overwintering in bark, the adults start the cycle anew the following spring.
While the galls can be unattractive on hackberries, the insects aren’t generally harmful to the trees. Additionally, gall maker populations are controlled in nature by several parasitoid wasps.