Nipple gall is so common on hackberry that it is often used to identify the tree. Once the galls form they protect the causal organisms and control is impractical. Even though their form and color may be alarming, galls seldom threaten the health of established trees. In some cases, the insect or mite is not protected by a gall and can be easily spotted on the leaf tissue. Aphids, for instance, may cause leaf curling, discoloration and defoliation.
Damage is usually more ugly than harmful.
Sprays would have to be applied prior to observable symptoms to be effective. The most important first step in deciding what to do about a plant problem is to accurately diagnose the problem. Take time to learn the common problems of your favorite plants. In many cases, sanitation practices such as raking and removing diseased foliage, or pruning out branch terminals that are deformed by aphids will give you adequate control.