Leaf galls are a frightening sight, but are not usually as serious as they appear. These bumps and deformations are usually the result of insects or mites feeding on the leaves. The gall itself is the plants response to the irritation. Its not unlike the bump you get when an insect feeds on you, expect the leaf gall is not going to go away.
Despite appearances, the insect is not living in the gall. In fact, it is very likely that once you notice the galls the insects have moved on. Before they do, they can do a lot of cosmetic damage to many plants and in particular trees. Many common trees are susceptible to leaf galls, especially in the spring. Maple, oak, elm, hackberry and others each are favored by a different insect that causes unsightly and intimidating galls. Damage will be greater following a mild winter, since more insects have survived and are hungry. Galls wont usually kill a tree, but they may cause early leaf drop. A healthy tree will send out new growth and recover.