Galls can be caused by a variety of organisms including bacteria, fungi, nematodes and mites. The most common cause of galls is insects. Insect galls occur in many forms, colors and shapes. They may be hair-like projections, spheres, rosettes or woody swellings and can occur on leaves, twigs or branches. The malformed plant tissue, or gall, forms in response to egg-laying or feeding by wasps, midges, mites, moths, aphids or beetles. The young insects develop inside, but cause little or no damage to the tree. Small exit holes on the outside of the gall indicate that the insects have already emerged, but the gall remains. Some beneficial organisms live in abandoned galls. Infested trees will continue to support populations of gall insects. Oaks may have more problems with insect galls than many other tree species.
Once a gall begins to form, nothing can stop its growth. It would be virtually impossible to time an insect application to prevent the insect from laying the egg. Since there isn’t an effective control, instead select gall-free plants. Leaf galls are harmless but unsightly. Galls on small stems and branches may be pruned out. Galls on large branches or trunks can cause weak points and possible limb breakage. When galls are extensive, removal is difficult because the tree can become disfigured if severely pruned.
More information on galls here: http://sfrc.ufl.edu/extension/4h/foresthealth/stress/galls.html
Top photo credit: Roi Levin, EPLS, Inc.