Galls are abnormal swellings of plant tissue, usually on leaves and stems. They may be caused by insects, mites, bacteria, fungi, or nematodes. Most insect and mite galls are caused by chemicals produced by the egg laying and feeding activities. The chemicals cause the affected plant cells to swell. Aphids, midges, wasps, psyllids, beetles and eriophyid mites can cause galls.
Stem gall. Photo: University of Maryland Extension
Galls may disfigure stems and foliage, but they do not seriously affect the health of trees and shrubs. Chemical control is usually not recommended. If a plant appears unhealthy, search for additional causes such as cultural problems or diseases.
Management
To prevent completion of the insect or mite life cycle prune out stem galls while they are green. Small holes in the gall indicate that the inhabitants have escaped repeating the cycle. To control leaf galls rake up and destroy infested leaves.