A gall is an abnormal growth on the leaves, stems and twigs, or flowers of a plant. This abnormal growth is the reaction of the plant to the feeding damage of an insect or to a toxin injected by the insect during feeding.
Galls come in a wide variety of shapes and colors and are caused by a wide variety of insects.
The good news is that galls cause very little damage to the health of the plant, therefore, treatment is usually not recommended. To reduce the insect population, galls can be pruned off. Eriophyd mites cause bladder and spindle galls on maple leaves. These galls change in color from yellow-green to red as the season progresses. Bladder galls eventually turn black.