Oak apple gall is an abnormal growth of plant cells, formed as a response to a cynipid wasp laying eggs in the leaf or stem. These growths are called galls because they contain large amounts of tannin, which has a very bitter taste. The developing larvae live in the gall and eat from it. When the insect reaches the adult stage, it emerges from the gall through a small exit hole. Generally, galls do not seriously harm the oak tree. They may be cut off from branches, but insecticides will not work because the insect is encased and well protected inside the gall. Fertilize, irrigate and prune trees to keep them healthy.