Woody oak twig galls that form marble-sized, dark brown, woody balls on small twigs are caused by a tiny gall-producing cynipid wasp. The wasp's egg laying and subsequent larval development result in gall formation. These galls do little damage in most cases and no control is required. Control is justified if infestation is heavy and on young trees. Insect populations vary from year to year. Heavy infestations of woody oak twig gall can be prevented by spraying the tree just as buds swell before new growth emerges in early spring. Timing is critical. Contact your local Extension office for insecticide recommendations.