Jumping oak galls are caused by a very small, stingless wasp that lays eggs on developing oak leaf buds early in the spring. The larva that hatches from the egg will start to feed and juices from the saliva will cause the gall to form. The larva will feed inside the gall which offers a measure of protection. The galls are quite small; about the size of a pinhead. The gall eventually drops out of the leaf and falls to the ground. The galls will then jump due to movement of the larva inside the gall. This helps the insect move into the litter under the tree or into cracks in the soil where the insect will eventually pupate and overwinter. The mature wasp will chew its way out of the gall the next spring to start the cycle over again.
White oaks and members of the white oak family can be affected. Though heavy infestations can cause leaves to brown (or turn black), curl and possibly drop, otherwise healthy trees are not appreciably harmed. Even if it were more serious, it is too late to treat by the time symptoms are seen. Often natural controls prevent damage in subsequent years. Keep trees healthy by watering during dry weather. (Ward Upham)