Hello,

Those look like a type of twig galls caused by a tiny cynipid wasp.

Wherever oak trees grow, they are hosts to a group of tiny insects called cynipid gall makers (Hymenoptera: Cynipidae) . These tiny wasps cause abnormal growths or deformities, known as galls, of all manner of sizes, shapes and colors on leaves, twigs, bark, buds, flowers, acorns and even roots of the tree. The twig galls are just one of several hundred types of galls that form on oaks in the eastern US. The galls are far better known than the insects that cause them. Most of the oak gall makers are tiny wasps in the Cynipidae family but there are a few in which flies are responsible. Galls are caused by powerful chemicals or stimuli produced by the insect that interact with the tree hormones to produce the gall. The inner wall of the galls is rich in protein and other nutrients, which provides the developing larvae inside the gall with concentrated food supply. The galls also serve to protect the larvae, but not completely, from parasites and predators.

Each gall-maker induces the growth of its own distinctive gall that is unlike the galls formed by other species. Galls are so common on oaks that many people regard them as typical structures of the tree. Many gall makers have complicated life cycles with a phenomenon known as heterogamy or alternation of generations. The offspring of these gall makers produce galls that are quite different than their parents but that are identical to the galls produced by their grandparents. The alternating generations often produce galls on different parts of the oak tree. And, more amazingly, the alternating generations of wasps often differ in body structure to the extent that they appear to be two different species

Oak galls seldom cause serious injury to the tree and I do not recommend attempting to control gall makers with insecticides. Oak tree and gall makers have been getting along together for 1000’s of years. Insecticides should only be employed when the health of the tree is threatened.

Control for these insects is difficult. Because of their complicated life cycle, spraying with an insecticide is usually not effective. Sprays would need to be precisely timed with when the females are laying eggs. This probably occurs when the new growth is expanding. There may be some value in pruning branches that contain galls but doing nothing is probably the best alternative.

Best regards,

Howard