UNUSUAL GROWTHS ON YOUR OAK TREE?

Are you seeing knotted twigs with round growths on your pin oak trees?

On closer examination, the affected branches have gray-brown, round galls with many small teeth or horns protruding from the galls. These galls are called horned oak galls. Heavily infested oak trees can become so heavily knotted that the tree becomes unsightly, particularly in winter, without benefit of leaf cover.

Horned oak galls are caused by a tiny (1/8 inch long) wasp. The female gall wasps emerge from twig galls in May and fly to oak leaves to deposit eggs. The larvae cause galls to form in the veins of the oak leaves. In midsummer adult wasps emerge from the vein galls, mate and deposit eggs in twigs. The twig galls usually require two years to reach maximum size.

There’s a couple times each season when oak trees can be sprayed to control horned oak galls. Infested pin oak trees can be sprayed in mid-May when wasps emerge from twig galls and again in mid-summer when the next generation of wasps emerge form vein galls.

Horned oak gall. Photo courtesey of John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org

Horned oak gall. Photo:  Lacy L. Hyche, Auburn University, Bugwood.org