Oak galls are tumor-like plant growths that appear on oak leaves or twigs. They are caused by tiny parasitic wasps or midges. While dramatic looking, rarely do any damage to the tree. As the tiny larva of these insects starts feeding on the tree (generally right after bud break in the spring) they release hormones into the tree that produce this tumor-like growth around the insect. This growth protects the larva and gives it a place to feed. Some of the most common oak galls in our area include the apple oak gall, which forms on the leaves of sawtooth oaks and shumard oaks and is round and papery. The vein pocket gall forms on pin oaks and is a swollen area along the main veins of the leaves. The oak marginal leaf fold gall looks similar but appears along the edge of the leaf and also affects pin oaks.
Most galls do very little damage to mature trees. Leaves with these galls can be physically removed early in the season before the insects mature, but this is impractical for large trees. Once the galls form, sprays are ineffective because the gall itself protects the developing larva. Sprays in early spring at bud break may have limited effectiveness but are nearly impossible for large trees since good coverage of the whole tree is needed. Generally, this pest can and must be lived with.