Horned Oak Galls on Pin Oak: Serious but Manageable

Pin oak in Illinois covered top-to-bottom with 1-3.5 inch galls.

Identification

Horned oak galls caused by cynipid wasps. Two-year life cycle: wasps emerge May, lay eggs on leaf veins, mid-July emergence of smaller wasps that mate and lay eggs in twigs. Galls develop over two years. Heavily galled trees can be disfigured, weakened, or killed.

Management

No effective insecticides. Prune and destroy galls before horns form (when wasps emerge). Reduces population. See Tulsa Master Gardeners link for images and details.