Stem Galls

Stem galls appear as bumpy, wart-like or conical growths on twigs, branches, or occasionally trunks of affected woody plants. Gall-causing insects include wasps, flies, mites, and plant-feeding aphids that trigger abnormal tissue growth. Galls vary greatly by causal organism and host species; some remain small and inconspicuous, while others are visible from a distance.

Most galls do not threaten tree or shrub health. Remove heavily galled twigs in winter dormancy to reduce pest overwintering. Prune to improve air circulation and reduce humidity favoring gall formation. Do not use pesticides, as chemical control is ineffective and destroys natural enemies.