Several species of small gall wasps (Family Cynipidae) form a variety of distinctive twig and leaf galls on bur oak growing in our region.
These galls are abnormal plant growths stimulated by the developing wasp larvae. The tiny dark adult wasps are seldom seen.
These conspicuous growths provide the wasps with food and protection. However, they raise concerns that something is wrong with the tree. The galls are harmless to the tree, and are only an aesthetic concern. Woody galls that form on twigs may persist on the oak tree for several years after the wasps have left them.
What You Can Do
Since galls on oak leaves cause mostly cosmetic damage, control is not usually required. Insecticide sprays would also be ineffective against the young wasps protected inside their galls. There are also a number of natural enemies that successfully attack gall wasps.
Woody galls on the twigs typically won't kill the branch, however, a large number of galls may reduce the amount of growth. Such galls can simply be removed by hand and destroyed.