Hearing the name wasp may send people running for cover. However, most wasps do not sting people. It is just a few bad guys giving the rest of the wasp world a bad reputation. The common maple bladder gall is a bladder or pouch-like growth on the leaves of maples caused by a eriophyid mite. The galls form on the tops of leaves and change in color from yellow-green to a beautiful red and eventually to black as the season progresses.
Oaks can have numerous types of galls.
Oak deformities are of various sizes, shapes, and colors on leaves, twigs, flowers, acorns and buds. Galls are so commonly found on oaks that many people think the galls are typical parts of the plants. Some early botanical drawings actually show galls as part of the normal plant. The good news about galls is the majority do little or no damage to trees. The trees are just well accessorized.