Those brown, golf ball-sized growths on your oak tree are called oak apple galls. Oak apple galls are green at first and turn brown as they age. They are up to two inches wide, but are usually the size of a golf ball and have a thin paper-like covering and spongy interior.

The oak apple galls are created by Cynipid wasps which have an unusual life cycle. Winged adult wasps emerge from these galls sometime in June or July. They mate and then drop to the ground. There, the females burrow into the soil and inject eggs into the roots of the oak tree. Wasp larvae hatch and feed on the roots for about a year and then form a pupa. Only wingless female wasps emerge from the underground pupae and crawl out of the ground and up the tree trunk sometime in early spring. They inject one egg into the midrib of a leaf that is just starting to grow. The larvae hatch inside the leaf and release a chemical as they grow that causes the leaf to mutate into a round green ball around the larvae. There the larva feeds while protected from predators. As the larva grows, so does the gall. When the larva is done growing, it pupates and later emerges as an adult wasp by drilling a small hole in the gall, and the cycle starts again.

These wasps only lay their eggs in oak trees and are particularly fond of black oaks, red oaks and scarlet oaks. Their activities do not harm the tree but many of these wasps on one tree could cause the tree to lose their leaves early that fall.