Galls are abnormal plant growths that are caused primarily by insects, but also by fungi, mites, nematodes and bacteria. Each gall insect has a specific plant host which produces a distinctive-looking gall. Of the 2,000 gall-producing insects in the United States, 1,500 of them are gall wasps or gall gnats. Of the over 800 different gall-makers on oaks, over 700 are gall wasps.
The pictured golf ball-size gall is referred to as an oak apple gall, specifically the Larger Empty Oak Apple Gall. (There are over 50 species of gall wasps that are known to produce oak apple galls in North America.) This particular gall is called “empty” due to the fact that the inner fibers disintegrate with age, leaving much of the interior empty. It housed the larva of a tiny gall wasp (Amphibolips quercusinanis) through the summer. An adult gall wasp laid an egg in a growing part of the plant (in this case a leaf bud) in the spring. The oak reacted to either a chemical secretion, the egg or the burrowing of the hatched larva (it is not known which of these agents is responsible) by forming a growth around it. This growth, or gall, provided the wasp larva with both shelter and food as the larva developed in a tiny chamber in the center of the gall. As the larva fed on the nutritious tissue in the walls of the chamber, it (the tissue) was constantly replenished by the oak tree. Eventually the larva pupated and the adult wasp, having emerged from its pupal case within the gall, chewed the tiny round exit hole you see in the gall before flying off to seek a mate.
As with the vast majority of plant galls, oak apple galls cause little harm to the overall health of their oak hosts.