About a month ago I came across this large, globular leaf gall on a northern red oak (Quercus rubra, Fagaceae). It was succulent, slightly translucent, and it had a small hole in the side. I tore it open to find out what was inside:
I found out this grape-sized gall was hollow, and the larva of some insect was loose inside. After some research I discovered this was the larva of a gall wasp (Hymenoptera: Cynipidae).
Tiny adult wasps lay their eggs inside plant tissues, and the young stimulate the formation of galls by excreting chemicals that alter plant growth (Shour, Jesse, and Lewis 2005). The gall protects the larva from predators, parasites, weather, and pesticides and provides a source of food (Shour, Jesse, and Lewis 2005). While these galls may be unsightly to people, they are rarely harmful to the trees.
Shour, M., L. Jesse, and D. Lewis. Insect Galls on Trees and Shrubs. 2005. Iowa State University Extension.