I found these mysterious balls attached to an oak tree in the winter. They are galls– abnormal growths of a part of a plant that has been injected with insect eggs. In this case, the galls were destined to be oak leaves before they were injected with wasp eggs. Instead of developing into leaves, they grew into these lumpy spheres, each protecting a single a wasp larva.
Apple galls are so named because they start out resembling green apples. The wasps who make these particular galls are nicknamed “oak apple wasps.” A oak apple wasp starts life as a larva feeding on the roots of an oak tree. During the second spring of its life, it matures into a wasp without wings which climbs the oak tree and injects an egg into the veins of a leaf. The egg contains chemical instructions that turn the leaf into a protective cocoon for a fully mature adult wasp to grow. In the summer, the wasp drills an escape hole and emerges into the big wide world to fulfill its duty, which is to spend the remaining few weeks of its life finding a mate to begin the process all over again.