Oak apple on an oak (Quercus sp.) tree. This swelling is a gall filled with the larvae of the gall wasp (Biorhiza pallida). Eggs are laid beneath the bark of the oak tree, and secretions from the developing larvae cause the tree's tissues to grow faster than usual. The larvae eat this rapidly-growing tissue as they grow. The eggs are laid in spring, usually in late April, and up to 30 mature gall wasps escape from each gall in summer.