Cynips quercusfolii Linnaeus, 1758
Size up to 25mm in diameter.
These marble like galls are found beneath the leaves of Pedunculate Oak (Quercus robur) and Sessile Oak (Quercus petraea). Cynips quercusfolii is a tiny black gall wasp, and within the plant gall its larva stage grows. On Pedunculate Oak the gall is smooth where on Sessile Oak it is rough and warty. They start off as yellowish-green, turning pink then red towards autumn. It turns brown just before the leaves fall.
The gall and the wasp larva inside matures on the ground and the adult wasp emerges between late autumn and early spring. Eggs are laid in the dormant oak buds where small purple galls are formed. These will bring forth the sexual generation which emerge in late spring and lay their eggs in the new season’s leaves to start the process over again. Seen summer until autumn. Common and widespread.