Diameter of gall up to 25 mm. This unmistakable gall is caused by the asexual generation of the Oak Marble Gall Wasp (Andricus kollari). They appear solitary or in clusters on oaks like Quercus robur and Quercus petraea, and start off green and gradually turn brown and woody. Each gall, which is a modified bud, contains a single gall wasp larva. The galls mature and the adult wasps leave them in the autumn. The females then seek out Turkey Oak (Quercus cerris) buds to lay their eggs. The subsequent larvae develop in a tiny egg-shaped gall which matures in March, and the sexual adults mate to begin the process all over again.
This is a non-native species which was introduced in the 19th century when the galls were used as a source of tannin for dyeing and ink manufacturing. Common and widespread througout Britain.
Class Insecta (Insects)
Order Hymenoptera (Ants, Bees, Wasps & Sawflies)
Family Cynipidae (Gall Wasps)
Species Andricus kollari (Hartig, 1843)
Photos Local field, South Staffordshire July 2020. © Peter Hillman