Class Insecta: Insects » Order Hymenoptera: Ants, Bees & Wasps » Family Cynipidae: Gall Wasps » Neuroterus numismalis (Geoffroy, 1785)
Width up to 5 mm.
These are unmistakable button-shaped galls which are found on the underside of various oak leaves, including Quercus robur, Quercus petraea and Quercus cerris. They are caused by the asexual generation of the Silk Button Gall Wasp (Neuroterus numismalis). These can cover the whole surface of a single leaf. Each gall contains a single wasp larva.
When the tree sheds it leaves during the autumn the galls mature on the ground and the adult wasp emerges in the spring. Eggs are then laid in the oak buds and the resulting larvae induce the formation of the sexual generation on young leaves. The resulting galls are harder to spot as they are the same colour as the leaves and are no more than a circular blister. The adult wasp emerges from May to July.
The galls can be seen July to October wherever the host oak (Quercus) is present.