Size 20 mm. These odd growths on the acorn are caused by a tiny wasp called the Knopper Oak Gall Wasp (Andricus quercuscalicis). The agamous (asexual) generation develop within the galls on the acorns of various oaks including Pendunculate Oak (Quercus robur) and Sessile Oak (Quercus petraea) as larvae. Green and sticky to begin with, the galls eventually flush red and then turn brown and woody.
The galls fall from the trees in late summer, and the adult gall wasps will emerge the following spring, although some may remain within the galls for up to four years. Eggs are then laid in Turkey Oak (Quercus cerris) buds which result in tiny cone-shaped galls on the male catkins. It is here the sexual generation develop, and when they become adults the cycle repeats itself.
The Knopper Gall arrived in Britain in the 1960s, and it did cause some alarm at first. It can be extremely abundant in some years but there are usually enough acorns left for the trees to survive.
Class Insecta (Insects)
Order Hymenoptera (Ants, Bees, Wasps & Sawflies)
Family Cynipidae (Gall Wasps)
Species Andricus quercuscalicis (Burgsdorff, 1783)
Photos: Local churchyard and woodland, South Staffordshire. © Peter Hillman