Knopper gall on an acorn. Close-up of a knopper gall on an acorn of a Pedunculate Oak (Quercus robur) tree. The acorn has been infected by the parasitic larva of a gall wasp (Andricus quercuscalicis), causing the formation of a knopper gall (odd-shaped growth). The larva develops within a hard inner gall, which provides it with both food and protection. Parthenogenetic (all female) adults emerge from the galls during the following spring. These females lay eggs on the turkey oak, Quercus cerris, causing galls to occur on their catkins, from which a sexual generation of wasps emerge.