Quercus robur, are caused by a minute gall wasp, Andricus kollari. Formed in buds, they can grow up to about 25mm in diameter. The gall starts life green in colour, but changes to brown by the autumn, when the adult wasps emerge. The empty gall, now showing a neat little hole in the side where the wasp emerged, remains on the twig. Like most gall wasps, the life cycle is complicated, with an alternation of a sexual and asexual generation each year.