Early autumn is a great time to go looking for galls. Most have had time to develop but those on leaves are still on the trees for easy spotting.
Two of the larger galls on oak are Knopper galls and Marble galls. Paul Hatcher described Knopper galls extremely well in a blog on this site in 2011. Trees affected often seem to have almost every acorn converted to a gall.
The galls are caused by Andricus kollari, a species of wasp that needs both Common oak and the Turkey oak (Quercus cerris) to complete it’s life cycle. Luckily for the wasp, the Harris Garden has both species of oak.
In spring fertilised females lay eggs in buds on the Common oak which develop into Marble galls. In late summer the single larva matures and bores it’s way out of the gall. This wasp is also female but is asexual – capable of laying eggs without fertilisation. The asexual female lays it’s eggs in the buds of the Turkey oak where they develop into smaller galls tucked inside the bud scales. The males and females from this generation mate before eggs are laid in the buds of Common oak again.
The Wildlife Trusts are conducting a survey of this introduced gall and have organised groups of volunteers surveying its occurrence.