At first when I saw these lying on the woodland path I thought that someone had been throwing dozens of hops about, though this seemed unlikely as none grow in the wood. I soon realised that they were scattered on the ground beneath every oak tree we passed.
Hop gall, also known as 'artichoke gall' or 'larch-cone gall' is produced when the female gall wasp Andricus fecundator lays her egg in a leaf bud on an oak in May or June. She lays a single egg in each bud and the gall starts to grow, reaching full size in the autumn. A single larva lives and feeds inside, protected by the overlapping bud scales. The adult insect, which emerges in the following spring (or sometimes not until 2 or 3 years later) is always a female.
This spring generation of females lays eggs on oak catkins which produce 'hairy catkin galls' and it is the generation of wasps that emerges from these inconspicuous galls in May and June that produce the hop galls.