Common spangle galls (Neuroterus quercusbaccarum) on the underside of an oak leaf (Quercus petraea) on Dundreggan. These are caused by the agamic generation of the wasps.
One species of wasp (Neuroterus quercusbaccurum) develops in tiny disc-like spangle galls, which are abundant on the undersides of oak leaves in the autumn. The galls drop to the forest floor, where the grubs develop over winter under the cover of fallen oak leaves. In the spring an all-female generation emerges. These are ‘agamic’, meaning that they are able to reproduce without mating. They lay their eggs in oak buds, producing currant galls on the catkins and leaves. The sexual generation of male and female wasps emerge from the currant galls in June, mate, and then lay their eggs on the undersides of the leaves. Spangle galls develop, and so the cycle continues.
We had loads of these last year. In october the floor was littered with these galls which fell off. The year before we had crown galls. It doesn't hurt the tree. This year there is a bumper crop of acorns.