Oak apples are at their freshest – spongy and sometimes rose-pink – around the time of Oak Apple Day, the 29 May. The day commemorates the Restoration of the Monarchy in Britain on 26 May 1660, when Charles II returned from exile.
The summer generation of the gall wasp Biorhiza pallida, has already emerged. As many as thirty of them live in separate larval chambers in the gall but they’re often joined by parasites and by inquilines: live-in lodgers that don’t attack the resident larvae. The variety of species sharing the gall probably explains the different sizes of the exit holes that they’ve made.
The fertilised females will go on to penetrate the ground and lay their eggs in the roots of oak trees. The larvae usually spend two winters developing underground, each in its own root gall, then emerge early in the year. This early spring generation will all be wingless and all female.
The unfertilised females climb into the branches of the oak and lay numerous eggs at the base of a bud. An oak apple forms from each bud and, as in my photograph, often several of them are clustered together. Males and females develop in separate galls.
These were on a small sessile oak growing by the towpath alongside the Calder & Hebble Navigation, downstream from Horbury Bridge.