Oak Apple Gall

Supposed to be quite common, but this is the first we've noticed in the wood.  We've previously seen spangle, artichoke, knopper, ramshorn, marble galls on our oaks.

The Biorhiza pallida wasp has an interesting life cycle.  Wikipedia summarises it:


The winter-hatching generation of  wingless agamic females (asexual reproduction) climb up the trunks of oak trees such as Quercus robur and Quercus petraea in the spring.  They lay batches of eggs in the young buds of the oak, injecting venom at the same time. This causes the leaf tissues to swell and soften. The eggs hatch and the larvae secrete further substances that encourage plant growth and a globular gall is formed. Reaching as much as 5 cm (2 in) in diameter, it is soft and pudgy at first, but later becomes dry and papery. The gall provides a nutritious, protective environment and there may be as many as thirty larvae developing inside. Males and females emerge from different galls after two to three months. After mating, the females descend to the ground where they make their way into the soil and lay their eggs singly inside the small rootlets of the oak. Small globular galls appear on the roots and after overwintering as larvae, another generation of wingless agamic females emerge and crawl up the trunk.