Oak apple gall wasp.

We discovered these colourful oak apples during a 10k yomp around the Shropshire lanes.
The oak apple gall wasp, Biorhiza pallida, is a tiny wasp that causes growths, or 'galls', on oak twigs. These galls can be found between May and June where the female has laid her eggs in the leaf bud. Inside the gall, there are a number of chambers, each housing a larva which eats its way out. Adults emerge in June and July.