Oak Apple Galls (2 images)
I came across this oak sapling today. It was completely covered in galls.
The Oak Apple Gall is caused by a tiny gall wasp, Biorhiza pallida. It can be found between May and June on oak twigs 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.