The leaves have only just started to open on this oak tree, a Sessile oak I think, yet it is already covered by many galls. These rounded disfigurations – called Oak apples – are caused by a tiny (5-6 mm) wasp in the family Cynipidae, called Biorhiza pallida. 

It is known that the galls are caused by the injection of venom by the wingless, parthenogenetic females, which cause the newly emerged leaves to soften and swell up. These females have emerged from galls growing underground, on the roots, and they have crawled up the tree to start a new generation in the Spring. (1) The eggs hatch and the larvae secrete chemical substances which also cause the tissues to grow and form into a ball; the apple gall.

Remarkably, all of the individual wasps developing within a given gall, of which there may be as many as thirty, are of the same sex. (2) Although the gall is made of plant material, because it is induced by the wasp it is said to represent the extended phenotype of gall-wasp genes (Stone and Cook, 1998). (3)

The tree was located near the Felmersham Gravel Pits, a Site of Special Scientific Interest between the villages of Felmersham and Sharnbrook, in Bedfordshire.

The life cycle of these amazing wasps is even more complex than I have outlined here, with individual asexual females able to produce both males and females from unfertilised eggs; alternating sexual and asexual generations and way of life that utilities both the below-ground roots and above-ground shoots of the tree.

1. https://en.wikipedia.org/wiki/Biorhiza_pallida
2. http://www.naturespot.org.uk/species/oak-apple-gall-wasp
3. http://stonegroup.icapb.ed.ac.uk/publications/gallwasps.html