This strange looking "fruit" on an oak tree is an oak apple gall. It is home to a group of Biorhiza pallida wasp larvae. This gall wasp has a complex life cycle, similar to the knopper gall wasp I posted a couple of years ago. A wingless unfertilised female gall wasp climbed the tree in early spring and laid either male or female eggs in a leaf bud and injected venom to stimulate the gall formation - each gall contains larvae of only one sex. The male and female wasps will emerge from galls after three months or so and the mated females then lay eggs in the ground at the base of the tree. These produce galls on the roots and the larvae overwinter there before the agamic females emerge to start the cycle over again.
I spotted this one high in a tree and had to use a telephotos lens to reach it.
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