Knopper galls.

You may have seen some strange looking acorns on the oak trees. These are knopper galls produced by the knopper wasp. They lay their eggs in the buds of English oak and the gall grows between the acorn and the cup. Depending on how many grubs the gall is holding depends on the distortion of the acorn. The acorns fall as usual from the tree in Autumn and if the acorn has not been predated, an adult female wasp will emerge from the gall in spring.

This cycle produces only females. These little wasps live complicated life cycles as they need both the English oak and the Turkey oak to survive. Once the female has emerged, she will fly off to find a Turkey oak and lay her eggs in the male catkins. This will create a second generation of wasps containing males and females and the eggs from this generation will then lay their eggs In the bud of the English oak. All without creating much of an effect on either oak tree....... Amazing stuff nature!