The oak knopper gall

During a morning walk to a local park I noticed a strange and unusual formation on a branch of an oak tree. A couple of developing acorns were grotesquely deformed and clearly not normal.

I remember as a young child climbing oak trees in summer and finding strange things like 'oak apples' on the twigs and wondering what they were. I found out years later that these deformations are caused by wasps. Each different shaped gall is caused by a different wasp.

Fascinating stuff!

The particular one that I found today is caused by the oak knopper gall wasp. It lays an egg in a developing acorn. This causes the acorn to grow into the weird shape before the new wasp emerges later in the summer.

After the wasp emerges, the gall then shrivels up and turns brown before dropping to the ground. You can see these brown wrinkled galls laying amongst the tree litter if you look closely. They look a bit like walnuts.