The marble galls undoubtedly distract some nutrients away from the plant. There is an argument that galls are some form of damage limitation on the part of the plants. Better to contain the herbivore rather than let it rampage all over the place doing damage to the much more important growing tips and developing stems. The larger soft pinkish galls also found on oak are the true 'oak apples', caused by a similar tiny gall wasp Biorhiza pallida. They will eventually harden and turn brown after their occupants have fed and left.
Gardeners' World Web User
28 Nov 2011 18:31
I often wondered what these knobs on my oaks trees were? Are they causing any damage? What about the squidgy pink ones I can find at the moment?