I write these nature notes as a part of my learning process about the natural world and I am constantly amazed at the intricate relationships that exist between species, in the case of most galls between insects and trees or flowers. Knowing nothing about oak apples (Biorhiza pallida) I turned to my reference book, "Britain's Plant Galls" by Michael Chinnery to find out how they come about. Here is a potted version of the incredible story.
The female gall wasp lays eggs in the leaf bud of an oak tree in early spring and the oak apple develops. Inside are several chambers each housing an individual larva. The larvae pupate and then, as winged adults, they chew their way out of the 'apple' in summer. The insects from a gall will be all of the same sex (I suppose that stops inter-breeding?). After mating the females lay their eggs in the tiny rootlets that are found along the main roots of the tree. These eggs hatch and stay underground in small, individual galls attached to the roots and remain there for about eighteen months. All of larvae are wingless, fertilised females. They crawl up the tree trunk, on to branches and lay eggs in a leaf bud to start the whole process going round again. Both sexual and asexual populations can occur on the same tree at any one time.
I think that is amazing; just how does a complex life cycle like that evolve?