Oak apple galls form when a female Amphibolips confluenta wasp injects her egg into a budding oak leaf. The tree responds by forming a gall around the insect. The gall protects the growing insect and provides it with sustenance.
I’ve seen hundreds of oak apple galls, but they’ve been old galls that were brown, dry, and papery, with a small hole where the adult wasp bored its way out. On Sunday we hiked to White Oak Sinks in the Great Smoky Mountains National Park. The fresh galls we found on the trail were green and pliable, and the wasp pupae were still growing inside.
Above: the inside is mostly air, with a seed-like structure in the center suspended by wispy fibers.
Below: With the seed-like middle structure opened, the white wasp pupae can be seen.
Here’s a very good natural history of Amphibolips confluenta, with pictures of the dried galls and the adult wasp.
October 25, 2006 update – Two of these pictures will be included in Oak Gallwasps of the Western Palaearctic, to be published by the Ray Society (Natural History Museum, London).