The object is about two inches across and very delicate with a thin, papery shell. This is an oak apple gall. A gall is an unusual growth of plant tissues caused by a variety of agents. Insects, mites, fungi, and bacteria can all produce galls in plants. This one is caused by a member of the wasp family, and it has a fascinating life cycle. The wasp is a tiny one, only about a quarter inch in size called Amphibolips confluenta.
Above is the only picture I could find of the beast. Sorry, it is a bit, ummm, dead. This comes from the University of Minnesota. Since the wasp is so tiny, and it has such a unique life cycle, there are not a lot of photos out there. I am bummed. :(
Anyway, on with the story. It starts with a young wasp hatching out of the oak gall. Here in Indiana, oak apple galls usually occur on red, black and scarlet oaks. The male and female wasps will mate, then the female will drop to the ground. She will burrow under the ground and inject eggs in the roots of the oak tree. The larvae will hatch and spend about a year munching away on the roots. Next they will rest in the form of pupae. Then, finally, the wingless females will hatch underground.
The wingless females will crawl up the oak tree trunk in early spring, find a newly developing leaf bud and lay an egg. The single larva will hatch inside of the leaf. It will produce a chemical that will start the formation of the gall. The gall will grow with the larva. The larva will eat and grow until it is ready to pupate, forming a bright green gall. Check here for a pic of the larve that lives inside.
This is a shot of the inside of the gall. Here the larva and pupa spend their time protected from the elements. There are dangers though. Woodpeckers, chickadees and squirrels know there is a tasty morsel inside these galls and will search for these yummy snacks.
Here is the inside of the gall without the outer shell. It feels soft and stringy.
After carefully scraping away all the gobbledeegook (like my scientific lingo?), the chamber is exposed. As you can see, it is no bigger than the end of a pencil.
The outer shell also has a small pinhole, indicating the wasp successfully emerged. Hopefully, it found a mate and the cycle will continue. Oak apple gall wasps usually will not cause too much harm to the trees, so it is not a dangerous pest to worry about.