We've got galls!

Plant galls can be caused by a variety of different organisms. Insects cause a lot of them though, like this wasp.

Actually, this wasp is probably parasitizing the gall-maker inside the gall. It is injecting its egg inside of the gall-maker's larva with a needle-like ovipositor. Once the egg hatches, the wasp's larva will eat the gall-maker larva from the inside-out. Wasps that do this are called parasitoids or hyperparasitoids (parasitoid of a parasitoid). Galls often contain several species of insect larvae: the insect that caused the gall in the first place, its parasitoids, and their hyperparasitoids. Oaks are hosts to an amazing variety of gall-making organisms.  Here are some samples of galls that we saw in Hartwick Pines State Park in Michigan.