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A group of three green galls growing on a Gary oak tree. Ranging in size from 2 to 4 in. (5 to 10 cm). There were several of these galls all over this one tree, maybe 30 in all. It looked more like a fruit tree than an oak tree. The one I opened up (pic 3) had a dense, white and juicy flesh.
Spotted on a Gary oak tree near the edge of a conifer forest at the Mima Mounds Natural Area Preserve. All the oaks I saw were under-sized (20 ft tall at the most).
These wasps form an important role in the ecosystem, with more than 20 known species that are parasitoids, inquilines, and hyperparasites that live on its life cycle, while the galls form a persistent shelter for various forms of fungi as well as many other insects. Several birds are also known to feed from the galls and their inhabitants.(Wikipedia)