I usually see these on Q. bicolor, also, but last summer, I found some on a Q. muhlenbergii sapling planted in my yard, too.

It seems to me that cynipids aren't always as rigidly host-specific as is often suggested. Felt and others often list just one host species, but that may simply be the species on which the first gall described was found. Even the oaks themselves don't always seem too concerned about what species they are--I'm told that red and black oak produce fertile hybrids, for instance. I suspect bur and swamp white oak are pretty closely related to each other as well. I see similar galls regularly on Quercus alba in Massachusetts. They do seem to be specific to the white oak group. Of course, it's also possible that we're talking about three different species that form similar galls each on their own oak species.

I think these are some of the oak "bullet" galls, but I can't remember offhand if that refers to a particular genus or is a more general category. John, with these fairly plain-looking round oak galls, it would be helpful if you could cut one open for a view of the inside. I'll try to sort through them sooner or later, if somebody else doesn't beat me to it. They all belong to cynipid wasps, though, I can say that much. There's only one spherical oak gall that's caused by a midge, and I haven't seen it on BugGuide yet.

I sliced a gall in half with a razor blade and added its photo as a third shot posted above.

See guide page for details... this is a gall of swamp white, bur, and overcup oaks. "The" oak bullet gall (Disholcaspis globulus), the one I was thinking of, is found on "various white oaks" but lacks the nipple that some of your galls clearly have. All the other ~spherical twig galls seem to be more host-specific and are not found on bur oak.

Thanks for your work in tracking this down. Those images on the forestry webpage you provided are a great reference. I am in awe of your grasp of this material... very much appreciated!

Thank you for providing all the necessary details/views for identification. Felt's gall book has 148 pages devoted to oak galls, and I'm just beginning to get the hang of navigating through them. Knowing the species of oak, the size of the galls, and what they look like on the inside takes away a lot of guesswork.