There is a distinctive oak gall that has shown up on BugGuide several times, and I have been unable to identify it with my gall references. The only way to get to the bottom of a mystery like this is to collect a bunch of the galls and see what comes out. But I have never encountered these galls in person and am unlikely to, since they have always been found on water oak (Quercus nigra), a tree whose range does not include New England. However, I was recently given a leaf found in North Carolina bearing a few of these galls.
The galls are 3-4 mm tall and are on the upper leaf surface. The points of attachment are visible as pale spots on the underside of the leaf:
These galls can probably be found anywhere that water oak grows; so far photos have been submitted to BugGuide from North Carolina, Alabama, Georgia, and Texas. The galls are still on the leaves through the end of October, and I’m not clear whether they drop off when mature–if not, it’s possible they could be found in leaf litter throughout the winter. (Well, even if they do drop off, they could still be found in leaf litter, but it would take some patient digging.)
Because the galls I have were collected in early September, I’m worried that they may not be fully developed. Also, having only five of them and all from the same leaf, the odds are pretty good that every one is parasitized. So, if you have water oaks in your area, I would love it if you’d keep an eye out for these this fall, and collect any that you see!
Edit: Dave Almquist just alerted me to this page, which identifies these galls as “maybe Xystoteras volutellae“; the name in the title has been corrected to Phylloteras volutellae. I’m not sure what that suggested identification is based on, since P. volutellae makes very different galls, and on bur oak (Quercus macrocarpa)–see the BugGuide page for that species.
Edit, August 2021: It turns out these galls are made by an undescribed midge in the genus Polystepha. I sent Ray Gagné some galls with dead larvae earlier this year and he confirmed this. No one has yet been able to rear adults. The midge is not specific to water oak; here are some galls of the same type found on Darlington oak (Quercus hemisphaerica) in Florida.