Last week while I was documenting a vernal pool for a client, I spotted a swamp white oak leaf that was covered with fuzzy brown galls, about 4 mm in diameter, on the underside. I stuffed the leaf in a sandwich bag, recognizing them as cynipid wasp galls that I had seen described but had never actually found before.
A few days later a tiny wasp appeared. It is a cynipid, so I will assume for the time being that it is Neuroterus floccosus, the species responsible for these galls, which are known as oak flake galls. In addition to swamp white oak (Quercus bicolor), they occur on bur oak (Q. macrocarpa), dwarf chinquapin oak (Q. prinoides), and overcup oak (Q. lyrata). Similar galls on white oak (Q. alba) are caused by N. exiguissimus.
It’s hard to see a tiny exit hole in that dense brown fuzz, so I’m not sure if this wasp was a straggler or the first of many to come… time will tell. This one was scurrying about continuously from the moment I let it out of the bag (despite having been in the fridge for a day), so although it may not be obvious, the above photo is an action shot.
(Added May 2) It was the first of many… a few days later I checked the bag and there were lots of little wasps, most of them running around nonstop like the first one. A few that had evidently emerged more recently held still and let me get some better close-ups. Below is a shot that shows the size of one of the wasps relative to the galls.