At the time I photographed this little scene (at Bell Smith Springs, Illinois) I was myself unsure of the drama playing out on the oak gall. I sent pictures to wasp expert Hege Vårdal to see if my preliminary guess of a pair of gall parasites was worth anything. Her reply:
I believe that you are on the correct track concerning the specimens. It is probably an inquiline and a parasitoid trying to reach the gall chamber. It looks like a unilocular (one-chambered) gall. Often the inquiline female kills the gall wasp larva when ovipositing in the gall or alternatively the inquiline larva kills the gall wasp larva when it hatches from the egg and then goes on to feed on the plant cells lining the gall chamber. Some Eurytomidae have been found to have both zoophagous and phytophagous periods during its larval development, but often it oviposits in or on a host larva and in this case it would be the gall wasp larva as the inquiline just deposited its eggs. Species of one subfamily of Eurytomidae, the Rileyinae are egg predators, but I don´t know of any cynipid hosts and I suppose there has to be more than one egg available (in this case the inquiline egg) to qualify as an egg predator, so my guess is that the eurytomid is ovipositing on/in the gall wasp larva.
So the larger cynipid wasp- a close relative of the insect that originally induced the gall- is usurping the gall for her own larva. Meanwhile, the eurytomid wasp (probably Sycophila) is a parasitoid whose target isn’t the gall but the larva inside. Either way, it wasn’t a good day for the gall’s primary inhabitant.
The easiest way to tell these two wasps apart is the antennae. Contrast the long, thin ones of the cynipid with the short, elbowed ones of the eurytomid. These wasps belong to rather different branches of the Hymenoptera phylogeny.
Thanks to all who played. I’ll do an easier one next week.