We have a small oak sapling behind the house that has achieved a height of about 32 inches (81 cm) and has maybe two dozen leaves. It also has a dozen or so oak galls, which are tumor-like growths, emanating from its branches in three places. These are caused by oak gall wasps, which—for the benefit of you taxonomy fans, belong to the Apocrita suborder of wasps in the superfamily Cynipoidea—and develop immediately after the females lay their eggs. That by way of introduction.
I have noticed, during the past week or so, that there is nearly-constant activity on and around these galls: Practically every time I walk by the little tree, I can see one or several very colorful wasps intent on patrolling the surfaces of the galls.
I decided to investigate further today and went out slowly and sat down quietly next to it this afternoon. During the ten minutes or so, before the next rainfall drove me back indoors, I was able to get a satisfactory series of images, sufficient for me to identify them.
My first feeling was that they were paper wasps, and I was happy to find that a perusal of BugGuide confirmed my tentative identification. They are most likely Polistes dorsalis, commonly known as brown and black paper wasps, and they appear to be nibbling on the outer surface of the galls.