I found this was hovering above the entrance hole of some sort of Field Ant's nest. Every so often, when the ant wasn't looking, the wasp would land on the ant's abdomen and lay an egg.

Every so often the wasp landed to take a rest, usually on the tip of a pine needle or piece of mulch that was sticking up nearby. In this picture it actually landed directly on the ground, in the fine dirt that had recently been removed from the tunnel.