Found on soil surface in large Formica sp. ant nest under a large heavy rock (some thatch around the rock) in prairie/oldfield. I didn't see the initial interactions between the beetle and the ants, but they ended up attacking it after their nest was disturbed. I removed the beetle and placed it on the rock. A couple of the ants continued attacking it for a minute or so (mostly around the pronotum area) and then seemed to lose interest and the beetle was left to crawl around on the rock undisturbed.