I’ve never seen anything quite like these insects. They are big - more than 1.5 inches long - and look like wasp-beetle-flies. When they put their wings up, you can see that their bodies are beetle-like. They fly like beetles, but their faces are more like fly faces.
These musk thistles are really lovely, but they’re also considered noxious weeds in CO and several neighboring states. So when I went back a few days later to take more photos, they’d already been poisoned with a blue wilting poison.