Great action shot of those beetles!
I did some investigating. Out east (in North America) the prevalent species is Tetraopes tetrophthalmus and they feed mainly on common milkweed, which is Asclepias syriaca.
Here in the west, the species is Tetraopes femoratus (red-femured milkweed borer), and they are found on Asclepias speciosa, AKA showy milkweed. These beetles have white rings in the antennas (not all-black) and a smaller middle black spot on the elytra. They supposedly also have some red on the femurs (upper legs), but I can't see that in my pic.








