A few summers ago, I used my Sigma 180 macro with a small, roughly 5x7 inch air-filled softbox as my diffuser, to photograph swallowtail butterflies. The softbox I used was made by Photoflex, but there are very similar ones made by LumiQuest, as well as other manufacturers. You can use this small softbox with a flash located on the hotshoe, or with the flash connected to the camera with a TTL remote control cord, and the flash held off to the side and above in either your left hand, or by an assistant, or on a flash bracket setup.
At close distances, the flash is very powerful,and one trick you might learn is that dialing the flash down in power gives ultra-quick recycling,and also shorter flash durations which will stop motion pretty well. Another thing to note: you can use the magnification scale on the Sigma 180mm lens as a rough guide to the exposure in f/stop that is needed at the different magnification ratios on the focusing scale.
THink somewhwrre in the 1/4 power to 1/8 power manual flash levels for butterfly close-uops at very close distances, at f/13 or so. The Siggy is pretty good at f/13,and that will help you to get a good background expousure during the daytime during the summer months. Set the camera shutter speeded to its fastest flash synchronization speed, often 1/200,and that will handle most of the daylight in the background.