UPDATE - This week, Alberta Agriculture & Forestry reminded their network of cooperators to watch for V. cardui larvae because the species has a broad host range and can feed on soybean (Action threshold=>25% defoliation), sunflowers, borage and dry beans in addition to several species of thistles (including Canada thistle) and mallow.
The Butterflies and Moths of North America website also includes some verified photos of larvae to compare to when scouting. Here's screenshot below of the photo provided by T. Stout to hasten the need to link to this valuable resource!
