Last year I grew butterfly weed (Asclepias tuberosa), a yellow flower variety. I just found a wild one growing in the side meadow - a pink variety though - with yet again a couple of dozen monarch caterpillars on it as last year. I was having difficulty taking a decent picture though because of how the caterpillars place themselves under the leaves, eating the leaves from below. They are always in a shadow. Perhaps they do this to hide themselves from predators lurking above.