i was wondering if butterflies can know when a flower's nectar is pesticidal and avoid imbibing death. but i suppose most milkweed is either grown by people who love butterflies or in areas that don't get bombed, verges along the railway or the edges of lawns that don't get mowed. i can't see a butterfly without feeling a flutter in my solar plexus. 
oops, i forgot to post this last night.