A hot day in mid-July and I am standing transfixed at the edge of a common milkweed patch, watching a bewildering number of colorful butterflies nectaring on the cluster of drooping, dusty-rose flowers. There are great-spangled fritillaries and tiger swallowtails, silver-spotted skippers and clouded sulphurs, gray hairstreaks and common sootywings, an American painted lady and…