Luckily, about the time Tony Troche and I found this plant, renowned Chicagoland entomologist and biologist Ron Panzer just happened to be walking by. While Ron also saw the Tall Green Milkweed, he had the careful eye to also note the presence of several Acadian Hairstreaks (Satyrium acadica). I would have known that this was a hairstreak, given the small size, grayish color, and small hairs on the hind wing, but that's where my identification would have stopped.
This small (up to 1.25") butterfly species is found in a narrow band from New England to the east side of the Rocky Mountains, most often near willows (Salix spp.) in wetlands. In fact, caterpillars of this species feed exclusively on willows. Adults feed on nectar from various plant species, including Meadowsweet (Spiraea alba), New Jersey Tea (Ceanothus americanus), thistles (Cirsium spp.), and milkweeds (Asclepias spp.), and fly from June to early July. While their numbers are considered globally secure, this is considered by many to be a rare butterfly.
I am constantly trying to expand my understanding and appreciation of our natural world. Thanks to great naturalists like Ron, who are passionate and eager to teach others, I can continue to expand my horizons.