Black Swallowtails are common garden butterflies across eastern North America. They also occur from Mexico into South America. I photographed this swallowtail on 8 August 2016 at the Open Hands Farm, near Northfield in Dakota County.
Black Swallowtails prove to be interesting butterflies. Males form leks. They gather at display sites where females mate with them (Wikipedia). Females are less brightly colored than males, and may mimic the distasteful Pipevine Swallowtail. The males’ bright dorsal spots do not mimic other species. These spots serve as territorial markers (University of Florida).
Caterpillars mimic bird guano. The caterpillars are often considered to be pests. They eat plants in the carrot family, including caraway, celery, dill, parsley and sweet fennel. They are not a serious problem for commercial agriculture. In home gardens, hand killing the caterpillars should keep them under control. Insecticides can provide control, but the caterpillars have a lot of insect predators and wasp parasitoids that provide natural control (University of Florida).