The body is ringed with black, yellow and white stripes. The chrysalis is smooth and light green or blue with gold markings. It can be distinguished by the dark band arching across the center of the hind wing. This species mimics the monarchs appearance to avoid heavy predation by birds who mistake this tasty mimic for the distasteful monarch. Caterpillars feed on willow leaves and mimic bird droppings in appearance.
Butterflies lack dark-colored wing veins and the white spots on the wings are more diffuse.
In the spring, surviving adult butterflies begin migrating north, breeding and developing through 2 to 4 or more generations as they spread north. About 100 million members of the last generation produced in the fall migrate to Mexico and live for about 9 months. Caterpillars have chewing mouthparts. Adults have siphoning mouths. Caterpillars feed exclusively on milkweed.