One spot on the white powdery dorsal wing means this Cabbage White, pieris rapae, butterfly is a male. The purple aster is host to him on this November day in my garden.
An interesting, easily observed behavior is the spiral flight. When a male shows interest in a female that has already mated, she indicates her lack of interest by rising, the two butterflies circling each other, until the male loses interest and drops back to the ground. The female then comes down more slowly and resumes egg-laying (Stokes 1983).
Two dots on the dorsal wing mean “female” Cabbage White butterfly. Such markings make this European immigrant easy to identify. The Cabbage White is one butterfly that, like the Sparrow bird, often goes unappreciated because of its abundance in the garden. Other similarly marked butterflies, such as the Alfalfa, a native species that I saw frequently as a child, have since all but disappeared in northern Illinois.