Cloudless Sulphur

Phoebis sennae — the cloudless sulphur — is the largest 'sulphur' butterfly in eastern North America and one of the most spectacular migratory Pierid butterflies on the continent. The species is widespread across the eastern and southern US (especially the Southeast and Texas), Mexico, Central America, and the Caribbean, with seasonal range expansion north to southern Canada in summer. Adults are 5-7 cm wingspan with the species' identifying coloration: MALES are bright lemon-yellow with no other markings (the 'cloudless' name comes from the absence of dark markings — most other sulphur species have dark wing borders or interior spots), while FEMALES are pale yellow-to-yellow-white with a few dark wing-edge spots and a small dark dot on each forewing. The species' major biological feature is AUTUMN MIGRATION. Every autumn, millions of cloudless sulphurs migrate south from the southeastern US (especially Georgia, the Carolinas, and Florida panhandle) to overwintering sites in southern Florida, Cuba, the Bahamas, and other Caribbean islands. The migration is one of the most visible mass insect migrations in eastern North America — flying populations include hundreds-to-thousands of individuals moving in directional southward flight at altitudes of 1-100 meters. Some populations cross the Gulf of Mexico (a 1000+ km open-water flight requiring sustained flapping over multi-day periods) to reach Caribbean overwintering sites — one of the most spectacular open-water butterfly migrations documented. The species is one of only four North American butterflies with sustained multi-state autumn migration: monarchs (Mexico), painted ladies (Mexico), red admirals (Mexico/Caribbean), and cloudless sulphurs (Florida/Caribbean). Spring northward migration is less synchronized — northbound populations fan out from overwintering sites and gradually colonize the eastern US over multiple breeding generations through summer. Larvae feed on plants in the genus Cassia (sennas) — the host plant constraint ties the migration biology to the geographic distribution of senna trees and shrubs across the eastern US and Caribbean. Adults feed on nectar from a wide range of flowers (especially red tubular flowers like trumpet vine, hibiscus, and salvia) using a long extended proboscis. The species is harmless to humans and a major beneficial pollinator.