Overview
The Anise Swallowtail (Papilio zelicaon) is a dazzling, sun-loving butterfly that graces open hillsides, coastal bluffs, and mountain peaks throughout western North America. Famous for its brilliant checkerboard wings of radiant sulfur-yellow and velvet-black accented with bright blue dusting and orange eye-spots, it is renowned among entomologists for its remarkable ‘hilltopping’ behavior.
Biology and Physical Characteristics
Papilio zelicaon is a medium-to-large swallowtail with broad, triangular wings terminating in slender tail streamers. The upperwing surface is predominantly bright sulfur-yellow, crossed by broad black bands and veins, with a prominent black border lined with yellow spots. The hindwing displays a dusting of iridescent blue scales and a conspicuous round orange-red eyespot centered with a round black pupil.
Behavior and Ecology
Males exhibit intense ‘hilltopping’ behavior: during warm afternoons, they fly to the highest physical summits, ridges, and hilltops in the landscape, aggressively patrolling the crest to defend courtship territories and intercept passing females. Its caterpillars feed on native carrot family plants (Apiaceae) and have adapted enthusiastically to introduced sweet fennel (anise), defending themselves with an orange, forked foul-smelling horn organ called an osmeterium.
Conservation and Human Impact
Assessed as secure (Least Concern), the Anise Swallowtail is widespread and abundant across its western range. Because its caterpillars thrive on invasive wild fennel (Foeniculum vulgare), it has flourished in urban and suburban landscapes, making it a familiar, beloved sight in gardens from Seattle to San Diego.
Interesting Factoid
To find a mate, male Anise Swallowtails race to the highest physical summit in the landscape—whether it is a 10,000-foot mountain peak or a suburban hilltop park—patrolling the summit ridge all afternoon in an acrobatic ritual called ‘hilltopping.’