California Native Plant Society
in collaboration with East Bay Municipal Utility District
Anise Swallowtail Details
All photos and verbage from May Chen's "Pollinator Post" on BringBackTheNatives website
The Anise Swallowtail, Papilio zelicaon (family Papilionidae), is a common swallowtail butterfly of western North America. It is found in fairly open country, most likely seen on bare hills or mountains, in fields oar at the roadside. Adult females lay eggs singly on the underside of host plant leaves. In the first two instars, the caterpillar is a bird poop mimic – dark brown, almost black, with an irregular white band at its middle. After that it becomes more green at each successive molt until, in the fifth (last) instar, it is predominantly green, with markings in black, orange, and light blue. Its major food plants are members of the carrot family, Apiaceae (including Angelica), and also some members of the citrus family, Rutaceae.
Anise Swallowtail caterpillars feed on plants in the Apiaceae family, including fennel, parsley, carrots, and dill, as well as poison hemlock. They also use native plants like Angelica, Cow Parsnip, and Yampah. Sweet orange (Citrus sinensis) is the only known non-Apiaceae host plant, likely because it contains similar chemical compounds.
When I arrive at Bay Farm this morning, my first order of business is to check the Fennel growing out of the Holm Oak bush. The last time I was here, I saw an Anise Swallowtail lay an egg on the Fennel foliage. Surprise, there’s a cluster of three fresh eggs attached on the same branch today! I have never known these butterflies to lay more than one egg at a time.
There are many more eggs on the same small branch of Fennel, in various stages of development. There are many free-standing Fennel plants along the trail; why is this particular Fennel favored by the butterflies? Higher nutritional value for the larvae? The plant has grown up in the shade of the Holm Oak, and it is sheltered from the winds that can be brisk along the shoreline here. Maybe these are important considerations for the mama butterflies?
Native to the Mediterranean, Fennel, Foeniculum vulgare belongs to the carrot family, Apiaceae which includes Anise Swallowtail’s original native host plants, such as Lomatium and Cow Parsnip. Since its introduction to North America, the plant has created a fascinating ecological shift. The butterfly has successfully adapted to using Fennel as a primary larval host plant. Because the invasive Fennel grows aggressively across disturbed areas, it provides a consistent, abundant food source that allows the butterflies to thrive in urban and suburban landscapes. While the plant supports multiple generations of butterflies from early spring through fall, land stewards and native plant conservationists still view Fennel as a threat to local biodiversity. In areas like the San Francisco Bay Area, native plants like Lomatium are crowded out by dense thickets of Fennel, which do not provide the proper habitat for other local insects.
5th instar Anise Swallowtail caterpillar on a young Coast Angelica, Angelica hendersonii
Demonstrating the fascinating defense behavior of the Swallowtail, a gentle tap on the head of one of the caterpillars instigates an immediate yellowish-orange horn-like structure. The osmeterium, is extruded from behind its head. It is almost immediately retracted. (The two forks of the osmeterium are of the same length, but look unequal from this perspective because of foreshortening effect.)
The osmeterium is a defensive organ found in Swallowtail butterfly caterpillars (family Papilionidae). It is a fleshy, forked structure that can be everted from the caterpillar right behind its head when it feels threatened. When extended, the osmeterium releases a foul-smelling secretion, often containing terpenes (essential oils) extracted from the caterpillar’s host plants.