California Pipevine Swallowtail (Battus philenor hirsuta)

The California pipevine swallowtail, Battus philenor hirsuta, is a striking black-and-iridescent-blue butterfly found in Northern California. It is the California subspecies of the wider pipevine swallowtail, distinguished in part by its hairier body, and it lives its entire life tied to a single plant: the California pipevine.

The butterfly

Adults are dark butterflies with shimmering blue to blue-green hindwings, most vivid in males, and a row of bright orange spots on the underside of the hindwing. Their toxicity makes them a model that several other California butterflies mimic for protection. Because adults range widely and fly fast, they are difficult to count reliably in the field.

An obligate relationship with pipevine

The pipevine swallowtail is an obligate specialist. Its caterpillars can feed on nothing but Aristolochia, and in California that means Aristolochia californica, the California pipevine. The butterfly cannot reproduce without it. If a local pipevine patch is lost, the butterflies that depend on it are lost with it, which is why protecting established host-plant habitat is so important.

Caterpillars, and how we count them

The caterpillars are dark, almost black, lined with rows of orange or red projections, and are often found feeding together on the vine. Like the adults, they store the pipevine's aristolochic acids and are distasteful to predators. Because adult butterflies cannot be reliably counted, surveyors census the caterpillars on the host plant instead. The presence of caterpillars feeding on pipevine is direct evidence of active, on-site breeding. Recent counts at the UC Berkeley patch recorded between 1,000 and 2,000 caterpillars in peak months.

Range and the Bay Area populations

The pipevine swallowtail occurs wherever its host plant grows in Northern California. In the San Francisco Bay Area, important populations cluster at a handful of sites, including UC Berkeley, the UC Botanical Garden, and the Tilden Botanical Garden. Together these may represent a meaningful share of the subspecies' local range, which makes each established patch valuable.

The UC Berkeley habitat

One of those established breeding sites, along Strawberry Creek on the UC Berkeley campus, is threatened by a major utility project. Read the full story and how to act on our Save the Pipevine page at whrfund.org/save-the-pipevine.

How you can help

Photograph and report adults, caterpillars, and pipevine plants you find, share your observations with us, volunteer for habitat restoration, and support new pipevine plantings across Berkeley. To get involved, contact us at wildreciprocity@gmail.com.

Donate and receive a sticker