California Pipevine Swallowtail (Battus philenor hirsuta) is a large butterfly, with a wingspan of 2.75 to 5 inches. It can be seen from March to October. Numbers peak in April-May. Host plants are Pipevines; in California, we have only one native Pipevine, California Pipevine (Aristolochia californica).

Eggs are laid in clusters, both on the stem and on leaves, but always close to the tender shoot tips. Caterpillars sequester aristolochic acids from the Pipevine plant, which makes them, as well as the adult butterfly, toxic to vertebrate predators. It overwinters as a chrysalis, and so adults are seen flying in March even before new leaves appear on the bare vines of the host plant. Overwintering chrysalises do not necessarily eclose the next spring; sometimes the butterfly will eclose after three or four years! Adults typically live for a month.

The egg-laying behavior of Pipevine Swallowtails is somewhat unique among butterflies (even among Swallowtails). The female conducts a very extended due diligence before choosing which shoot it will lay eggs on. As you read the description below, contrast the Pipevine Swallowtail with the Anise Swallowtail, which typically flies in to the host plant, chooses a spot, lays an egg, and flies off again, sometimes in less than 10 seconds.

A female Pipevine Swallowtail will typically first fly several low circuits above the host plant. Next, it will do a couple of touchdown circuits, where it briefly alights on maybe 15 or 20 different leaves. Most will be leaves of the host plant, but a few times it will land on the host plant’s host plant (the bush that the Pipevine is twined around). Female butterflies are equipped with a scratch-and-sniff chem lab -- in the form of chemoreceptors on their legs and antennae -- that they use to identify whether scratching a given plant causes it to emit the volatile organic compounds that should be emitted by its host plant.

The female then shortlists 8 to 12 different spots on the host plant. It will do multiple touchdown circuits -- often 3 or 4, but sometimes more -- where it touches down in turn on each spot, sampling its chemical composition repeatedly. Only then does it choose the spot where its precious cargo of eggs will be laid.

Pipevine Swallowtails are found across North America and Central America, but this subspecies is endemic to California.