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 November. Numbers peak in April, with a secondary peak in July. 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. Caterpillars sequester aristolochic acids from the Pipevine plant, which makes them, as well as the adult butterfly, toxic to 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 chrysalids do not necessarily eclose the next spring; sometimes the butterfly will eclose after three or four years!
The egg-laying behavior of Pipevine Swallowtails is somewhat novel among butterflies (even among Swallowtails). The female conducts a very extended due diligence before choosing the shoot on which it will lay eggs. 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, all within 5 or 10 seconds.
A female Pipevine Swallowtail will first fly several low circuits above the host plant. Next, it will do a couple of touchdown circuits, where it briefly alights on maybe 20 or 25 different leaves. Most will be leaves of the host plants, but a few times it will land on the host plant’s host plant (the bush that the PIpevine is entwined 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 10 or 12 different spots on the host plant. It will do multiple touchdown circuits — typically at least 3 or 4, but sometimes more — where it touches down in turn on each shortlisted spot, sampling its chemical composition repeatedly. Only then does it choose the spot where its precious cargo of eggs will be laid.
