This is a What’s in Your Watershed? archive edition, meaning it was originally published back in 2014 and it to this day one of our most popular articles. Photos differ from the original article.
The iridescent blue-black pipevine swallowtail butterfly will only lay her eggs on the California Dutchman’s-pipe vine, and I was delighted when one of them chose the plant in my backyard this March. The caterpillars that hatched a few weeks later were tiny and seemed fragile, but their vibrant black and orange colors accented with spiny protrusions give a clear warning message. As the caterpillars eat the pipevine, they encounter and ingest a toxic chemical that makes them distasteful or poisonous to predators such as birds and even to parasitic wasps that attack them.
Through the intricate slow dance of co-evolution, the pipevine swallowtail has co-opted the alkaloid chemicals that the plant produces to defend itself. Few other pests will chew on the pipevine plant because of its toxic leaves, but the pipevine caterpillars sequester the aristolochic acids in their bodies. The relationship between the plant and the caterpillar is a finely tuned balancing act. In response to the caterpillar’s feeding attack, the concentration of acids in the leaf increases after about 48 hours, making them harder to digest. The caterpillars will then move to a new leaf before entirely consuming the first one.
The baby caterpillars are munching machines, and in just a few days molt by shedding their too-tight skin—actually their external skeleton. This process is repeated four more times over about five weeks, and soon they will crawl away from the host plant to form pupae in secluded places. The adults will likely emerge in several weeks, although it is possible that the pupae might stay dormant until the following February or March. Each butterfly lives for only about a month, feeding on nectar from many types of flowers while seeking mates. The adults are also poisonous, and the female passes some of the chemicals on to the eggs, protecting them from predators as well.
The pipevine swallowtail butterfly also occurs across the eastern and southern portions of the U.S. and into Central America, where the caterpillars feed on closely related plants. In other part of the world, several species of butterflies mimic the warning coloration of the pipevine swallowtail, benefiting from their achievement without the cost of processing toxic chemicals. The California population may be a separate subspecies, and its pipevine plant occurs only in our state.
The California pipevine gets its name from its odd, pale green and purple veined pipe-shaped flowers. It is often planted by butterfly gardeners, but patience and space are needed. A San Francisco State University study of pipevine butterflies in both gardens and natural areas showed that the butterfly usually lays eggs on plants in the sun that are at least five years old, and that reproduction is most successful in patches of pipevine over 185 meters square in size. My yard has vines in both sun and shade, but only the sun covered vines have ever had eggs, although the shaded plant is older and larger. I will likely never find the cryptically colored pupae, and may never know for sure if they successfully emerge as adults. But if I see a pipevine swallowtail butterfly in my yard next month, I will feel I have helped perpetuate this brilliant example of coevolution.