The explosion of wildflowers throughout March and April ushers in a favorite Sonoma County phenomenon — the emergence of the pipevine swallowtail butterflies (Battus philanor).
While they represent just one of many pollinators now visiting our nectar-rich fields, these black and iridescent blue lovelies stand out. They are large and slow enough that we humans can easily follow their progress as they visit flower after flower in search of nectar. The tiny scales on their dark wings catch the light, reflecting blue metallic hues above and displaying bright orange spots underneath.
And, if we know when and where to look, we can track their entire life cycle during the coming months.
Butterflies in general are the poster-children for insect metamorphosis. Eggs hatch into caterpillars that, after consuming vast amounts of plant material, form a pupa from which the adult emerges weeks later, apparently by magic. While this typical life cycle is easily described, the real magic is hidden in the details. Pipevine swallowtail butterflies, especially, employ some pretty mind-boggling tricks during their short lives.
Take their caterpillar host plant, for example. Here in California, female pipevine swallowtails lay their tiny orange spherical eggs almost exclusively on California pipevine or Dutchman’s pipe (Aristolochia californica), named for its unusual pipe-shaped flowers. California pipevine is a member of the “birthwort” family of plants, many of which contain potent alkaloid toxins known as aristolochic acids.
These chemicals are so toxic they can produce cancer and kidney failure in humans, but they also protect the plant from being eaten. Except for pipevine swallowtail caterpillars, who foil the plant’s defensive system by sequestering the aristolochic acid in their bodies. This makes the caterpillar and the adult butterfly unpalatable to predators.
Monarch caterpillars also sequester chemicals called cardenolides from milkweed, which is why most birds also avoid monarchs and their lookalikes).
California pipevine is a climbing perennial that grows throughout Sonoma County, so it’s not hard to find the swallowtail’s eggs and caterpillars this time of year. The flowers that appear in early February have mostly faded, so look instead for the plant’s heart-shaped leaves, about the size of your hand, and light green stems that twine around other plants in moist woods and along rivers and streams. The leaves and stems are covered in thousands of tiny hairs called trichomes.
Look for small clusters of orange butterfly eggs on the stems or on the underside of the leaves. If you’re lucky, you may be able to witness a female pipevine swallowtail gently touching her abdomen to the plant and releasing one perfect egg at a time, a process called ovipositing.
When the caterpillars first hatch, they are smaller than a grain of rice, but with their voracious appetites they can grow to over two inches long in a just about a month’s time (the largest one I’ve seen was about the size of my pinkie finger).
At first the tiny critters hang out together, but as they grow, they spread out on the pipevine plant, sometimes chewing so loudly it is audible to the human ear. During this time, caterpillars go through several stages (called instars) and change from a mostly orange-red color to blackish-brown with orange protrusions.
They are scary looking but fun to hold in your hand. They don’t sting, and their skin is soft — why have bristly spines if you’re too toxic to eat?