by Jeanne McRight
Do you have Golden Alexanders growing in your pollinator garden, or are carrot family plants (such as carrots, celery, parsnip, chervil, coriander, dill, fennel, parsley) included in your veggie patch? Then you are lucky! These plants are hosts for Papilio polyxenes – black swallowtail – caterpillars, and the butterflies will be busy with egg-laying in late spring. Watch for these gorgeous butterflies but also keep a sharp eye out for their beautiful and interesting caterpillars.
Black swallowtails have ingenious defenses against predators. Here's what to watch for:
Toxicity and terror
Members of the Apiaceae (parsley family) produce furanocoumarins, organic compounds thought to help deter the plants' herbivores and known to protect the plant against various harmful fungi (Larkin, 2010). Black swallowtail caterpillars are specialist feeders that have co-evolved with their parsley family hosts. They are among the few butterfly larvae that have adapted to the high levels of furanocoumarins found in these plants. The compound protects them at the young instar stage from insect predators (Hall, 2021).
As the caterpillars grow and develop through their instar stages, larger caterpillars are able to scare off predators with truly frightening red waving appendages that emerge from their heads when disturbed. Just imagine biting into a nice juicy hot dog and suddenly it sprouted angry red horns! Surely you'd drop it and run!
Camouflage and mimicry
As they go through developmental stages (instars), they change appearance. For example, when they first hatch, black swallowtail caterpillars are prickly and black with a white splotch around the middle, closely resembling a bird dropping.
Later instars will become less prickly and drop the bird-dropping disguise in favor of a different type of cryptic coloration - alternating stripes of green, white and yellow that help break up their profile in the dappled light of the delicate foliage of their host plants.
Adult black swallowtails use a different strategy to deter predation. They no longer retain the toxicity they had in their early developmental stages, but their colours mimic those of the poisonous pipevine swallowtail butterfly (Battus philenor). Pipevine swallowtails feed on pipevine (aka Dutchman’s pipe), a plant that contains the lethal toxin aristolochic acid. This type of mimicry is known as Batesian mimicry. According to Mark Salvato, “Batesian mimicry involves a palatable, unprotected species (the mimic) that closely resembles an unpalatable or protected species (the model)." (Salvato, 1997). Another example of this is the viceroy butterfly, a mimic of the poisonous monarch butterfly.
Subterfuge
Finally, my favorite clever swallowtail ploy, described in Hall's Featured Creatures blog: some caterpillar predators are able to detect their prey by the smell of the prey's feces. Black swallowtail caterpillars pick up their feces and throw them a distance away!
We live in a world of wonders, but they are not infinite. We can help by reducing our lawns, growing gardens full of native plants, and joining the numbers of caring humans worldwide who actively work to support the preservation of biodiversity on this planet.
Resources
Salvato, Mark (April 15, 1997), Book of Insect Records, Chapter 28: Most Spectacular Batesian Mimicry. Department of Entomology & Nematology, University of Florida
Hall, Donald W. (pub. 2011, rev. 2021), Featured Creatures blog: Eastern Black Swallowtail. Entomology and Nematology Department, University of Florida https://entnemdept.ufl.edu/creatures/bfly/bfly2/eastern_black_swallowtail.htm
Larkin, Deirdre (2010) Fed on Fennel. The Medieval Garden Enclosed blog. The Cloisters, The Metropolitan Museum of Art, NY, NY. https://blog.metmuseum.org/cloistersgardens/2010/09/15/fed-on-fennel/
Salvato, Mark (April 15, 1997), Book of Insect Records, Chapter 28: Most Spectacular Batesian Mimicry. Department of Entomology & Nematology, University of Florida