Nature News: Black swallowtails prefer carrot family plants

Published June 16th with and in the Portsmouth Herald, the York Weekly, Foster’s Daily and other Seacoast papers

An early instar of the black swallowtail caterpillar with its bird-dropping camouflage Sue Pike photo

While out weeding the garden the other day I excitedly approached my fennel.  I like to grow fennel, not just because I like to eat it, but because black swallowtail butterflies (Papilio polyxenes) do too.  And sure enough, munching their way along two robust stalks were a host of black swallowtail caterpillars.  Black swallowtails prefer to lay eggs on carrot, celery, dill, fennel, parsley, Queen Anne’s lace and other members of the carrot family. In my garden they seem to prefer the fennel over all others. Because of this they are considered pests by commercial growers of these crops.

These are one of our most common butterflies.  Like monarch butterflies black swallowtails are lovable, charismatic insects.  The adults are large black butterflies with two rows of yellow spots along the margins of the wings, and those characteristic drooping “swallow” tails on the hindwings.   The caterpillars are, I think, just as attractive. 

Black swallowtail caterpillars, like most insects, have an arsenal of defenses against predators that they employ at different stages of their life cycles, I will focus on camouflage and mimicry here.  As they molt and go through progressive developmental stages, called instars, they change appearance quite dramatically.  

When they first hatch out, black swallowtail caterpillars are prickly and black with a white splotch around the middle.  At this stage they closely resemble a bird dropping, terrific camouflage for the young butterfly.  This kind of cryptic coloration isn’t unusual, other species of swallowtails use this approach to predator avoidance, as do some spiders, even frogs. 

That white splotch might have dual functions.  It is caused by uric acid deposits that are thought to protect the caterpillars from a toxic chemical (furanocoumarin) found in their diet. Members of the Umbelliferae family, like fennel, dill and carrots, all of which are host plants for black swallowtail caterpillars, produce furanocoumarins-thought to help deter insect predators and various fungi.  Black swallowtail caterpillars are specialist feeders that have co-evolved with their carrot-family food and have adapted to the high levels of furanocoumarins found in these plants (most other types of butterfly larvae cannot feed upon the Umbelliferae).

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.  

Here is a caterpillar molting-shedding its bird-dropping coloration for the bold yellow, black and white striping of the mature caterpillar. This caterpillar will go through a few more molts as it grows and readies itself for metamorphosis into the adult butterfly.

In New England, vast meadows and fields are few and far between and are getting scarcer as farmland is replaced by suburban sprawl.  We can all help by planting native plants in our gardens, avoiding pesticides (which are taking a heavy toll on many pollinators) and turning our lawns, which are typically as biodiverse as a corn field, into meadows populated with native plants, havens for butterflies, bees and other pollinators.