Beneficial Insect Files: Black Swallowtail Caterpillar

Beneficial Insect Spotlight:  The Black Swallowtail Caterpillar

The Black Swallowtail caterpillar is also known as the Eastern Black Swallowtail, the American Swallowtail, Parsnip Swallowtail, or simply, Parsley worm.  The Black Swallowtail caterpillar is a common butterfly larvae throughout most of the United States.  This species lives and dines on members of the parsley family (including carrots, dill, fennel and parsnips), which is how it got its nickname, Parsley worm.   If you want to attract the black Swallowtail, be sure to plant many of these for them to enjoy.

Adult Black Swallowtail butterflies lay small, yellowish eggs one at a time on the underside of leaves.  Within a few days, tiny caterpillars begin to emerge from their eggs, first eating the egg shell and then turning their ferocious appetites to the host plant.  The Black Swallowtail caterpillar moves through at least four instars, the technical name for stages of development before entering into the pupa stage and transforming into butterflies.  During each instar, the caterpillar molts its exoskeleton, which begins to change its appearance.  Newly hatched B.S. caterpillars, also called first instars, are small black, fuzzy looking worms with dark spikey hairs and a lighter band near the middle.  As they progress from first to fourth instar, the caterpillar loses the black hairs and forms the unmistakable banded pattern of the mature caterpillar.  In the last instar, no hairs are visible at all.  After reaching the final stage of its development, the caterpillar attaches itself to a branch or other structure by spinning silk and eventually builds a chrysalis in which the transformation into the pupa begins.  Within just a couple of weeks, the pupa undergoes a metamorphosis and emerges as an adult butterfly.  In our region, Black Swallowtail are able to reproduce at least three generations throughout the year.

Swallowtail caterpillars are beneficial insects; however, if you are a parsley farmer, you may have a different opinion!  Caterpillars forage heavily on their host plants, but they also serve as a food source for songbirds and other wildlife.  After their metamorphosis into butterflies, they become pollinators, which is important because one-third of the world’s cultivated crops depend upon the work of pollinators like butterfly and bees.  In addition, butterflies are beneficial to humans for their aesthetic qualities.  Many species are brightly colored and so capture the attention and marvel of many people.  Just watching the whimsical flight of butterflies is enough to lift the spirits!

I came home after work yesterday to discover this big guy munching down on the parsley.  I knew immediately what he was and ran inside to grab the camera.  Judging from his appearance, I’d say he is in his third or fourth instar and almost ready to create a chrysalis.

After trying to get a good angle on him, I accidentally bumped him.  I was shocked to see two large orange antenna poke out of his head, much in the same way as a slug’s eyes pop out.  I thought this was his way of checking me out, until I noticed a very foul odor.  I’ve smelled that smell before when I was a kid playing with certain kinds of beetles.  Smelling my hand, I realized that he had just sprayed me with some sort of chemical.  After repeated failed attempts to wash the smell off, I hopped online to discover an interesting fact about Swallowtails.

Swallowtails possess a defense mechanism whereby a Y-shaped organ called the osmeterium inverts itself as a bright warning to prey.  The osmeterium also produces a chemical secretion containing terpenes, which are foul-smelling and serve as a further deterrant.  I just so happened to set off his defenses!

What’s even more weird is that when I woke up this morning, I saw a black worm crawling on a used washcloth on the floor in the utility room.  I’m not one to kill bugs, so I picked it up for a better view.  Under closer inspection, I realized it was another  (albeit tiny!) Black Swallowtail caterpillar in its first instar!  As I picked it up from the rear, it threw its head back towards my fingers and it’s osmeterium popped out to warn me.  Luckily, the younger caterpillars don’t smell as bad.  I took him outside and put him on the parsley, which I hope he appreciated!  Now, HOW did he get in the house?  Riding on someone’s pant leg?  I found him again (I think!) in the evening when I returned from work, so I snapped a pic.

In case you need convincing of the benefits of the black swallowtail caterpillar, take a look at how magnificent an adult is!