A garden is the best alternative therapy.

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!


Comments on: "Black Swallowtail Caterpillar" (15)

  1. Amber Stoots said:

    I just had to say thank you. I went to the park with my friend and her kids and found one of the Swallowtail Caterpillars in its third stage as you say. Her daughter loves bugs so we picked it up, I started smelling something that smelled like trash and dirty clothes, the whole time thinking it was me. The little one said ” she smells like gummy worms” ha ha not sure how she smelled that. We were just so curious about the orange things coming from its head and now we know. I am going to call her now and tell her so she can go look for more and we know for sure they are not dangerous. Thanks!

    • roundrockgarden said:

      Like you, I found out the hard way, by getting sprayed! Yeah, I don’t think they smell like gummy worms at all! 🙂 Thank you for your nice comment, Amber.

  2. Linda Miller said:

    Enjoyed hearing about your black swallowtail caterpillars. I have a lot of volunteer dill and recently it has been covered with these caterpillars. I have a wooden box with a screen door into which I put vases of dill and all the caterpillars I collect–these will now overwinter in a safe place–sorry birds!! One year I did not have a secure door on the box and over the winter the mice at the chrysalis. Very enjoyable hobby –no expenses!!

    • roundrockgarden said:

      I like your thinking! What a fantastic hobby. These cats can go through some dill pretty quickly! I guess up north it’s too late for them to go through their metamorphosis before the cold weather sets in?

  3. Kitty Ratledge said:

    I am doing an Integrated Pest Management of the Parsley Worm and am not finding enough material ontheir control….reason I chose this worm was it’s striking beauty. Seems as though many feel the same way and go out of their way to encourage it to multiply.

    Anyone have input on the control of this striking worm and beautiful butterfly?



  4. I have overwintered some Black swallowtail before. This year I have 2 that have already emerged . I’m keeping them until we have a warm day … I worry since the nights temps go down to the 40’s. This coming friday 64but night 37 so should I let them go . Today through thurs possibilty of rain. I’m feeding them sugar 1 to 9 parts water.

  5. I raised and released monarchs for about 8 years. When I found a swallowtail cat, I’d bring it in and raise it as well. The trouble was the male monarchs patroling my yard would chase away any egg laying female swallowtails.

    This year I’ve cut my milkweed back tremendously and have been able to find several swallowtail eggs on my dill patch as a result. They are indeed beautiful, and very different than monarchs because they have variations in color. Their chrysalis to me looks like the face of a fox. The color of the chrysalis depends on what they are near. I’ve had green ones, brown, beige and even blackish ones.

    After a very work-heavy school year, working in my garden and caring for these beautiful creatures of our planet is extremely therapeutic.

  6. How did you get rid of the bad smell on you, and on your clothes?

  7. Thanks:) Whew, that was a nasty smell!

    • roundrockgarden said:

      I read somewhere that a baking soda paste (water and baking soda) helps absorb much of the smell. I agree, though, it is a nasty smell and a great defensive mechanism these cats have developed!

  8. Isn’t nature just incredible? It’s just so beautiful, and complex.

  9. Just found one of these laying to waste my parsley. In trying to move it, I provoked the Yellow Antenna Of Foulness. No idea what it was til I found your blog, but it seems I escaped being sprayed. Folks here mention “raising” butterflies from caterpillar stage. Can you guide me to where I might learn more about this?

  10. This is great! I’m so glad to know there are more people like me, who value bugs and take them outside to begin new lives! I’ve got quite a few pupa of these butterflies in my parsley, and I’m so excited!

  11. Thanks for all of this, all of you! We just started a rooftop garden and found 4 of them on our Parsley. The birds must have got to them so the next two we found we brought in the house to keep safe. One is in an early pupa stage, the other is in the pre-pupa stage. I have them in a smaller container right now. I plan to release on our roof as soon as they come out, should I be worried if they become butterflies while I’m not home that they may die if not released immediately?

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