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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!

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Comments on: "Beneficial Insect Files: Black Swallowtail Caterpillar" (21)

  1. He sure is a brightly colored fellow.

  2. Great photos! I guess they told you…or warned you. I have seen that caterpillar before and didn’t know what kind it was…good to know.

    • roundrockgarden said:

      @ Amy: they tried to tell me, but i’m a persistent, curious fellow! :)

      @ Melody: thanks for visiting.

  3. Interesting critters, aren’t they? I found some of them last year on the dwarf lemon tree. they had orange “horns” and were scary looking. But they turn into gorgeous butterflies.
    Dropped for GBBD and got hooked on your site. You have lots of good info.

  4. I recently discovered that if you don’t want your caterpillars carried away by big black wasps, you better cnip off and flowering heads of dill, fennel, et cetera. Last year I had 20 small cats on one fennel, and the next day only a few. Wasps, I imagine, who bring them back to the nest and put their eggs into them.

  5. This is a fabulous blog! You are amazing. Just forwarded this post to someone asking about swallowtail butterflies. I’ll add you to the CTG blogroll if I haven’t done so already.

    • roundrockgarden said:

      Thank you so much, Linda! I’m quite a fan of Central Texas Gardener. I read the blog quite frequently through Google Reader. I have some new photos of pupating swallowtail caterpillars to post today. I’m glad you like my blog, and thanks for referring a new reader! ;)

  6. I have found a black swallowtail caterpillar on my parsley patch. I would like to either move him or protect him and watch him grow. Do you know when and how I could do this successfully?

    • roundrockgarden said:

      Sorry for the late reply. If you want to watch him grow, probably the easiest thing to do is remove the portion of plant that the cat is on. Prepare a little home for him inside and remember to provide fresh pieces of the host plant on a daily basis. When the cat is mature, it will stop eating and will look for a suitable location to pupate. You can provide some pieces of upright, sturdy twigs for this purpose. Keep watch and within a week or so, the butterfly will emerge. I found more information here for you. Thanks for visiting!

  7. Angela Goldberg said:

    Wonderful article and information!!! I found these curious creatures all over my curly parsely that I had planted in a large pot. My 9 year old son and I searched on the internet for pics of different caterpillars and came upon your pics and related article. Your information is right on the money!!! Very educational for a inquisitive 4th grader and myself as well. We both thoroughly enjoyed learning about this beautiful creature and look forward to hopefully observing the pupal stage and subsequent metamorphosis. Thank you!!!

    • roundrockgarden said:

      How wonderful! I’m happy you and your son found the entry helpful and educational! It’s so fun learning about these wonderful creatures. If you’d like to see one pupating, I captured some photos early in the summer. This is what you’ll be looking for: http://roundrockgarden.wordpress.com/2010/06/18/pupating-black-swallowtail-cat/. Oh, and you won’t find them on their host plant. They like to move to another location to pupate, so you may have to do a little searching, but likely not too far away!

  8. I am not a fan of these little critters. They wipe out my parsley & dill no matter how much I plant. Being anti- insecticide, especially on an herb, what do I do to control them. Picking them off & squishing in not an option.

    • I am also not a fan. They went to town on my meyer lemon tree I got this year as a gift. It only has about five leaves left on it and every one of them has been chewed on. I’m hoping it will survive as it has lots of green lemons on it.

      • roundrockgarden said:

        That sounds more like the giant swallowtail – the largest butterfly in North America! Lucky! :) (sorry about your tree – I hope it rebounds)

  9. Can you tell me when wasps lay eggs on caterpillars? We found what we believe is a black swallowtail caterpillar on a dill plant that is about 1″. We are hoping that it has not become a host for wasps eggs. Is there anyway you can determine this before the wasps eat their way out of the caterpillar?

    • roundrockgarden said:

      Unfortunately, i don’t think there is anyway of knowing. I collected eight monarch chrysalis last fall to save them from a temporary freeze. After a couple of days, all of them dropped little maggots out of them, so they were all victims. Good luck to you.

  10. ciscogt said:

    we have some as pets

  11. M.Saxatilis said:

    Thank you for this wonderful post. My ten year old daughter asked me to help her identify a caterpillar she found in our parsley. Like any good parent, I googled it, I tried an identification website and eventually landed on your posting. We saw your crisp, detailed pictures and knew we nailed it! And your detailed explanation of the instars is terrific.

    We now have two caterpillars in a butterfly habitat that appear to be in their fourth instar. My daughter is taking daily pictures of their progress. With any luck we’ll have two beautiful black swallowtail butterflies this summer.

  12. Good work! Only think I would change is the statement that they “build” a chrsalysis. After attaching themsleves to a suitable support, their skin splits open and eels away revealing the chrysalis which formed under the skin.

  13. Samantha said:

    Thanks for this detailed post – we have one on our parsley and (s)he’s been really interesting to watch!

  14. Very helpful, thank you. We have 6 caterpillars currently residing in our dill patch. Every day we go out and observe them. They are quite fascinating to watch, and silly looking at times.

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