Black Swallowtail Caterpillars and Adults

2014 August 23

Our neighbor found four of these eating the dill plants in her garden on August 3, 2013. She wasn’t particularly bothered by them (it is very easy to grow considerably more dill than you actually need), but she was curious what these large, colorful caterpillars were going to turn into.

For an idea of their size, here’s one on an 8-year-old child’s little finger.

This is one species I remember from my childhood downstate, where we used to find them pretty regularly on the carrots in our garden: they are Black Swallowtails, Papilio polyxenes (probably the subspecies asterius). The green body with black stripes and yellow dots is pretty distinctive, as is their diet: they are one of only a very few caterpillars you will find eating carrot greens, parsley, dill, Queen Anne’s Lace, and other members of the carrot family.

One of their defenses against getting eaten is that, if you squeeze them, they will pop out these two orange “antlers” (I couldn’t get pictures of them fully inflated because they only popped out all the way for a few seconds. They are about half-inflated in this next picture).

These antlers are not only brightly-colored and startling. They also stink pretty bad. The general consensus is that the horns smelled like rotting dill.

I put them in a cage to try getting them to pupate, along with some dill and parsley to eat if they weren’t ready to pupate yet. It turn out that even though they were found on dill, they actually preferred parsley. They also didn’t really get along, and would tussle with each other whenever they met.

After a few days, they did pupate. Three of them made brown chrysalises like this one, that were fastened by their abdomen tip, but with a thin harness of silk around the middle holding them against the wall.

But, one of them was still bright green after pupating.

I had read that they only had one generation per year, and that they would overwinter as a chrysalis. So I thought that it would take until spring to see the adult form. But, it turned out that the green chrysalis (and one of the brown ones) suddenly emerged as adults after only two weeks!

Again, I’m not as good at photographing butterflies as I’d like, so these were the best pictures I got. So, anyway, it was still only August 19 at the time, so we let them go just in case they were going to have time to raise up a second brood for the winter. We kept the other two chrysalises to try to overwinter them, but unfortunately they never emerged in the spring.

But, just a couple of weeks ago (early July), Sam and I spotted an adult Black Swallowtail out in the back yard, so some of them did get through the winter from somewhere. And then, on August 10, 2014, Sandy found some of these in the garden on her fennel plants.

That’s what they look like as very young caterpillars, less than a half-inch long. The coloration makes them look like a bird dropping at this age (up until about their second molt).

On the next molt, though, they were changing to the green-black-and-orange coloration.

This time, I actually got a picture of the “horns” at full extension. As you can see, at this age the horns are almost half the length of the caterpillar’s body.

And at the moment I’m writing this (August 21), there are five big, fat swallowtail caterpillars on the fennel that look like they are ready to pupate Any Moment Now. I’m happy to see this, because until last year I’d never seen Black Swallowtails this far north. I think they have just recently expanded their range up into this area.

Given what they eat, there is a good chance that Black Swallowtail caterpillars and adults are toxic. Their foodplants mostly produce polyacetlyenes and coumarins in their greens, which are responsible for the flavor and aroma that they have, and are also poisonous in quantity. Given that the caterpillar’s “stinkhorns” smell like the plants they eat, it sure looks like they are concentrating the toxins from their foodplants as defensive chemicals. They probably mimic the bird droppings when very young because they haven’t picked up a good concentration of chemicals yet, and then switch to the warning coloration once they’ve had a chance to build up their toxicity.

7 Responses
  1. August 23, 2014

    Great photos and research as always.

    These guys are a case where evolutionary biologists could have some fun. The caterpillars wrestle with each other when they meet? What a waste of time! If it were me, I’d explain it in the textbooks with something like, “Not all adaptations make sense. In this case, the caterpillars are wrestling because they’re stupid. All that wiggling around is bound to attract predators and it’s not like they’re going to be able to do anything meaningful to each other. Losers.”


  2. August 24, 2014

    Actually, I’m not so sure the wrestling is pointless. Their preferred food plants have these long, thin leaves that don’t have much bulk to them, and there really isn’t that much foliage per plant. For example, a single caterpillar strips most of the leaves off of a fennel plant by the time it matures, and if they shared the plant with another caterpillar it would get stripped down to bare sticks in fairly short order. If, when they meet, one of them can knock the other off of the plant they are on, then the winner of the fight is more likely to be able to mature without running out of leaves and having to hunt for a new plant.

  3. August 25, 2014

    These are my favorite pests, by far. I pretty much let them do their thing because they are just so beautiful and they don’t usually do a lot of damage. We’ve been seeing them in Ontonagon for as long as we have been growing here (since 2007). The only year we had a real problem with them was 2012, when we had to squish a few to protect the fennel, but that was the year of the caterpillar.

  4. August 25, 2014

    Love those antler thingies — I had no idea! Next year I’m going to grow dill again (the raised bed disintegrated a couple of seasons ago after a mere 20 years) just so I can find me a caterpillar to give a gentle squeeze to and see this for myself.

  5. August 26, 2014

    Andrea: Thanks for the note. Now I’m not sure if we didn’t see them before because they hadn’t arrived in Houghton yet, or if it’s just because we weren’t planting anything in our garden that they liked as well as fennel. Some of the range maps I’ve seen show them not being in this area, though, so it is somewhat likely that they just took 6 years to get from Ontonagon to here.

    Anne: While they will eat dill, parsley, and carrots, all of the ones we found in our garden last month were on fennel in spite of all those other plants being available just a few feet away. So, including Andrea’s experience, it looks like fennel is the best choice to draw them in.

  6. August 27, 2014

    Hi there! I subscribed to a bunch of blogs including yours, about 2 years ago, and yours is the only one I still read! I look forward to all of your posts. Great info and pictures; I’ve learned so much from you! I live in NJ and my 6-year-old son have been raising Eastern Black Swallowtails for 3 summers now. They are very forgiving and nearly foolproof, as long as you have their food plants on hand, and don’t use pesticides (don’t buy conventionally grown herbs once the caterpillars eat all your herbs!). I grow dill and parsley (among other things) — the adults seek it out (apparently they have amazing eyesight) to lay their eggs since the larvae only eat plants in that family, which includes fennel and carrots too. We’ve learned a lot about these butterflies in the last few years, though I don’t fully understand the breeding cycle, meaning, I don’t know for sure if it’s just one generation per year. I say that because our third group of swallowtails are about to pupate within the next day or so. It’s a lot warmer here in NJ, though. The smelly antlers are called osmeterium, and my little guy LOVES when they pop out, even though it stinks! Regarding chrysalis color, we’ve read — and found — that generally the chrysalis will turn brown if it’s attached to a twig (we place them in the habitat once the caterpillars reach the size of my son’s finger), and they will usually remain green if they attach themselves to the top or side of the enclosure, or pupate on the floor. Camouflage. I overwintered 2 our first year, since we found the caterpillars in October. Once they entered their chrysalis phase, I placed them in the fridge in a jar and swapped out damp paper towels at the bottom of the jar a few times over the course of the winter. I pulled them out in May, and them emerged a few days later. One, unfortunately, was deformed, but the other was just fine. The deformed one was (so far, knock on wood) our only failure, but it was so depressing. I try not to push human qualities onto animals, especially insects, and I am not a unicorns and butterflies sort of girl…I love creepy crawlies for all of their creepy crawliness! But I’ve become a huge fan of the members of lepidoptera. Their short life cycles are a perfect story of redemption: as youths, they trash our crops, but they repent as adults by pollinating.

    An aside: I have a proper DSLR camera with a few lenses, as well as a USB microscope and love getting pictures of arthropods. But recently, I picked up an $8 slip-on macro lens for my iPhone, and I am amazed at the quality of the pictures I’m able to get. It’s often way easier for me, and less intrusive to bugs to take a picture with a phone rather than a huge camera. I feel like I’m patting myself on the back, so I apologize if I come off that way! But I got some really rewarding swallowtail pictures this season, and while your pictures are awesome, I just wanted to tell you about this, in case you’re looking for something a little easier — or at least another tool in your case — for getting pictures of insects and such.

    Sorry to babble, and thanks again for an informative, entertaining blog!

  7. August 29, 2014

    Thanks, Chrissy!

    As it turns out, I don’t have an iPhone, and my cheap cell phone has a seriously minimalist camera that wouldn’t be much good for bug photos even with a slip-on macro lens. But, I’ve just purchased an Olympus TG-3 (which, according to UPS, was just delivered today) to serve as my “camera-that-is-always-with-me”[1]. I got it because, aside from being shock resistant, dustproof, and waterproof to 50 feet, it is also supposed to have some pretty impressive macro capabilities for a point-and-shoot camera. Including built-in focus stacking, where it automatically takes a series of pictures, and then processes them together into a single extended-depth-of-field image. I expect to still use the digital SLR most of the time, but this one should be good for when I spot something interesting that I can’t catch and bring home.

    [1] “The best camera is the one that you have with you.”

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