Black Swallowtail Larva and Fascinating Behavior

Black Swallowtail (Papilio polyxenes) caterpillars on parsley
Black Swallowtail (Papilio polyxenes) caterpillars on parsley

These two Black Swallowtail larva caught my eye while I was planting and pruning in my garden.

This must be the first brood of the usual two or more broods that this species reproduces from June onward. The adult butterflies are seen from April to October.

The adult female butterfly lays just a few eggs on one plant, so the caterpillars can’t eat a great many plants. It rarely becomes a pest in the garden.

Black Swallowtail (Papilio polyxenes)
Head shot and the osmeterium isn’t visible. 

Hostplants for Black Swallowtail Larva

The host plants for this caterpillar are in the carrot family. They eat carrots, celery, dill, parsley and caraway. None of which are native to North America. This leads me to believe this is an introduced species. The young caterpillars eat both leaves and flowers. Later instars (larval stages) prefer to eat just the flowers.

Black Swallowtail (Papilio polyxenes) - back end
The back end of a Black Swallowtail larva.

Black Swallowtails are found most readily in open spaces. Such as fields, gardens, marshes, deserts and along the coasts. Seldom are they found in forest interiors. As former agricultural land returns to forest, the Black Swallowtail is becoming less common.

A fascinating behavior of Black Swallowtail larva (and all Swallowtail butterflies) is their curious and amusing defense mechanism.

Black Swallowtail (Papilio polyxenes)
Wave your osmeterium in the air, wave it like you just don’t care…

The Orange Osmeterium

All Swallowtail larva have an organ called osmeterium. This organ is attached immediately behind the head. When threatened (or poked gently with a stick, which I did), the caterpillar raises its osmeterium and waves it about. The osmeterium is laden with butyric acid and emits a foul-smelling odor. Some say it is similar to fresh vomit. I didn’t get that close so I can’t say what it smells like. I doubt I ever will.

Black Swallowtail (Papilio polyxenes)
Not very impressive, yet.

This caterpillar’s orange osmeterium was quite small. Perhaps this is due to youth. Larger Black Swallowtail larva have organs that are quite impressive. They must terrify birds or some other unfortunate creature who dares to tangle with this caterpillar.

Info bit:

I used this field guide – Caterpillars of Eastern North America: A Guide to Identification and Natural History (Princeton Field Guides)


More Butterfly Information

Butterfly Life Cycle

The 6 Butterfly Families and Identifying Butterflies 

Butterfly Family: The Swallowtails 


  1. OK thanks for the suggestion to poke the little guys and see them wave their osmeteriums about. I think I smelled the stinky substance too, but maybe by the time it got to my nose it was so diluted (?) or whatever reason, it smelled *good*: kind of fruity like pineapple soda or pineapple candy! did it a few times on different days, consistent smelling results… –Lorel

    • I thought the same when I poked this other different looking caterpillar, the smell was delicious, by no means was it foul smelling. He actually was munching on my “Guanabana” tree, which smells delicous as well, but it has a different scent/aroma than the one emitted by the fellow. He also changed in color, he had colored rings, and he could change them from yellow to orange as he would evert his osmeterium. It was amazing, but I wasn’t sure if that scent was safe or not.

  2. I find them readily on my Florence fennel plants. Which is in the same family as carrots and dill….

  3. You got me thinking there when you mentioned this caterpillar’s host plants. A little digging revealed it is indeed a native eastern butterfly. I started racking my brain to come up with native carrot family members and the only ones I could dredge up were Queen Anne’s lace (the wild carrot plant) and poison hemlock (a deadly toxic herb that resembles Queen Anne’s lace; not the hemlock tree). I’m not sure what the range on these plants are, and I’d certainly love to learn if they’re other, native parsely/carrot member?

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