What's That Bug? does not endorse extermination

Unknown caterpiller with warning stalks?
Location: Harrisburg, PA.
September 28, 2011 7:26 pm
I was wondering if you had an idea as to what this little fellow is. The first photo shows the results of prodding him with a stem to move him into better camera view. The orange stalks appear to be some sort of defensive measure. Could you inform me as to what they actually do? Any help would be appreciated.
Signature: Joseph Grabko

Black Swallowtail Caterpillar

Hi Joseph,
One of your photos shows this early instar Black Swallowtail Caterpillar on an Italian parsley leaf, one of the garden herbs that serves as a host plant for the caterpillar that is often called a Parsley Worm or Carrot Worm.  Fully grown Black Swallowtail Caterpillars have a striking green, black, yellow and white pattern that your individual doesn’t yet possess since it is an earlier instar.  Caterpillars molt five times between the time they hatch from eggs until they metamorphose into a chrysalis, and each of the stages is called an instar.  The orange stalks you mentioned are a scent organ called an osmetriumthat is characteristic of Swallowtail Caterpillars from around the world.  As you indicated, it is a defense measure that will dissuade predators like birds.  Your individual is one of the earliest instar images we have received of a Swallowtail Caterpillar displaying its osmetrium.

Black Swallowtail Caterpillar displaying Osmetrium

What's That Bug? does not endorse extermination
Location: Pennsylvania

3 Responses to Black Swallowtail Caterpillar

  1. Antikythera says:

    I think I saw one of these guys in my garden today. He was quite small, about a centimetre long, and the white band and orange spikes were not well defined. But I’ve seen black swallowtail butterflies in the garden this summer, and the caterpillar was on (of course) the parsley that I picked for dinner. Don’t worry, he didn’t end up in the potato salad, I threw him into the peonies on his piece of parsley.

    • bugman says:

      Chances are that this caterpillar will not survive as it will not substitute peonies for parsley as a food source.

  2. Antikythera says:

    I realized that too late. I’d forgotten, and was reminded while reading about swallowtails, about how specific their dietary requirements can be.

    This morning I found a caterpillar on the parsley that might be the prepupal phase of the black swallowtail. I have to do some more research and observe it a bit more closely. It’s much larger than the first one. I mistook it for a monarch caterpillar, but of course those don’t eat parsley either. It has black and green stripes, and I can see remnants of the little orange spikes on the black parts. If I can find my camera battery (we’ve just moved) I’ll take a picture and submit it.

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