Caterpillar Quiz: How to Tell the Difference between Monarchs and Eastern Swallowtails

“Hello, I planted dill and it is dying.  The bad news is that tons of Monarch caterpillars are on it.  I’m not sure what to do, or how to keep the dill alive.  Any suggestions?”
–Jennifer L.
Eastern Swallowtail caterpillar on fennel, one of its host plants.  Photo by Monika Maeckle

Eastern Swallowtail caterpillar on fennel, one of its host plants. Photo by Monika Maeckle

First of all, Jennifer, a Monarch caterpillar would not be found eating dill, since it only hosts on milkweed species.  Host plants–the plant a caterpillar eats and lays eggs on–are often the best clue to what kind of caterpillar is visiting your garden.

In their caterpillar stages, Eastern Swallowtails and Monarchs are often confused with each other.  That’s no surprise, since later in their development, both sport stylish green, yellow, cream and black-striped suits. 

Monarch caterpillar on milkweed

Monarch butterfly caterpillar on milkweed, its host plant. Photo by Monika Maeckle

These two very different butterflies–Monarchs and Swallowtails–grace our Central and South Texas skies regularly.  The migrating Monarch appears in spring and fall during its annual migration.   The Eastern Swallowtail seems to be present just about year-round, except in extreme cold.  

As butterflies, you can’t mistake these beauties for each other.  The Monarch, Denaus plexippus, exhibits orange-and-black markings that resemble a stained glass window.  

Monarch butterfly at the San Antonio River Milkweed Patch

Monarch butterfly, with wings closed, on milkweed, its host plant. Photo by Monika Maeckle

Monarch butterfly nectaring on milkweed.  PHoto by Monika Maeckle

Monarch butterfly, with wings open, nectaring on milkweed. Photo by Monika Maeckle

The dark blue-and-black Swallowtail,  Papilio polyxenes, boasts elegant cream, gold and orange dots.   Both are large, lovely and can be drawn to your gardens with the right plants.

Eastern Swallowtail

Eastern Swallowtail, wings closed, nectaring on milkweed. Photo by Monika Maeckle

Swallowtail nectaring on milkweed.  Photo by Monika Maeckle

Eastern Swallowtail, wings open,  nectaring on milkweed. Photo by Monika Maeckle

Yet as caterpillars, the Monarch and Swallowtail are often confused with each other, as the email that opens this post suggests.  Here’s a few tips that should help you pass the “caterpillar quiz” in distinguishing the Monarch and Swallowtail caterpillars from each other.

1.  Note the plant the caterpillars are eating.

Checking out the plant a caterpillar is eating generally is the easiest way to tell what kind of caterpillar you’re watching.

Monarchs only lay their eggs on and eat milkweed, members of the Asclepias family.  Swallowtails will host on members of the Apiaceae family, which includes parsley, Queen Anne’s Lace, carrot, celery, fennel and dill.

Swallowtails will also host on plants in the citrus (Rutaceae) family, including rue bushes and lemon, lime and orange trees. If you find a green-striped caterpillar noshing on fennel, it’s a Swallowtail; a stripe-suited chomper chowing down on your Antelope Horns is a Monarch.

Swallowtail showing tubercles

Who goes there?!? Note the yellow “tentacles” which the Swallowtail shows off when bothered. Monarchs don’t do that. Photo by Monika Maeckle

2.  Check the tentacles/antennae.

Monarch caterpillars have tentacles on either end of their bodies.  The ones in front are technically antennae and have special sensory cells, while the ones on the back are “just for show”–to throw off predators.

Swallowtails, on the other hand, don’t always show their antennae.  When bothered or poked, yellow tentacles pop out of their head and emit a distinctive, sickly sweet odor.  Kids are always impressed when you provoke the Swallowtail’s tentacles.

3.  Note the body shape.

Monarch caterpillars’ body type is consistent in its breadth, while Swallowtail caterpillars are thicker in general, and mass into a “hooded” shape at the head.

In answer to Jennifer’s question about what to do about a lack of dill, I suggest planting plenty of it–some for yourself, and some for the caterpillars.   Dill tends to die as summer heats up, so you can also try some of the other Swallowtail host plants.  Rue and fennel have worked well for me, thriving even in our Texas heat.

An emergency run to a local nursery might also be in the cards to pick up some caterpillar food.  Just make sure it hasn’t been sprayed with any systemic pesticides.

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13 thoughts on “Caterpillar Quiz: How to Tell the Difference between Monarchs and Eastern Swallowtails

  1. Hi all,

    Just wanted to let you know that the swallowtail cats love queen annes lace and yellow Alexander. We found one munching on the yellow flowers growing in our field. Come to find out they eat most plants and weeds in the carrot family.
    They also love parsely!

  2. Good info! The only thing is the picture of the Monarch catterpillar in this article is actually not a monarch but a Queen.

  3. Hi Karen,
    Thanks for writing. You are mistaken about the caterpillar. It is a Monarch. Queens have three sets of filaments, or tentacles/antennae. See the article at the end of the post. –Monika

  4. There is no such thing as an eastern swallowtail. The larvae eating the dill is a black swallowtail larvae (sometimes referred to eastern black swallowtail). The black swallowtail and the black form of the eastern tiger swallowtail are hard to differentiate in the adult butterfly – but you would never find an eastern tiger swallowtail larvae on dill. Black swallowtail larvae do bare a resemblance to monarch larvae. The larvae you have in the top photo is a black swallowtail larvae – classic shot.

  5. We have a ton of dill in our backyard, we just discovered what we thought were monarch cats on the dill, but see on this discussion that they are swallowtail cats, just fine by me. We have half a dozen, my husband found one by accident and we started looking. We have an insect and bird friendly backyard, and are excited about this!

    • Congratulations! The birds will find the caterpillars sumptuous, so don’t be surprised if they disappear without a trace. If you want to have some fun, bring one inside. Put some dill in a vase with newspaper underneath and you can watch them grow and develop into the chrysalis. Good luck!

      • I wish I had seen this post just yesterday when I spied (6) Eastern black swallowtails on my dill. They’ve disappeared without a trace just as you had mentioned above. NOW what can I do to find them again. Am I to assume the birds I feed, came in for a snack or did they move onward? Thanks for answering my question.
        Lora

  6. I went or to check on my garden today. I have a raised bed. While looking at my dill, which has began to flower, I saw my dill is almost gone. I have one plant, and I stopped counting at 22 caterpillars. I was checking to see what they were, thinking they were monarchs, but have discovered they are swallowtails. Almost as pretty as the monarch. My dtr suggested I should get rid of them, but I’m rather excited to see what happens with them. I’ve got cucumbers and beans near by. . .oh and parsley, but they haven’t bothered them yet.

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