4 keys to telling apart monarch and swallowtail caterpillars

We stress that it is important to plant milkweed because it is the only plant that monarch butterflies lay their eggs on and that their caterpillars eat.

However, some people have come to us confused that monarch caterpillars are eating the dill in their yard.

Taking a closer look though, it’s not monarch caterpillars eating their dill at all! It is black swallowtail caterpillars!

(Butterfly or moth? Can you tell the difference?)

Here are some ways to figure out which caterpillar is in your yard.

1. Look at the plant the caterpillar is eating.

It’s not just a “sometimes” rule, it’s an “always” rule. Monarch butterflies only lay their eggs on species of milkweed, members of the Asclepias family. It could be butterfly milkweed, common milkweed, swamp milkweed, whorled milkweed, but it’s always milkweed.

Photo of a monarch caterpillar on milkweed

Monarch caterpillar on milkweed

Swallowtails’ host plants are in the Apiaceae family, which include parsley, Queen Anne’s lace, carrots, celery, fennel and dill as well as plants in the Rutaceae family, which include citrus plants.

Photo of an eastern swallowtail caterpillar

Black swallowtail caterpillar on dill

There isn’t any crossover between host plants, so you can definitely tell which caterpillar you’re looking at by what it’s munching on.

2. Check out the stripes.

Monarch caterpillars have thin stripes of black, yellow and white. Black swallowtails have their stripe pattern includes dots of yellow, or sometimes orange. Monarchs never have dots, only stripes.

Photo of monarch caterpillar

Monarch caterpillar

Black swallowtail caterpillar

3. Note the antennae.

Monarchs have what appears to be black antennae on both ends of their body. The ones in front are antennae, and the one in back are to confuse predators.

Swallowtails don’t have obvious antennae. We don’t suggest bothering caterpillars, but swallowtails will only show their antennae when they are poked or prodded. Then their yellow antennae stick out and emit and a strong odor.

4. Look at body shape.

Monarch caterpillars have a consistent width all the way along their bodies. Black swallowtail caterpillars are wider in general and also have a wider head than body.

Photo of a monarch caterpillar

Monarch caterpillar

Photo of a black swallowtail caterpillar

Black swallowtail caterpillar

11 Comments

  1. Carol on August 27, 2019 at 12:49 pm

    What about yellow tiger swallowtail caterpillars? What do they look like? Different from black swallowtail caterpillars?

    • kiley on August 27, 2019 at 2:13 pm

      Tiger swallowtail caterpillars are green and look quite different.

  2. susan tipton on August 26, 2019 at 5:59 pm

    Thanks. I was able to understand quickly. In time to move a caterpillar who had finished a lone, unidentified plant to what I know is milkweed!

  3. Susan Thompson on August 21, 2019 at 4:23 pm

    That was the easiest thing I’ve ever looked up. Thank you for making it so clear.

    • Buz Peoples on August 26, 2019 at 12:46 pm

      Wonderful description and comparative photos of these two caterpillars.

  4. Carol Bauer, professional gardener on August 13, 2019 at 5:49 am

    Very helpful easy to understand information. Thank you

  5. Christie on August 5, 2019 at 7:52 pm

    I have found several different size monarch catapillers dead on the milkweed big ones and little ones not sure why. I was hoping for them to turn into butterflies but no luck I’m am sad. If anyone knows why they are dying I would like to know. Thank you

    • Katie on August 25, 2019 at 10:25 am

      There is a monarch disease called Ophryocystis elektroscirrha (OE).

  6. E.J. Necco on August 4, 2019 at 6:59 pm

    I have a Black Swallowtail caterpillar that I found crawling on the grass below the fennel. I put it back on the fennel and a couple of hours later, I find on the ground again crawling on toward the brick. Is it trying to find a place to develop it’s chrysalis? Maybe not on the fennel?

    • kiley on August 13, 2019 at 12:35 pm

      That could be

    • Chris on August 19, 2019 at 4:25 pm

      I would grow parsley for the Swallowtail caterpillars. They always crawled off somewhere more sturdy to make their chrysalis. I even found attached to the siding.

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