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!
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.
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.
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.
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.