Earlier in the week Charlotte and I stopped by Cedar Rock Gardens for their ongoing 25% off all plants sale. We paused near the fennel plants when Charlottes asked, “is that a Monarch caterpillar on the fennel?” A teeny, tiny yellow, green, and black caterpillar was tucked in, sleeping in the foliage. She found two more so we purchased all three plants. Little did she know, that question is similar to one of the most frequently viewed posts on my website “No, that is not a Monarch Caterpillar on Your Parsley Plant!”

Black Swallowtail caterpillar recently molted with discarded skin

Black Swallowtail (Papilio polyxenes) caterpillars, especially in their early instars, are easily mistaken for the yellow, white, and black caterpillars of the Monarch butterfly. Butterflies oviposit their eggs on specific plants to their specific species. These plants are called larval host plants, in other words, caterpillar food plants. Monarchs only deposit eggs on plants in the milkweed family ( Asclepiadoideae) while Black Swallowtail females only deposit eggs on members of the carrot family (Apiaceae). Plants in the carrot family include dill, parsley, fennel, carrots, parsnips, and Queen Anne’s Lace. In days gone by, the Black Swallowtail was commonly referred to as the Parsnip Butterfly.

You will never see a Monarch caterpillar on fennel or parsley. Conversely, you will never see a Black Swallowtail caterpillar on your milkweed plants 🙂

Yellow, white, and black striped Monarch Caterpillar on Common Milkweed (Asclepias syriaca)

Yellow, green, and black striped and dotted Black Swallowtail caterpillar on fennel plant

Cedar Rock Gardens is located at 299 Concord Street in West Gloucester. For more information, visit their website here.

Happy Summer Solstice and Happy Pollinators Week!