A black swallowtail caterpillar crawls on the stem of the native water hemlock.

I am blessed to be working outdoors most days, tromping through prairies and wetlands.

Doing so allows me to observe many creatures in their natural habitat and speculate on some behaviors.

Across the area I have been seeing large numbers of black swallowtail butterflies this summer.

It wasn’t until this week, however, that I ran across any caterpillars.

The black swallowtail’s host plants lie in the carrot family. Queen Anne’s lace and wild parsnip are in that family, and at Midewin, we have millions, but I don’t see the caterpillars utilizing them most of the time.

I had a revelation this week when I was walking in one of our many dried-up wetlands and saw dozens of caterpillars concentrated in one area. After taking a photo, I took note that they were all feeding on the native water hemlock (Cicuta maculata), which also is in the carrot family.

The wild parsnip and Queen Anne’s lace are both introduced plants to North America, and while they do provide a food source, the swallowtail caterpillars clearly had a preference for the native hemlock.

What does it mean? Perhaps not all plants are created equal. I will eat junk food, but I’d rather have a home-cooked meal if given the chance, and I will be better off for it.

Butterflies such as the black swallowtail have co-evolved with native plants for thousands of years and we, as land managers, should take that into account when assessing appropriate habitat.

After my encounter, I ordered some extra water hemlock seed for my Limestone Park District pollinator project.

The experiment continues.

Reach Trevor Edmonson at

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