My Favorite Butterfly

I've known the Baltimore Checkerspot (Euphydryas phaeton) since childhood. There was a sizeable colony next to my paternal Grandmother's house in Massachusetts. Growing up, I was attracted to the beauty of the vivid red and yellow spots set off by a rich, velvety black. One of the first things that I learned about Bluff Spring Fen was that it was home to a colony of this species. This was on my first visit in March of 1982, and I immediately wanted to go back later to see it flying.

Turtlehead

In addition to being a very beautiful butterfly, The Baltimore is noteworthy for its interesting life cycle. This has helped maintain my fascination as an adult. For many years, the common wisdom was that Turtlehead (Chelone glabra) was the exclusive caterpillar host plant. A nice simple story- but too simple, it turns out. After taking many twists and turns, my understanding of the life cycle has settled on a pattern that I am reasonably confident of.

Baltimore Checkerspot eggs on a turtlehead leaf

At Bluff Spring Fen, female Baltimore Checkerspots lay their eggs on two speacies of plants: Turtlehead and Mullein Foxglove (Seymaria macrophylla). The larvae hatch in mid late July, spin webs on the stems of the host plants and feed communally for the rest of the summer. In early fall, they migrate to the base of the plants where they overwinter.

A web containing young Baltimore Checkerspot caterpillars on Mullein Foxglove

In Spring, the young checkerspot larvae become active again. Most migrate away from the plant that they spent the previous summer on. As with many youngsters, they become a bit less fussy about what they eat as they get older. We have found them feeding on about a half-dozen plant species at the Fen. Most, like penstemon and swamp betony, are in the snapdragon family, as are turtlehead and mullein foxglove. They will use a few unrealted plant species, such as valerian and even seedlings of ash trees.

Older Baltimore Checkerspot larva. This one is posing on horsetail. It's actually eating swamp betony, the curly-leved plant in the background.

Sometime in late May or early June, the caterpillar wanders off of its food plant, spins a button of silk on a stick or leaf, and hangs upside down from it. After a while, its skin splits, reavealing the developing pupa or chrysalis underneath. The adult butterfly develops beneath the skin of the chrysalis. Eventually the skin splits, and the adult emerges to complete the life cycle.

Baltimore checkerspot larva, and the pupa that formed from it

Here in Illinois, the Baltimore checkerspot is an uncommon species. It is relegated to fens and sedge meadows where its host plants grow. Twently years of butterfly monitoring at Bluff Spring Fen has revealed a population that has apparently increased in size. Last Sunday I counted 36 individuals in an hour. The number will probably be even higher on my next census. It's a very healthy population.