Hosting butterflies on the doorstep
Walking out the door one September morning, I saw a tiny black blip at my feet. It was the final molt of a pipevine swallowtail caterpillar, the old, squashed skin (and face!) that drops when a butterfly caterpillar becomes a chrysalis. So, I looked up. And there was the chrysalis, fresh and glistening, hanging from a brick above my head. But the best part was when I made my kid look at the black blip, and then he too looked up to find the butterfly-in-progress who dropped it. He knew.
The fascinating relationship between host plants and butterflies
The little wad of caterpillar skin tells a big story: the story of a native plant and animal interaction, and of the astonishing specificity of a larval host plant. Here, the plant is wooly pipevine (Aristolochia tomentosa), and the larva is the caterpillar of a pipevine swallowtail butterfly (Battus philenor). Pipevines, also known as Dutchman’s pipe, are the only plants a pipevine swallowtail will lay eggs on because that’s what her caterpillars can eat. The plant and animal evolved together in the habitat native to this place, Nashville, Tennessee. This is why I grow plants that make new butterflies. And why my boy and I knew what the blip was.
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A butterfly host plant at the door means we get to see, just by taking a step outside, every stage of a butterfly life cycle. It’s an ideal show-and-tell for what a host plant does. We can watch as a fluttering pipevine swallowtail mom extrudes one tiny orange egg at a time, her abdomen curved to aim at a leaf just so. We can see the hatchlings emerge, watch them eat their own eggshell, and then start eating the leaf. When they get bigger, we can even hear them munch! With a host plant, leaf damage is a good thing.
Best of all, we get to see a caterpillar become a chrysalis, and a chrysalis become a butterfly. When I showed my mom the pipevine action, she freaked. I’d been talking about host plants for years, but she’d never witnessed proof. “You should tell people,” she yelled, “people who want to make butterflies! They can just plant this vine!”
Every butterfly species needs particular food plants for the larval stage, as do 90 percent of all insects that eat plants. They are specialists. Monarch butterflies are a famous example; they cannot reproduce without milkweeds (Asclepias spp.). But all specialists have adapted to digest particular chemicals in certain plant families, genera or species. This is why every butterfly garden needs not just nectar plants for adults, but also caterpillar host plants for babies.
For example, one of the easiest species to “make” at home is black swallowtail (Papilio polyxenes). Its range is huge, and the caterpillars eat plants in the carrot family — also huge — which includes not only the gorgeous native perennial golden Alexander (Zizia aurea), but related, non-native herbs, such as parsley (Petroselinum crispum), fennel (Foeniculum vulgare) and dill (Anethum graveolens). Other easy show-and-tell species are Gulf fritillary and variegated fritillary butterflies, which use passion flowers (Passiflora spp.) native to your area.
Keep an eye out for an overwintering chrysalis
Note that if a swallowtail caterpillar becomes a chrysalis late in the fall, it’ll overwinter to emerge in spring. Larvae abandon their host plants when it’s time to pupate, so there could be a camouflaged chrysalis on any random stem (or porch brick), or even down in the leaf litter. And because butterflies overwinter as either eggs, larvae, pupae, or adults, they all need protection from pesticides, leaf blowers and excessive tidying, so that each big story can get the happy ending and new beginning it deserves.
Joanna, author of This Is How a Robin Drinks: Essays on Urban Nature, knows that trees and shrubs can make butterflies too, but they aren’t as convenient for show-and-tells that freak people out. Follow her blog at sidewalknature.com.
