A recent study of declining butterfly populations published in the journal Science made headlines across the country when it came out in March. The New York Times even launched an interactive tool so readers could see how butterflies had been impacted in their specific area. In the abstract of their findings the scientists concluded that “the prevalence of declines throughout all regions in the United States highlights an urgent need to protect butterflies from further losses.” So, what’s a gardener to do?
Plant caterpillar food, says Mary Anne Borge, the editor of Butterfly Gardener, a publication put out by the North American Butterfly Association for its members. When gardeners think about supporting butterflies, “we always think about providing nectar for butterflies for the adult butterflies,” says Borge. “And we think of other critters that might munch on the leaves of plants as pests. But butterflies are not butterflies through their whole lifecycle. They are caterpillars first. So if you really want to help butterflies, you really need to provide not just nectar for the adults, but you also have to provide food for the caterpillars.”
And it turns out, caterpillars are also very particular about what they can eat.
Butterflies strategically lay their eggs on particular plants, so that when their caterpillars hatch they will find themselves on a plant they can digest. The best known example of this host plant and caterpillar relationship is the milkweed-monarch connection (in part because monarchs are so distinctive and they have just one host plant), but there are hundreds of other caterpillar-plant relationships, for example, the zebra zwallowtail’s only host plant is the pawpaw tree (Asimina triloba). Some caterpillars like monarchs and zebra xwallowtails can only eat one plant, while others can happily feast on a dozen, but caterpillars can rarely eat ornamentals imported from abroad.
What you plant to feed your regional caterpillars will, therefore, depend on where you live. The National Wildlife Federation has a plant finder tool where you can plug in your zip code and search for butterflies and get a list of their host plants you might use in your garden. But Bolger also suggests gardeners keep an eye on plants slightly outside of their region, too. “I found it very interesting to actually look at the regional data from that report in Science. There’s some interesting trends that they hint at in the writeup for the study: Butterflies that historically have had a pretty decent population in the southern United States who are now increasing their numbers in the north,” says Bolder. “The only thing that enables that is having the caterpillar food plants that they need.”
