The largest butterfly in North America has expanded its range from the South and Midwest during the last decade and is now appearing in much of New England with regularity. The eastern giant swallowtail (Heraclides cresphontes), which features wide yellow stripes across its brown wings, has made it as far north as northern Vermont and as far east as Bar Harbor, Maine, but it isn’t expected to go much farther.
“We never even dreamed of seeing one here in Vermont, and then in 2010 somebody shipped us a photo of one in their garden, and that became our first record,” said Kent McFarland, a conservation biologist at the Vermont Center for Ecostudies who has studied the swallowtail’s range expansion. “Then we had a couple more sightings that summer. The next season, a lot more were reported, including caterpillars, so we knew they were reproducing. We figured that a harsh winter was going to knock them back, but we’re still finding them.”
In the 1800s, an entomologist wrote that giant swallowtails were resident in Massachusetts, but they had become extirpated by the turn of the century. With the exception of a few scattered records in the 1900s, the butterfly disappeared from the region until very recently.
Based on data from the internet portals iNaturalist and eButterfly, the Champlain Valley of Vermont is now a stronghold of the species, with dozens of sightings reported. In just the last two years, the species has been observed 16 times in New Hampshire and 10 times in Maine as well. It has also been reported numerous times in southern New England.
Why the eastern giant swallowtail is expanding its range so dramatically is unknown. In a research paper McFarland published in 2021, he and his co-authors speculate that it has to do with climate change and the range of its host plant, a shrub called prickly ash, which is one of the only plants that the butterfly’s caterpillars will eat. Prickly ash is found in scattered locations around the Northeast. As the climate has warmed and winters have become milder, the butterfly has moved as far as it has been able to find prickly ash.
“They’ve expanded wherever there’s a host plant, and they can expand north because of the warming climate,” said McFarland. “It’s all about winter climate change; they can withstand some pretty cold temperatures.”
McFarland doesn’t expect the butterfly to continue its range expansion much farther north, however, because prickly ash isn’t found north of New England.
“They’re trying to keep going north, but it’s a dead end for them,” he said. “They strike out for new territory and have shown up in the Canadian Maritimes and Quebec City, but they’ve outrun their host plant.
“The caterpillars can feed on gas plant, too, and a lot of people are planting that in their gardens, so it might spread around a little bit more,” he added. “Somehow they find it and lay their eggs on it. It’s a little like a bird feeder – we’ve made islands of refuge for them with gas plants.”
This article was adapted from an article originally published in ecoRI News.