Wild Side: Big news – new butterfly found

A giant swallowtail butterfly.
Photo courtesy of Mass Audubon

A giant swallowtail butterfly.

The Vineyard’s butterflies have been a subject of study for many years. The first records I’m aware of go back more than a century. In 1943, a monograph published by the Maria Mitchell Association treated all the Lepidoptera (moths as well as butterflies) on the Vineyard and Nantucket. In more recent decades, naturalists have spent literally thousands of hours observing butterflies on the Island. It’s safe to assume that whatever lives here, or shows up with any regularity as a vagrant, has been found.

So it is big news, at least from the Wild Side’s perspective, when a new butterfly species is added to the Vineyard checklist, which, like all checklists, will never totally stabilize. New species will continue to colonize the Vineyard; species will be lost, and rare vagrants will continue to drop in. But almost by definition, such changes to the checklist are interesting, and the most recent one is a stunner: the aptly named giant swallowtail, one of the largest and most strikingly marked butterflies occurring in North America, has paid a visit.

Credit for this find goes to two observers whom I had no idea shared my interest in butterflies. Tom Hodgson and Sarah Mayhew independently found and photographed this species on August 17 in West Tisbury. While it’s impossible to say for sure, I suspect that only one butterfly was involved: the two sightings were fairly close together in the Panhandle area, and given how showy this species is, it’s easy to imagine multiple observers finding the same individual.

Tom was a conspicuously active volunteer field worker in a study of Vineyard bees conducted over the past two years by entomologist Paul Goldstein. Sarah has emerged as one of the most proficient bird photographers on the Island. Both observers did what good naturalists do when they run into something fancy: recognized it wasn’t usual, made every effort to document their sighting, and reported it promptly to more experienced observers. This was a very tidy job.

The giant swallowtail, which truly is big even for a swallowtail, ordinarily lives in the Southeast and Midwest; range maps in field guides show its usual distribution reaching north only to about Delaware on the East Coast. But like a number of other butterfly species, giant swallowtails occasionally engage in mass movements well outside their normal range, sometimes even establishing short-lived colonies far from where the species is normally found. Both last summer and this one have seen such a movement, and from late July into August, a torrent of reports built on the Massachusetts butterfly listserver.

The pattern of the expansion of this species into the region is an interesting one, with the first Massachusetts sightings coming from the western part of the state, followed by a steady push to the north and east into the Boston area, southern New Hampshire, and Maine, and beyond. Some of these butterflies probably represented the offspring of last summer’s invaders; others were surely new arrivals, apparently entering New England from the west, crossing the Appalachians, rather than by moving north along the coastal plain.

As a result, we had to wait longer than most of the state to get ours. But once the migration began to spill southeast onto the coastal plain and Cape Cod, it became a question of when, not if for the Vineyard. There has not yet been a report from Nantucket, which is obviously harder for a butterfly to get to than the Vineyard. But these things can fly: it wouldn’t surprise me to hear of a giant swallowtail on that island, too.

As for Martha’s Vineyard, I’d say more giant swallowtails are not just possible but likely this season. But don’t expect the species to colonize the Island any time soon. It’s a question of food plants for the caterpillars: the presumed host in our region is prickly ash, which does not occur on the Vineyard. Still, across this butterfly’s wide range, its caterpillars use a fairly wide variety of plants; I could imagine giant swallowtails finding a plant here that worked, conceivably even one that the species doesn’t use elsewhere. Of course if it successfully colonizes the mainland of southern New England, the giant swallowtail could become a regular visitor to the Vineyard. So the near future of this butterfly on the Island, and in the region, merits watching.

Giant swallowtails differ from our usual swallowtails in being mostly yellow below and mostly black above. Our other species may be either mostly black or mostly yellow, but it’s the same on both top and bottom. Because (like most other butterflies) they are attracted to flowers for nectar and because they are migrating, giant swallowtails are as likely on a butterfly-bush in Edgartown as on a patch of goldenrod in Aquinnah. So keep your camera handy!