Illustration by Adelaide Tyrol
It’s a bird, it’s a plane, it’s a...? In September of 2012, I spied something fluttering wildly on the lavender phlox in front of my house. At first I thought it was a hummingbird, but as I moved closer I discovered it was a huge butterfly – the largest I’d ever seen, with a wingspan of about six inches. I rushed into the house to get my camera.
The butterfly was a challenge to photograph, its wings a blur as it hovered and darted from flower to flower, sipping nectar with its long tongue. The upper side of its wings were black, with a band of yellow spots from wingtip to wingtip. Another yellow band led diagonally from each wingtip to each wing “tail.” The tails were long, with yellow spots edged in black. On the underside, the coloration was similar to a tiger swallowtail – pale yellow with thin black stripes. I consulted my butterfly guides and determined the fabulous creature was a giant swallowtail, a cousin to our common Canadian tiger swallowtail.
Until recently, giant swallowtails, North America’s largest butterflies, were only occasional visitors to southern New England, and absent from our region. But since 2010, these big butterflies have been seen in increasing numbers in the Northeast.
According to Kent McFarland, a conservation biologist with the Vermont Center for Ecostudies, the giant swallowtail is known to wander, and feeds on a wide variety of flowers. He theorized, “they probably had good productivity in the South and their population was able to explode and move north in the warmer weather of recent years.” McFarland noted that this sudden range expansion is not unprecedented. “There are accounts of giant swallowtails in the Northeast from the late 1800s. Samuel Hubbard Scudder, a famous lepidopterist, had records of giant swallowtails up into Quebec. There were big flights into the Northeast that lasted for several years, then disappeared, perhaps due to colder weather.”
Giant swallowtail caterpillars have been found in Vermont on prickly-ash, a shrub in the citrus family. This is the only known host plant native to our region, and the caterpillars' presence is evidence these butterflies have reproduced in the Northeast. People have also found them on potted citrus trees on their porches, said McFarland. (In Florida, where giant swallowtails are abundant, these caterpillars are a pest in citrus groves and are known as “orange dogs.”)
When the caterpillars first hatch from tiny orange eggs, they look like bird droppings, a disguise to foil predators. As they munch on leaves, they grow to two inches long, and become brown or olive with white patches and small purple spots. At this time they resemble small snakes, with a swollen thorax that mimics a snake head. When attacked, they project a red, Y-shaped gland that looks like a snake’s forked tongue. The gland contains a pungent mix of chemicals that are toxic to small predators like spiders and ants.
The caterpillar’s skin eventually hardens into a brownish case or chrysalis, where it spends the winter. Inside the chrysalis, attached to a branch or fencepost with a silken thread, the caterpillar transforms into a butterfly. In spring, the butterfly splits the chrysalis open and emerges. Holding onto the shell of the chrysalis, it pumps fluid into the veins of its wings until they are fully extended. Once the wings are hardened, the giant swallowtail takes its first flight. These butterflies have a distinctive flight pattern – they appear to be “hopping” through the air, and they tend to fly high and fast. They produce two broods annually in the North.
Fresh giant swallowtails have been seen in southern Vermont and the Champlain Valley in May in recent years, evidence that they have overwintered successfully here.
Did giant swallowtail chrysalises survive this year’s cold winter, or will the species retreat south again? “We’ll have people looking,” said McFarland. “It will be fascinating to find out.”
Keep your eyes open for the spectacular giant swallowtail this spring and summer. You can report sightings here.
Susan Shea is a naturalist, conservationist, and freelance writer who lives in Brookfield, Vermont.