"Giant" Backyard Discovery Returns!

one of the late-instar caterpillars

Before 2010, a Giant Swallowtail Butterfly had never been seen in Vermont.  Now, for the second consecutive autumn, this species has laid eggs in the Montpelier backyard of Abby Colihan.  A mild winter, sandwiched between two hot summers, has allowed the Giant Swallowtail to thrive in areas where it had been absent just a few years earlier.  

The range expansion of this species has limits, even in a warming climate.  The main host plants of caterpillars include those in the citrus family, and the only native host plant documented in Vermont so far has been Prickly Ash.  But Prickly Ash grows only in the Champlain Valley and southern parts of Vermont, so for Giant Swallowtail to lay eggs in Washington County, they need a little help.   Perhaps that is why the “gas plant” in Abby’s garden has been used two years in a row!

I counted a total of eight caterpillars on the small shrub; two were late-instar (older) caterpillars, and six were early-instar (young) caterpillars, which indicates that at least two female Giant Swallowtails laid their eggs there at different times.  At a glance, they look like bird droppings, but upon close inspection their intricate pattern is uniquely beautiful.  

Keep your eye out for adult Giant Swallowtails this month, as they may still be on the wing.  And one cold winter could halt their northward expansion.  Here is a photo of an adult that visited the North Branch Nature Center this past May:

Giant Swallowtail in the NBNC butterfly garden

 And here is a picture of one of the early-instar caterpillars, which was around 1/2 inch long:

An early-instar caterpillar

Photos and text by Larry Clarfeld