For the second time in three years, the Vineyard (and, indeed, all of southern New England) is enjoying an exceptionally early and somewhat bizarre start to the butterfly season. Cold-blooded like all other insects, butterflies are scarce or absent on our winter landscape, with the earliest of our resident species typically becoming active in mid-April. But following a notably mild winter, some of our early-season species were on the wing weeks ahead of their usual schedule. And the entire region is now experiencing a robust and exceptionally early influx of migrant butterflies from the south.
The first hint that something odd was up may have come as early as the start of January. As the new year began, a mild winter already appeared to be in the cards, and the Island was being scrutinized by dozens of field observers scouting for or participating in the annual Christmas Bird Count. One result was the discovery of several orange sulphur butterflies — a mid-sized yellow species with black borders on its wings that is common on the Island, especially on agricultural land. A hardy insect, it is routinely on the wing here until about Thanksgiving and the species had been found here in late December. But the January reports caught my eye.
Also catching my eye were orange sulphurs that turned up during a warm spell in the second half of March: I found this species at several widely spaced locations on March 21 and 22. If you compare this pattern to the previous record early date for the Vineyard, April 14, you can tell something is up. Orange sulphur differs from most of our native species in that it often doesn’t enter into a state of deep dormancy to survive the winter. Such dormancy, called “diapause” by scientists, is a common strategy among northern butterflies for dealing with extreme cold: its metabolism slowed nearly to zero, a butterfly (usually still in egg or larval form) ceases its development for several months, freezes solid and sleeps right through deceptive winter thaws, resuming growth only when warm weather is firmly established.
Not so the orange sulphur, essentially a southern species that has become common on the Vineyard only in the last 80 years or so. Larval orange sulphurs, research has shown, often don’t enter diapause, remaining alert enough to resume feeding and growth during winter warm spells. The strategy is dicey, no doubt, at our latitude: if the winter is a long and cold one, larvae may run out of stored energy and die. But if the winter is mild enough, survival rate is high and adults may mature, emerge, and lay eggs at any point during the season, getting the species off to a head start when spring comes.
This is how orange sulphurs behave in the South, cycling through multiple generations continuously through the year. A few more warm days during January and February, and we might have observed precisely this pattern on the Vineyard. In effect, our sulphurs this winter showed a pattern approaching that shown by populations in Virginia or North Carolina — a hint of how things may change as the climate warms in coming decades, making winters like this past one routine.
Then, in early April, another unusual phenomenon began to emerge, originating in the Midwest but rapidly moving east into our region. Three migratory butterflies — American lady, red admiral, and question mark — staged an early and stunningly large invasion of the northeastern U.S.
Though the scale and the early timing were exceptional, the pattern itself is not. Unable to survive a deep freeze, all three of these species either migrate southward or die off as winter arrives in the northern part of their range; in spring, they migrate northward again, vastly expanding their breeding range. Mid-April is a typical arrival time for these butterflies in Massachusetts; this year, the vanguard showed up a good two weeks early, and numbers rapidly grew. The wave was slow to appear on the Vineyard, probably a reflection of the Midwestern origin of the influx (in contrast to a more usual pattern of movement up the eastern seaboard). But all three species are here now, widespread and apparently growing rapidly in numbers; for example, during a few sunny hours in the field last Saturday, I encountered more than two dozen representatives of these species.
Other butterflies are also earlier than usual. Trustees of Reservations ecologist Russ Hopping, for example, turned up a hoary elfin at Wasque on April 3, beating the previous early record by more than a week. Butterflies, unfortunately, are small animals that are often in rapid motion. And many of our species are small, cryptically colored, or both. So even the most impressive butterfly events escape the notice of the inattentive. But if you pay attention to your surroundings and make a point of checking flowers for butterflies that may be nectaring on them, you can hardly miss at least a glimpse of this spring’s odd insect event. Butterflies are on the wing, well ahead of schedule, and their numbers are growing fast.