Handily the high point of my year so far was getting good — well, decent — photos of a butterfly I’ve been unable to find in a decade and a half. The bug in question is a little brown job, roughly penny-size when it folds its wings up over its back, called the frosted elfin.
This understated insect has, as a caterpillar, a very specific taste in food. In our region, it’s wild indigo (some other populations of the butterfly feed on wild lupine). But despite the abundance of wild indigo on the Vineyard, the frosted elfin appears to be rare, rare, rare.
My Island interactions with this butterfly are easily summarized. For about five years around the turn of the century, I knew of a very small colony that persisted in Edgartown. Each year I’d pay a visit or two, finding anywhere from one to three frosted elfins.
I’ve also seen the species twice in Correllus State Forest. But the last of those sightings, each of a single individual, was in 2001. And at the tiny colony in Edgartown, my last sighting was in 2002. And that’s it, despite continuing my annual visits to the colony and a campaign of searching other likely habitat.
Interestingly, though, it was back at the site of that colony that I recently found another frosted elfin. I suppose it’s possible that it wandered in from elsewhere on the Vineyard. But given how rare the species appears to be, the odds of me and a wandering frosted elfin randomly colliding at this particular site can’t be great.
No, I think the little devils persisted at that site, but in such small numbers that I was unable to find them despite looking and expecting to find them. To be sure, they are small, well-camouflaged insects. But they’re often active enough to be fairly conspicuous for their size. So it’s a good exercise in humility for me to suppose that these butterflies simply evaded me for 15 straight years.
We’re near, but surely not at the northern end of, the range of the frosted elfin; the species occurs in much of the East. Everywhere, it is essentially a barrens insect, limited (along with its larval food plants) to areas with lean soils and frequent fire or other disturbance.
And it’s patchy in its distribution: Small populations will persist, sometimes for many years, but much seemingly suitable habitat goes unoccupied. In short, the occurrence of this species on the Vineyard is just a well-developed example of that general behavior.
One of four generally similar elfin species on the Vineyard, the frosted elfin is marginally larger than its cousins, and it flies a bit later in the year than our two most common elfins (brown and hoary); all my smattering of records are from May.
All the elfins are active insects, prone in varying degrees to vigorous aerial chases with mates or rivals, and to patrolling a territory to find mates or roust rivals. Since most of my Vineyard sightings have been of single individuals, I can’t really say how lively their social interactions are. But I do know that the frosted elfin is the most ambitious patroller of this group, and the most powerful flier.
I watched the elfin I recently found for about 40 minutes. Its activity centered around a clump of flowering shrubs. After a few minutes of alternately nectaring and basking in the sun, it would take wing and rip around most prodigiously for a couple of minutes, darting out in ever-expanding orbits before zeroing back in on the flowers. I guess when you’re that rare, you need to work hard to find a mate!
Perhaps the frosted elfin was more common on the Vineyard at some point in the past, though there are few records other than my own, and an important 1943 monograph on Vineyard and Nantucket lepidoptera considered it absent, or at least unrecorded, on the Vineyard.
I doubt the elfin is a new arrival. All barrens species are pretty good at dispersal, because their required habitat is invariably patchy and transient. But the frequency of movement between the Vineyard and the Cape must be very, very low for such a scarce and frail insect. I suspect this odd butterfly has persisted, probably mostly under human radar, since rising seas shut a younger version of the Vineyard off from the mainland.
A few weeks ago, I was ready to call the frosted elfin extirpated from Martha’s Vineyard. I still can’t feel optimistic about the prospects for such a small colony, or for others like it if there are any, to persist indefinitely. But the fact that frosted elfins are still here, despite decades and perhaps centuries of rarity, suggests that some finely tuned abilities in this insect are at work.