I’ve been trying — with little success — to clean up a butterfly mystery. The subject is a homely-looking brown job that most people probably wouldn’t even recognize as a butterfly: the dreamy duskywing.
“Duskywing” is the common name for members of the drab genus Erynnis, a type of “spread-wing skipper” (so called because of their bouncing flight and because they often perch with their wings spread wide). Five duskywings occur on the Vineyard. Juvenal’s duskywing, especially, and sleepy are quite common; Horace’s and dreamy are rare; wild indigo duskywing falls in between in abundance. These are in general spring butterflies, though Horace’s and wild indigo have a second generation each year, with adults flying in July and August.
But more precise knowledge, especially of the less common species, is complicated by the fact that duskywings are notoriously hard to identify. Dreamy and sleepy stand apart as a somewhat distinctive pair, but telling one from the other is a subjective experience. Dreamy is smaller, more delicate, less clearly marked, more “frosted” with gray scaling on its wings. It tends to fly lower and slower than sleepy does, and they associate with different plants. But these are all impressionistic traits; the only objective mark that helps is the presence of elongated palps — protruding mouthparts — on dreamy, a tiny detail that is hard to see or photograph.
As a result, I don’t even trust my own records for dreamy duskywing, let alone those of observers of unknown skill! During my first few years on the Vineyard, I expected dreamy duskywing to be common here, based on my off-Island experience, and that surely resulted in some erroneous records. Eventually, growing more familiar with this genus, I realized that this species is rare here, and that may have led to the opposite problem: Wishful thinking, resulting in more incorrect records. These records may be valid — I’m a careful if imperfect observer, and the late-May and early-June dates are plausible — but there is no way to tell for sure.
Even the historical record for this species is blurry: Characterized as “not rare” on the Vineyard in a monograph published in 1943, dreamy is also described as having two generations per year, with the first appearing in the “early season.” This makes no biological sense, and must simply be wrong. Several specimens taken back in the 1930s, which I haven’t examined, reportedly bear April dates, and I suspect reflect misidentified sleepy duskywings.
The only dreamy record I’m really confident of was a single individual I photographed near the northwestern corner of Correllus State Forest on June 7, 2001, the only physical evidence for the presence of this species on the Vineyard since those dubious 1930s specimens. I’ve visited the area where I got that photograph regularly every spring, with no luck, and had reluctantly concluded that dreamy duskywing was gone from Martha’s Vineyard.
So I was happy to get unambiguous photographs of this species on May 26, in almost the exact same spot as the photo 15 years ago. The small size of this species, and especially its diagnostic, elongated palps, show clearly in the shots I obtained, though I only captured the drab underside of the wings.
On Saturday, I returned to the site of the 2001 and 2016 photos, a firebreak replete with young aspens in West Tisbury, hoping for more pictures. Butterflies were plentiful, and I had good looks at Juvenal’s and wild indigo duskywings. But dreamy? Well, I saw three good candidates — very small duskywings flitting low around aspen clumps — but all three darted away on wind gusts before I could get a decent look. I will have tried again at least once by the time this column appears. But given the history, I’m not optimistic; this species was surely present during my searches over the past decade and a half, but in such small numbers that I didn’t stumble onto one.
Why is dreamy duskywing so rare here? The answer is probably a shortage of suitable habitat. Like all butterflies, dreamy duskywings associate with particular food plants used by their caterpillars. While dreamy duskywing is known to use a number of larval hosts, including birch, willow, and locust, the most common one, and the one I think they use on the Vineyard, is aspen. The tree is a “colonizer,” popping up in clear-cuts, wind-throws, or burn scars, but disappearing as larger tree species get re-established. It may have flourished in sheep pastures, but as the Island reforested, aspen grew uncommon. And there may be long distances between one patch and the next. Given the weak flight of this butterfly, that adds up to very small, isolated populations, each one vulnerable and with little prospect of reinforcement from other populations.
Still, dreamy duskywing managed to persist in its one known Vineyard site for 15 years, rare enough to elude a motivated observer but common enough so males and females kept finding each other to mate. And there may be other, unknown populations out there. This is clearly a resourceful butterfly, and despite its rarity, it may hang on here for many years to come.