I don’t know whether it’s a source of frustration or of ever-expanding interest. But the natural world never stands still, and most of the time, once you’ve answered a question about a wild species, it’s time to start answering it again. Your knowledge is always, always obsolete.
This principle is much on my mind right now as it applies to one of my favorite butterflies, the juniper hairstreak. Fifteen years ago, I felt I had a good read on this distinctive insect on the Vineyard: it was rapidly expanding from a beach-head in Oak Bluffs and Edgartown, and would soon occupy suitable habitat across the Island. But the expansion of the species on the Vineyard seems to have stalled, and I can’t quite tell if its numbers are plummeting, or if I’m not looking for it enough, or in the wrong places.
If you look where it occurs, the juniper hairstreak is fairly easy to find, and very much worth the effort. Like other hairstreaks, it’s a small butterfly, about the size of a dime when it perches with its wings folded over its back. Also, like other hairstreaks, its underside bears an elaborate pattern of stripes. But this species is distinctive because the basic ground color of its wings is olive — it is the Island’s only predominantly green butterfly, and fresh ones, especially, are stunningly beautiful.
The juniper hairstreak associates very closely with red cedar, though it uses relatives of this plant elsewhere across the butterfly’s continent-spanning range. Males spend most of their day perched on the tip of a twig, usually in the upper branches of a cedar. Females lay their eggs — tiny, ridged, pale green spheres — on twig tips as well, and the larvae, or caterpillars, spend their entire period of development eating cedar leaves. The species goes through two full generations each year on the Vineyard, with adults from the first one, which overwintered as pupae, active mostly in late April and May and adults from the second one flying in late July and early August.
Sometimes you can spot the profile of a juniper hairstreak perched on a twig-tip. During the appropriate season, I habitually scan cedar trees with my binoculars, looking for exactly this. But the best way to find this butterfly is to grab the highest branches you can reach on cedar trees and give a few hearty tugs. The main thing that happens is that a shower of prickly cedar leaves rains down on you, with much of the debris making its way scratchily down inside your shirt. But if you happen to be tugging on a cedar with hairstreaks on it, you’ll see them jolted into brief, swirling flight. Track them, wait for them to land, and enjoy binocular views of, arguably, our prettiest insect.
The juniper hairstreak’s history here is easy to recite since it’s quite short. Though looked for, this butterfly was unknown on the Vineyard until the mid-1990s, when a visiting naturalist named Paul Miliotis found some in a large stand of cedars on East Beach, Chappaquiddick. That alone was a bit of a puzzle: how long had they been there, and if they were recently arrived, why on earth did they arrive there instead of East or West Chop, both of which have plenty of cedars and both of which are closer to the mainland?
By 1998, my first spring on the Island, juniper hairstreaks proved to be all over Oak Bluffs; on some occasions, I found as many as 25 at a single location. I began shaking cedars elsewhere on the Vineyard, eventually finding this species as far south as Katama and as far west as West Tisbury. The pattern was one of expansion: where they were absent one year, they were present the next. But somewhere along the line, this pattern seems to have reversed. The cedars remain common, but it has been years since I’ve found the species outside of Oak Bluffs. Even there, it seems to be scarce at locations that formerly supported it in large numbers.
I found a juniper hairstreak this past weekend, probably an adult from the first generation, running late because of our protracted winter. I’ll check the area where I found it later in the season, hoping to find a solid population. But I’m wondering: was the abundance of this species circa 2000 a temporary thing? If so, how many times has the species expanded and contracted — possibly even disappeared and reappeared — on Martha’s Vineyard, and what prompts that pattern? How much of its current scarcity reflects a real decline in numbers, and how much reflects a shift of the population from places where I can find it to places where I can’t? This is a volatile butterfly, playing by its own rules, and I just can’t figure it out.