Wild Side: Changing seasons

Tracking the last of this year’s butterflies.

As autumn progresses and the natural world begins shutting down for winter, I like to play a game of “the last X of 2020.” The last chimney swift. The last meadow katydid. The last garter snake.

You never really know it’s the last one, of course, until a few weeks have passed and you haven’t seen another one. But this game keeps me mindful of the process of seasonal change. And saying goodbye to my friends in the natural world seems to be part of my own psychological process of getting ready for winter.

A dependable “last” in early October is the Eastern tailed-blue butterfly, Cupido comyntas. A tiny insect, white with dark spotting below and a topside that is either bright blue (males) or dull gray (females), this is one of the Vineyard’s most common and widespread butterflies. The eponymous tails are delicate threads extending from the margin of the hindwings, one thread per wing. (They often break off, though, so don’t bank too heavily on them as a field mark.)

This is a season-long species in southern New England, with adults first appearing in mid-April (April 14 is my earliest record), and three, possibly four generations coming and going into autumn. I’ve recorded this species twice on Oct. 19, but its numbers have usually waned enough by the third week of September to make the species challenging to find.

The “Eastern” in its name is a bit misleading. Yes, the range of this butterfly is primarily Eastern, from the Gulf of Mexico north into southernmost Canada. But this butterfly occurs south into Mexico and Central America, and populations, probably introduced unintentionally by human travel and commerce, also occur in California, the Pacific Northwest, and the deep Southwest. Except for the Rocky Mountains, where it is absent, the Eastern tailed-blue now occurs across the continent.

Adult Eastern tailed-blues vary in size, probably depending on how well they fed during their larval stage. A big one, perched as usual with its wings folded up over its back, might be the size of a thumbnail; a small one, more like the nail on your pinky finger. A rather weak-flying species, this butterfly is usually found close to the ground, patrolling low vegetation in nearly any kind of open habitat.

Caterpillars of this species eat a wide range of members of the legume family. The specific caterpillar host plants vary regionally; on the Vineyard, Eastern tailed-blues associate closely with bush-clovers (Lespedeza) and tick trefoil. Female blues lay their eggs on the flower heads of these plants, and I suspect that it is mostly on the flowers, rather than the leaves, that the caterpillars feed.

Adults that emerge in late April and early May give rise to another generation in late June; a third generation takes flight in August. My records suggest a subtle, final pulse of activity by fresh-looking individuals in late September; this may reflect a partial fourth generation. Between these generations, adults may be scarce or entirely absent (though the species is still out there, as eggs or larvae that are hard to find).

The final generation of the year waits until spring to complete its development. Mature caterpillars survive the winter tucked into crevices on their food plants (some accounts suggest that caterpillars may overwinter inside seed pods). In early spring, these individuals rouse themselves, feed a bit more, pupate, and then emerge as adults to start the process over. Very little warmth is needed to get the ball rolling in the spring; this is consistently one of our first butterfly species to be on the wing.

In contrast to some butterfly species, the Eastern tailed-blue is not at all fussy about its habitat. Bush-clovers occur quite widely on the Vineyard, and Eastern tailed-blues can be found nearly anywhere these favored larval food plants occur. Fire lanes in Correllus State Forest are a good place to find Eastern tailed-blues. The Land Bank’s Old County Arboretum, more old meadow than arboretum, situated across the road from the West Tisbury School, supports a robust population; if you hit the peak of a flight period there, you can sometimes see hundreds of this tiny butterfly puttering around near the ground.

Small populations of the caterpillar food plants seem sufficient to support the butterfly, and so this species can also be found along roadsides, in yards, and around the edges of farm fields. Its small size and low, slow flight may make this butterfly hard to detect. But the Eastern tailed-blue ranks among our most ubiquitous butterflies.

Until mid-October, that is. That last, partial generation fades away as the nights get frosty, and these butterflies grow harder and harder to find. And then one day, though you may not know it at the time, you see your last one for the year.