Wild Side: The pearl crescent

Attractive, and one of the only butterflies still around in October.

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A female pearl crescent. —Matt Pelikan

Mid-October is nearing the end of the season for butterflies on the Vineyard (indeed, insect life of all kinds is winding down for the winter). But there is still time to catch up with one of our most attractive butterflies.

The pearl crescent, Phyciodes tharos, is named, regrettably, for an obscure characteristic: A whitish (“pearly,” to some entomologist’s vivid imagination) half-moon set near the edge of the mottled underside of the hindwing. It takes a close look at a cooperative individual to see it.

But among the species that occur on Martha’s Vineyard, the pearl crescent is easily recognized without recourse to subtle details. An inch or perhaps a bit more across its open wings, the pearl crescent has a bright orange ground color to the tops of its wings (what you usually see), laced with in intricate web of black lines and spots.

You can readily determine the sex of a pearl crescent at a quick glance. The basic pattern of the top sides of the wings is the same for both males and females; but in the former, the black lacework on the wings is much more open than on a female. The difference can be pronounced, and one might be excused for mistaking males and females as belonging to two related but distinct species.

Like all butterflies, the pearl crescent’s distribution is governed largely by the plants its larvae, or caterpillars, feed on. In the case of this butterfly, that means asters: any of several of that huge and diverse group of wildflowers. Adult female pearl crescents lay their eggs on the undersides of aster leaves.

The ubiquity of its larval host plants means that the pearl crescent is very much a generalist in terms of habitat use. The species surely can’t breed in a manicured lawn (though it might stop by to take nectar from flowers). But any setting with a decent mix of native wildflowers present probably supports pearl crescents at least some years: Roadsides, pastures, and farm field edges, woodland clearings, and grassland are among the habitats where this species breeds on the Vineyard.

Pearl crescent has one of the longest seasons of any butterfly known on the Vineyard. My records suggest that the species runs through three generations each year, producing three distinct peaks in abundance: in spring (about the second week of May into mid-June); midsummer (mid-July into mid-August); and early fall (early September into early October).

A few hardy individuals hang on into mid-October. I’ve never seen this species in November on the Vineyard, but as our growing seasons extend with the changing climate, it seems like just a matter of time until somebody establishes a November record. When that occurs, it might reflect either a long-lived individual that just hasn’t gotten cold enough to die yet (such an individual would be visibly worn). Or it may reflect a partial fourth generation (in which case the butterfly will be pristine).

Butterflies — indeed in insects general — often vary widely in abundance from year to year, or generation to generation. And the pearl crescent illustrates this beautifully; I can’t think of another butterfly that shows such dramatic variation in numbers.

At one extreme, this species can be almost inescapable at the peak of one of its flight periods, turning up in nearly any kind of reasonable habitat, often in huge numbers, and wandering widely enough so that individuals turn up in surprising spots (puttering down Circuit Avenue, for example). I recall counting 250 pearl crescents at Nat’s Farm, along Old County Road in West Tisbury; since I walked only a small portion of that vast field, the number of crescents present was surely many times that figure.

At the other extreme, there have been entire years when the species simply can’t be found in respectable numbers, even at points in the year when you’d expect it to be abundant. Adult pearl crescents can also be totally absent between generations, when all the individuals present are in larval form, feeding on asters.

Predation or, more likely, parasitism might have something to do with this variability, but I tend to think the main explanation is weather. Periods of drought are often associated with low numbers of pearl crescents (and often other butterflies, as well); it seems likely that if the weather exceeds a certain limit for heat and dryness, pearl crescent caterpillars die in droves, either from dehydration or because the drought slows plant growth and renders the caterpillar’s food plants tough and indigestible.

But these swings in numbers appear to be perfectly normal for this species, which always rebounds quickly from a failed generation. Half-grown larvae — however many survive — are preparing to overwinter now, to emerge next spring. And if you are lucky, you may still catch up with an adult of this interesting insect.