Wild Side: American copper

Evidence suggests these little butterflies were introduced here from Europe.

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An American copper (Lycaena phlaeas). —Matt Pelikan

As the days shorten and the butterfly season begins to wind down, the Vineyard appears to be enjoying an unusually good late-season showing by one of my favorite butterflies, the American copper, Lycaena phlaeas. As of this past weekend, the species was still going strong in my yard in Oak Bluffs, with as many as a half-dozen individuals sipping nectar from asters, courting, and chasing one another in territorial disputes. Last Sunday, amid mild autumnal conditions, I found individuals of this species in all six Vineyard towns.
A small butterfly, about the size of a penny when its wings are folded above its body, the American copper is colorful, active, and easy to find. Above, its forewings are orange with heavy black spotting, while the hindwing is gray with an orange rear border. A similar pattern, only muted and paler, appears on the underside of the wings. In flight, the orange seems to sparkle as the wings beat. A rapid flier, the American copper nevertheless tends not to move around much; a disturbed individual is likely to dash about for a few seconds, then return to the same leaf or flower it was previously perched on.
While it is a familiar sight in our region, though, referring to Lycaena phlaeas as the “American” copper is simply wrong. Butterflies that most biologists lump into this one species occur quite broadly in temperate, alpine, and subarctic habitats around the Northern Hemisphere. (I’ve observed this species as the “small copper” in England.) As many as two dozen subspecies have been described, differing subtly among each other in structure and coloration, occupying different geological areas, but integrating with each other where multiple subspecies are in contact. Four of these subspecies occupy northwestern portions of North America.
But mystery surrounds the origins of the copper population in Eastern North America — that is, our population. These butterflies are geographically isolated from the unarguably native counterparts farther west in the continent. Physically, our coppers resemble the Scandinavian subspecies more closely than they do the populations of northwestern North America. And the plants preferred as food by the caterpillars of our coppers are European ones (members of the genus Rumex, most notably sheep sorrel, Rumex acetosella, a ubiquitous weed of gardens and lawns).
That evidence all suggests that our “American” coppers are in fact non-native, introduced here along with their host plants by European colonists. But other entomologists point out that a number of native butterflies have switched from using native host plants to feeding on introduced ones. And our subspecies is not a perfect match for any of the European populations. So it may be impossible to determine with certainty whether our coppers are introduced or have been here all along.
Whatever its origin, the “American” copper is well established on the Vineyard, widespread and often very common. It is one of a relatively small number of butterflies that can reproduce successfully in a typical yard, though because its caterpillar food plant has become so universally naturalized, you can also find American coppers in less developed areas. And it has a very long season, running through three generations each year with, if warm weather lingers long enough, a partial fourth generation of adults flying in late fall.
My extreme dates for this species on the Vineyard are April 14 and Nov. 22; more typically, the species can be found here from late April into late October or early November. Its small size may make it hard to spot, but once you develop the knack for finding tiny butterflies, you’ll learn that the American copper ranks among the easiest butterflies to find on the Island.
The life history of the American copper is as interesting as the species’ taxonomy. Like other insects that live permanently in areas with cold winters, American coppers have developed a life stage (in this case, partially developed larva) that can withstand freezing temperatures. And like any such insect that goes through multiple generations in a single year, it has had to develop a way for larvae early in the season to complete their development, while allowing later larvae — the last generation hatched in the fall — to enter the dormant state known as diapause and overwinter.
Part of the secret appears to be day length. Shortening days in autumn increase the chances that a larva with creep under the leaf litter and enter diapause rather than continuing to develop to maturity. But this response is somewhat flexible, tempered by the effects of temperature. In mild falls, odds increase that some offspring of the year’s third generation will continue their development, emerging as a fourth generation of adults in late October or even November. It is this partial fourth generation that accounts for the smattering of November individuals — always reflecting pristine, freshly emerged adults — shown in my field records.
While future weather is never certain, it looks so far as if the American copper is finding congenial conditions this autumn, and conditions may be ripe for the species to be active well into November. And in any case, we have another couple of weeks or so in which to enjoy the current generation of adults.