A butterfly made for harsh Martha’s Vineyard habitat

Edwards' hairstreaks are tiny — smaller than a nickel as shown here. They love milkweed and are often spotted at the end of a branch. — Photo by Matt Pelikan

The Vineyard sandplain is a harsh environment, with dry, sterile soil, a hot sun, frequent drought in the summer, a history of wildfire, and the occasional dousing with storm-driven salt spray for good measure. So the plants and animals that live here are hardy and resourceful, and many species have evolved highly specialized life histories to counter the habitat’s hostility.

One of the most specialized insects to inhabit the sandplain has its high season during July: Edwards’ hairstreak, named for the 19th-century biologist who first untangled the butterfly’s lifestyle, apparently requires the presence of both a specific kind of shrub and a specific kind of ant. But as rigorous as they are, these requirements often overlap, and both the shrub and the ant are common here. So Edwards’ hairstreak, while its distribution is rather patchy, is common to abundant across much of the Island.

Like its relatives, the Edwards’ hairstreak shows an intricate pattern of markings on its wings — orange and blue spots at the wing’s margin, plus a row of tiny white circles that are the best field mark. While its caterpillars may eat other species of oaks in other parts of the butterfly’s range, Edwards’ hairstreaks on the Vineyard invariably associate with scrub oak — a shrubby plant of open, dry, sandy areas. And the butterfly also occurs near colonies (visible as robust, sandy mounds) of the ant Formica integra (like most ants, this one has never received a common name).

Here’s how the odd inter-species partnership works, according to published sources, not my own observations: female Edwards’ hairstreaks, active during July, lay their eggs on the rough bark of scrub oak shrubs. The eggs lie dormant through the winter, prompted to hatch by the onset of warm weather in spring. The tiny, slug-like caterpillars feed on the buds and young leaves of scrub oak, growing rapidly. As both the caterpillar and the vegetation begin to mature, the larvae switch to a nocturnal feeding schedule, which helps protect them from discovery by predators.

They spend the days tended by ants in shelters the ants assemble from twigs and leaf litter. The ants help protect the caterpillars; in exchange, the caterpillars develop glands that produce a sweet liquid that the ants eat.

As bizarre as this partnership is, it turns out to be quite common in the insect world. Ants of many species tend insects of one kind or another, and among the hairstreaks (a largely tropical family of butterflies), partnerships with ants may be the rule rather than the exception. Documenting this behavior was the high point in the career of William Henry Edwards (1822-1909), one of America’s most famous insect biologists. It was in the case of the hairstreak that now bears his name that Edwards first demonstrated such a cooperative relationship in a butterfly.

Hairstreaks generally (eight, possibly nine species occur on the Vineyard) pose a real challenge for the observer. For one thing, these are small butterflies: perched with its wings raised over its back (the almost unvarying posture), a hairstreak is smaller than a nickel. And although a hairstreak, well seen, shows an intricate pattern of markings, the overall impression of the butterfly is dull gray, anything but eye-catching. In the air, a hairstreak is fast and erratic: keeping track of one as it rips around takes good eyes and a lot of practice.

Fortunately these insects, which are rather territorial, often return to the same perch after they’ve been disturbed. If you flush one, in other words, immediately freeze: the butterfly will quickly lose interest in you and often return to its perch in a few seconds. Moreover, the habits of these butterflies are quite predictable: Edwards’ hairstreaks, for example, generally choose an exposed perch, such as the tip of a branch extending into a clearing, almost always between two and four feet off the ground. When you’ve found the right habitat and the right food plant, you can often find the butterfly, too, by walking slowly and scanning for the profile of a hairstreak on a suitable perch.

Hairstreaks, with energy-intensive lifestyles, are also avid nectar-drinkers. So another way to find these butterflies is by checking the blossoms of good nectar sources such as milkweed or dogbane flowers in appropriate habitat. A promising and accessible place to look for hairstreaks is the large stand of common milkweed next to the pond at the state forest headquarters. But milkweed or butterfly weed in any part of the Island with scrub oak is likely to attract a few Edwards’ hairstreaks.

Uncommon in mainland Massachusetts, Edwards’ hairstreaks finds exactly what they like on the Vineyard, flourishing amid the taxing conditions of the sandplain. And the next few weeks furnish the short annual window during which a Vineyard observer can find and enjoy this species and its close relatives.