BOXFORD — The butterfly tracker leaves the tree-shaded trail and walks among knee-high plants in a sunny clearing on Weir Hill in North Andover.

Ecologist Russ Hopping points to the horizon. Mt. Wachusett is visible on clear days, and beyond it, on especially clear days, Mt. Monadnock, he says.

But Hopping, with fast-focusing binoculars hanging from his neck, is here for the nearby views.

He leans over and touches a wild indigo stem.

If the thumbnail-sized frosted elfin butterfly is anywhere on this 300-foot hill, it will be here, he says. Indigo is the only local plant on which the female lays her eggs.

In the same breath, he says he’d have to be wildly lucky to spy a fluttering, brown-and-silver-winged frosted elfin, and would register the sighting with the U.S. Fish and Wildlife Service.

“They are absolutely rare,” says Hopping, who grew up in Boxford excited by the natural world. He still lives there and still is excited by nature and its ways.

For 27 years he has been an ecologist with The Trustees of Reservations, taking the pulse of the nonprofit’s 117 conservation and historic preservation properties in the state.

The frosted elfin butterfly is not only rare but has an exceedingly short day in the sun — two or three weeks to do what it can to give future frosted elfins their little moment.

This afternoon in June is getting late in the season for a sighting.

A group of volunteers with the Fish and Wildlife Service were out earlier in the day and left their calling card nearby, a pink ribbon tied to an indigo plant — marking the patch as searched. In mid-May, prime time for elfin seeking, a group of nine volunteers spent the day searching for them on Weir Hill and reported seeing 12.

A field of predators

People aren’t the only ones looking for elfin, however.

Dragonflies, rising and falling in their herky-jerky flight, look for butterflies to eat. Birds, including the resident eastern phoebe, dine on elfin, too.

The butterflies may end up deemed threatened or on the endangered species list. The Fish and Wildlife Service is taking stock of their numbers, surveying the insect’s range in advance of a 2023 decision on their federal status.

Butterfly watchers see frosted elfin far less than other species. Weir Hill is the only Trustees property where the butterfly is currently documented as being in existence.

When pressed for a guess as to the hill’s elfin population, Hopping says maybe 50.

Some four years ago a Tufts University study caught and tagged frosted elfin here, estimating the population at 70.

They are, in fact, a mystery. A century ago butterfly collectors overestimated their numbers by confusing them with other elfins.

What distinguishes the frosted variety is a small black spot.

Meagan Racey of the Fish and Wildlife Service says the “condition of nearly 90% of its populations across the eastern U.S. — including sites in North Andover — is unknown.”

The butterfly is found in pockets that rely on fire, she says.

That includes Weir Hill. The healthy, sloping field that Hopping searches was scorched in a controlled burn he arranged in recent years. It now supports an array of plants beyond the indigo, including wild blueberry, dewberry and huckleberry.

Controlled burns

Just a week earlier, with local firefighters present, Hopping and The Trustees set fire to 17 acres on the hill nearby.

American Indians used fire here to enrich the plant-life, making the grounds better for hunting and berries and growing crops. The frosted elfin was undoubtedly here on the double drumlin, a north-south geological hill of gravelly soil formed 12,000 to 15,000 years ago during the last Ice Age.

“I think of the frosted elfin as an indicator species,” Hopping says.

They indicate for a community of mixed plants, birds and animals that require a certain habitat.

He’s fascinated by the rare, one-host species idea, he says. And the single plant that serves as host relies on fire. How did such an arrangement evolve, he wonders.

The elfin eggs become green, camouflaged caterpillars and feed on indigo stems before burrowing at the base and becoming a buried chrysalis over the winter. They emerge in spring, pump up their wings and flutter, mate and die within a couple weeks, the females leaving eggs on indigo buds to let the cycle start anew.

Weir Hill is the only property of the 117 that The Trustees manage in Massachusetts where the elfin has currently been documented. The state is home to 120 species of butterflies, and on some of The Trustees’ property pretty much all of the species can be found.

Weir Hill is home to about half the 120 species, says Hopping.

This Trustees property has been restored to its centuries-old condition, before it was largely cleared for timber and sheep pastures. It is again forested, lots of black oak and hickory stands, and interspersed with clearings opened by fire.

Searching

On this day, a fine early June afternoon, Hopping sees tiger swallowtail and black swallowtail butterflies.

He sees dragonflies and birds, including a kingfisher, bluebird, robin and downy woodpecker.

He points high above at a Cooper’s hawk. Its flight zig-zags, the bird-eating raptor being harassed by an eastern kingbird.

There would be no frosted elfin sightings on this day.

But Hopping did get to see the progress in the burn area. Already, little shoots of grass are growing.

Before long wild indigo will sprout. And, perhaps, fluttering among its leaves will be elfin, the frosted variety.