By Sidney Scarlett, Communications and Marketing Coordinator
There’s a particular kind of quiet above treeline in Glacier National Park in August. The wind moves rock dust across scree fields. Marmots whistle from somewhere out of sight. And if you happen to be standing on the right slope, at the right elevation, on the right kind of broken, sun-baked talus, you are standing on top of one of the most concentrated food sources in the Northern Rockies. You’d probably never know it.
Beneath your boots, tucked into the dark, cool spaces between fractured rock, millions of army cutworm moths are waiting out the day. They arrived here from hundreds of miles away, born on the wheat and alfalfa fields of the Great Plains, and they’ll spend the summer doing something almost nobody sees: hiding by day, feeding on alpine wildflower nectar by night, and quietly converting sugar into fat. By late summer, a single moth is roughly 70 percent lipid by dry weight, an energy density that, remarkably, makes them direct competition with elk and huckleberries for a grizzly bear’s attention.
This is the strange, under-told relationship at the center of a new study published in Biological Conservation by researchers from Washington State University, Glacier National Park and the University of Arkansas. It’s a paper about moths. It’s also, more consequentially, a paper about grizzly bears, about how little of the map actually matters to them in this high alpine region, and about the collision course between backcountry recreation and the tiny slivers of habitat that keep bears alive.
An insect most people never think about
Army cutworm moths (Euxoa auxiliaris) aren’t the most charismatic of creatures. They don’t have a fan club, outside of us dedicated bear-biology enthusiasts. Unless you’re an entomologist, a wheat farmer irritated by their larval stage, or a grizzly bear, you’ve probably never given them a second thought. But for decades, wildlife biologists working in the Yellowstone and Glacier areas have known that some grizzlies—particularly in years when whitebark pine seed crops fail or ungulate carcasses are scarce—travel enormous vertical distances to reach high alpine talus slopes and dig for moths. It’s some of the most physically demanding foraging behavior grizzlies exhibit anywhere in their range, and some individual bears may get the majority of their seasonal calories from an insect the size of a fingernail.
What’s been missing, until now, is a rigorous, broad-scale picture of where exactly this is happening, and why moths choose the rock piles they do.
To answer the first question, researchers didn’t just watch for bears. They went looking for the moths themselves, one shovel-full of rock at a time. Over the summer of 2020, field crews hiked into 18 survey units across Glacier and hand-excavated small pits in the talus, digging, as the study’s authors put it, in a manner that approximates how a grizzly bear forages: clearing rock down to bedrock or soil in patches roughly a meter across, then checking for moths hiding in the cavities. They repeated this at hundreds of plots, logging not just presence or absence, but the physical characteristics of each site, including rock size, talus depth, slope position, and underlying geology.
Meanwhile, a second team took to the air. Flying before dawn in a helicopter equipped with a high-definition, thermal-imaging camera, biologists surveyed the same alpine terrain across two summers, documenting grizzly bears as heat signatures against cold rock, then confirming foraging behavior on video. It’s a labor- and cost-intensive way to study wildlife, but it solved a problem that has dogged bear research for years: how do you reliably document an animal that spends its summer in some of the most remote, vertical, and inaccessible terrain in the Lower 48?
Why moths pick the rocks they do
The results describe a habitat requirement that is almost absurdly narrow. Moths weren’t just anywhere in the alpine zone. They were concentrated in talus that offered a very specific combination of features: rocks large enough to leave generous air gaps underneath, talus deep enough to buffer daytime heat, sites more than 10 meters from exposed bedrock, and a preference for the blockier fracture patterns of limestone and igneous rock over the flakier rubble produced by mudstone. Elevation mattered too. The odds of finding moths climbed sharply above roughly 2,800 meters (about 9,200 feet), and sites with even faint nearby vegetation, a proxy for nectar availability, were more likely to host moths as well.
In other words: moths need rock piles that behave like a walk-in cooler, deep enough and coarse enough that temperatures a few inches below the surface barely fluctuate, even while the exposed rock above bakes in August sun. It’s a remarkably specific architectural requirement, built by geology and erosion over centuries, and it turns out to be increasingly rare. When researchers mapped this habitat across all of Glacier’s roughly 4,000 square kilometers, suitable moth habitat covered just 12 square kilometers, three-tenths of one percent of the park.
When you map the moths, you map the bears
Here’s the part that should make land managers sit up. When researchers layered their grizzly bear observations over the moth habitat model, the overlap wasn’t incidental. It was, statistically, the whole story. A model using Army Cutworm Moth occurrence alone predicted grizzly bear foraging locations nearly as well as a far more complex model incorporating slope, terrain ruggedness, solar exposure, and vegetation combined. Moth habitat wasn’t one factor among many explaining where bears forage in this area. It was, overwhelmingly, the factor.
That finding reframes what “grizzly bear habitat” even means in these landscapes. It’s tempting to think of habitat protection in broad strokes: big blocks of undeveloped land, connectivity corridors, roadless areas. All of that matters enormously. But this study is a reminder that within those broad blocks, there can be critical pinch points, tiny, specific, almost invisible places where an entire summer’s worth of a bear’s energy budget gets decided. Lose access to a handful of these talus slopes, and you haven’t just inconvenienced a bear. You may have removed its best shot at putting on fat from a non-human resource (such as unsecured garbage) before a den season that can last five months or more, especially as climate change renders other lower-elevation food sources’ availability increasingly unpredictable.
The recreation collision
This is where the science stops being an interesting footnote about insects and starts presenting a management problem. Glacier National Park has absorbed roughly a million additional annual visitors since the mid-1990s, and a growing share of that traffic isn’t staying on valley-bottom boardwalks. It’s pushing into the alpine, onto scrambles and scree routes and off-trail objectives that, for hikers and climbers, look like blank rock. For a grizzly bear trying to feed undisturbed at dawn, those same slopes are a pantry.
The timing compounds the conflict. Peak moth-foraging season for bears runs through the exact weeks when alpine recreation peaks: long, dry days in late July, August, and September, when talus slopes are free of snow and scramblers are most active. The study’s authors documented grizzlies fleeing short distances in response to survey helicopters passing overhead in a small fraction of observations. It’s not hard to imagine that a hiker cresting a ridgeline unannounced, at close range, produces a far more disruptive encounter, both for the bear and for the person.
None of this necessarily means closing the high country. But it does mean that vague, park-wide messaging about “bear activity” undersells the precision of the problem, and probably the precision of the solution. If moth-foraging habitat occupies less than one percent of a landscape and can now be mapped with reasonable confidence, land managers have something increasingly rare in conservation: a genuinely surgical option. Seasonal, site-specific closures or use restrictions, rather than blanket alpine advisories, could meaningfully reduce disturbance at exactly the places it matters most, while leaving the vast majority of backcountry terrain open. There’s already a working example not far away: the Confederated Salish and Kootenai Tribes close the McDonald Peak Grizzly Bear Management Zone in the Mission Mountains Tribal Wilderness to all recreational use every year from July 15 to October 1, precisely because grizzlies concentrate there to feed on army cutworm moths.
A small insect, a large question
It’s worth sitting with how easy this relationship would be to overlook. Nobody’s writing grant proposals about protecting moths for their own sake, and nobody’s putting up trailhead signage warning hikers about insect aggregations. But the research points toward something bigger than one insect or one park. Grizzly bears in the Northern Continental Divide and Greater Yellowstone ecosystems are, by most measures, a conservation success story: populations that were pushed to the brink in the 20th century have grown and expanded their range. That recovery has always depended on unglamorous, often overlooked infrastructure — whitebark pine stands, spawning cutthroat trout, army cutworm moths in a rock pile most of us will never notice we’re walking over.
As alpine recreation continues to grow and climate change reshapes snowpack, moisture, and the very microclimates that make these talus slopes livable for moths in the first place, that infrastructure is going to need active attention, not just goodwill. The good news is that this study hands managers something concrete to work with: a map, a threshold, a testable hypothesis about where disturbance does the most damage. Whether that translates into meaningfully different rules on the ground in Glacier, or in the moth-rich talus of the Greater Yellowstone Ecosystem, will say a lot about whether conservation science actually gets to shape how we recreate in bear country, or just gets filed away as one more interesting paper about a bear’s remarkable, improbable diet.