A recently posted video highlights among the more fascinating food-getting behaviours of the North American grizzly bear—and is also raising concerns familiar in our contemporary age of social-media hype about the potential impacts of drawing attention to sensitive wildlife habitat.
The video diary in question comes from wildlife filmmaker Casey Anderson and an arduous dayhike high up in the Absaroka Range of the Greater Yellowstone Ecosystem (GYE) in the western United States. Posted last month to his Endless Venture YouTube channel, it documents a notable ursid tradition that, in fact, is about to get back underway over the next couple of months in this Rocky Mountain fastness.
Anderson and cameraman Riley McClaughry observed a whopping 47 grizzlies before noon on one day in August 2025. "That even broke my all-time record, which included one day in Alaska at some of the hottest salmon streams where I saw 46 bears in one day," Anderson told the Wyoming news publication Cowboy State Daily.
And the dozens and dozens of grizzlies captured in Anderson's video were hanging out in a spectacular but austere landscape: barren rockfields and tundra slopes above timberline in the Absarokas, a rugged volcanic range that exceeds 13,000 feet and forms some of the roughest and most alpine scenery in the GYE.
Why were so many grizzly bears congregated in that windswept realm at the top of the world? Well, as tends to be the case with the omnivorous bruin, food was the draw—but food of rather a surprising character. All those grizzlies had mountaineered their way up to the talus in quest of an insect: the army cutworm moth, to be exact.
The army cutworm—also known by the catchall name "miller moth"—pursues a remarkable life history. Found widely in western North America, it comes into the world as a hungry (hungry) caterpillar down in lower elevations, including the vast croplands and rangelands of the Great Plains. The label "cutworm" refers to the larval form's habit of browsing plants—including commercial crops such as wheat and alfalfa—down to the ground.
Those squiggly caterpillars, which have overwintered upon hatching in the preceding fall, undergo metamorphosis in late spring, turning into the adult "miller moth" with a taste not for vegetative matter but for nectar. And those full-grown moths aren't fans of the summer heat that builds over the Plains. Their predilection for colder temperatures and their thirst for the sweet stuff of flowering plants compels army cutworms on an impressive migration, which in the case of Great Plains moths sees them wing hundreds of miles westward—and upslope—to the subalpine and alpine heights of the Rockies.
They seek out particular mountainscapes that deliver the magic mix of phenological window, flowering-plant diversity, and rockland shelter that summering moths require. Often, as in the high Absarokas, these are wind- and sun-exposed westerly and southwesterly slopes that don't accrue a heavy, long-lasting seasonal snowpack and melt out early. Supping on flower nectar at night, the heat-adverse moths clamber into the cool depths of scree and talus—the frost-shattered and gravity-riven gravel and boulder fields occupying much square footage in the high country—to snooze the sun-dazzled day at 10,000-plus-feet away.
As Robert Chaney noted last year for Mountain Journal, radar tracking shows that army cutworms summering in the GYE mountains may be coming from points as distant as Nebraska and the Northwest Territories of Canada.
Many Greater Yellowstone grizzlies—and, indeed, others in the Middle and Northern Rockies—muscle their way up above timberline in midsummer to "greet" these far-traveling bugs in the form of scarfing them down. Scarfing them down by the tens of thousands per day, in fact.
Why such a fuss, on the bears' parts? Well, no documented griz food packs such a caloric punch—close to eight kilocalories per gram—as the fraction-of-an-ounce army cutworm. Feeding among the ground-hugging blooms of the high country and snoozing in the rocks, the moth fattens up, becoming 70 percent or more lipid content. The army cutworm is, in the memorable description of researcher Erik Peterson (via Glacier National Park Conservancy), "bear butter." It's thus a standout among other top-quality, lotta-bang-for-your-buck bear nosh such as stone-pine nuts, acorns, fish, and meat.
Those army cutworm moths are gorging on nectar to fuel late-summer and early-fall flights back down to the lowlands in order to lay eggs. The grizzlies, in turn, are hoovering them up with an eye toward the next winter's big sleep. Indeed, the alpine aggregations of army cutworms in high summer coincides with the opening stretch, for grizzly bears, of hyperphagia: a period of intensified feeding aimed at really packing on the pounds before it's time to retire to dens and nap away (and fast away) the cold, snowy months.
When you consider that the army cutworm is so fat- and protein-rich, so utterly calorie-loaded; that it idylls on particular mountainsides in such dense concentrations; and that it's a heck of a lot easier to subdue than an elk or a bison (both important food sources for Greater Yellowstone grizzlies, but especially in the form of scavenged carrion), it's not surprising that so many bears go mountain-climbing in the summertime.
As the late, venerable grizzly researcher David Mattson explained, the bears most heavily feed on moths in the early morning and the evening, when army cutworms are returning to or embarking from their talus hideaways, respectively. They'll often nap in a daybed during midday and afternoon, when the moths are mainly deep down in the rocks. But, as Anderson's video shows, grizzlies will also go "mothing" into the daytime, employing their massive strength—powered by the shoulder muscles that form their distinctive humps—to lift up cobble and boulders and slurp up aestivating army cutworms.
Studies from both the Greater Yellowstone and Glacier National Park to the north in northwestern Montana show that goodly proportions of the total local grizzly population may spend extended periods of time in the summer slurping moths in very concentrated areas. The energy boost represented by each army cutworm, and the fact that the moths are so abundant within those high-elevation hotspots, help explain why bears gather in dense numbers this time of year, even though nothing short of a human being is so risky to a grizzly bear as another grizzly. That's certainly true of grizzly cubs, yet mother bears with litters rub shoulders with dangerous adult males (boars) among the "moth gardens" above timberline: all focused on inhaling as many of the bear-butter packets as possible, while they're available.
Indeed, while we think of grizzly bears as mainly solitary beasts, a variety of high-quality, concentrated, and seasonally available food resources see them congregating in close quarters. That includes salmon streams in places such as southern Alaska, northwestern British Columbia, and northeastern Russia as well as fruiting berry thickets and the supersized carcasses of beached whales.
Kerry Gunther, Yellowstone National Park's lead bear biologist, told the Union-Bulletin in 2022 that he marveled at the sight of a dozen-ish grizzlies eating moths in close quarters. "Seeing that many bears close together with no conflicts," he said, "they had a hierarchy worked out."
Grizzly bears, like most other ursids, happily eat a variety of insects and other invertebrates, and in fact the army cutworm isn't the only one that'll lure them up into higher elevations. As David Mattson pointed out, large numbers of the now-extinct grasshopper species called the Rocky Mountain locust, infamous for crop-hammering swarms in the U.S. West and Midwest, were sometimes wafted up into the mountains and entombed in snowfields and glaciers. There is more than one "Grasshopper Glacier" in the Rockies (including in the GYE), and historical reports suggest grizzlies likely scavenged—and perhaps still scavenge—melted-out locusts from these icy reservoirs.
There have been concerns for some years now about increasing human use of the high-elevation backcountry some Rocky Mountain grizzlies use in summer to feast on army cutworms. Climbers aiming to "bag" peaks are among the relatively few user groups who've typically been up in these mostly roadless, farflung heights, but there are indications that an increasing share of visitors may be making the journey specifically to see these bear aggregations, spurred on by social-media imagery and other online hype.
In a May 30 interview with Yellowstonian's Todd Wilkinson on this subject, retired longtime head of grizzly recovery for the U.S. Fish & Wildlife Service Chris Servheen said, "Bears at moth feeding sites have a human visitor problem. They are very vulnerable at these sites and the bears using them need solitude, and to be left alone. They have no visual cover and cannot find habitat security when people are there and approaching them. Increasing numbers of people at such sites will displace bears, increase stress levels for the bears that do not immediately run, and drive the most vulnerable bears off these sites."
One way to prevent people from stressing grizzlies seasonally congregating at critical food sources? Close off the area in the question. That's what the Confederated Salish and Kootenai Tribes have done in the wilderness area they manage in Montana's Mission Mountains, prohibiting human visitation during the summer moth-munching window in that mountain range (which happens to be one of the first places this grizzly foodway was scientifically documented).
In his video, Anderson, to his credit, does head off any response to it that might suggest Yellowstone grizzlies are somehow overpopulated, and stresses that the great bear—despite its ferocious reputation—isn't out to harm people.
The lofty moth gardens that Greater Yellowstone, Glacier, and some other grizzlies visit each summer might be thought of as sacred ground for bears, representing as they do such a crucial energy source. Given the stakes, they're surely best thought of as a high mountain gathering place for grizzly bears that, for the most part, humans—practicing a kind of etiquette of the wild—ought to leave be during that special window of time.
Natural World
Predator vs Prey
Predator vs Prey
