ABSAROKA RANGE—Andy Pils was silent as he scanned with his Vortex spotting scope, its lens focused on a steep, talus-covered mountainside some 2.5 miles away. 

Two weeks before, 10 grizzly bears were clustered together in the same area flipping rocks and lapping up moths, but on this early August day, zero grizzlies were visible. Pils soon realized why.

“Oh, shit,” Pils said. “I see a guy walking up there.”

Shoshone National Forest wildlife biologist Andy Pils scans a mountainside 2.5 miles away to look for grizzly bears feeding on an army cutworm moth site in August 2022. (Mike Koshmrl/WyoFile)

The longtime Shoshone National Forest wildlife biologist continued scanning. Actually, he said, there were two guys and a dog. It was late morning, and the canine-human crew was headed for a summit. 

“Those guys are in full view of that slope, so I’m sure all the bears moved off,” Pils said. “They’re right where we saw the bears two weeks ago.” 

A little bit later, Pils saw the displacement in real time. At 9:53 a.m., the first two grizzlies of the day entered his view. But they were too concerned about the hikers to bother with their calorie-dense bug breakfast.

“They’re actually running from the people,” Pils said. 

Within moments the two grizzlies trotted off in tandem and disappeared from sight. In Pils’ view, there was no question as to why. The peak-baggers and their canine companion had inadvertently spooked them off.  

Two hikers trek along the ridgeline of a mountain in the Absaroka Range on Aug. 8, 2022. (Mike Koshmrl/WyoFile)

Weeks before Pils installed a sign to warn of the bears’ presence, and the hikers should have been able to see the fleeing grizzlies if they were looking in the right direction, but that was tough to ascertain from this distance.

While a rarity to witness, the scene the biologist observed in the Absaroka Range high country wasn’t unexpected. Despite a reputation for standing their ground or becoming aggressive with humans, grizzly bears often flee from people. Grizzlies walked or ran away from people 80% of the time when former Montana State University graduate student Erika Nunlist observed 43 human-bear interactions at two army cutworm moth congregation sites in 2017 and ‘18

There’s no indication grizzly bears are altogether abandoning these nutrient-rich talus slopes — regionally, grizzly use of moths is actually increasing. But the Shoshone National Forest, which houses all the region’s known moth sites, has a management plan underway that sets out to keep the extraordinary alpine food source viable in the face of increasing human pressure. That’s partly as a result of more interest in peak bagging in the Absarokas. But it’s also because heaps of grizzlies gorging on thousands of insects a day out in the open draws spectators. Nearly 60% of the mountain travelers Nunlist surveyed at one high-use site for her study identified “photography” or “bear viewing” as a reason for being there. She understands the allure. 

At least 11 grizzly bears feed on army cutworm moths on a mountainside in the Greater Yellowstone Ecosystem in the summer of 2022. This image was captured during an aerial survey. (Mark Gocke/Wyoming Game and Fish Department)

“It’s just totally crazy,” Nunlist said. “When there’s 22 bears on a slope that’s maybe a football field or two [in size] and you can just watch them, it’s just really amazing. It never got old.” 

Management conundrum

Wildlife managers are in a tricky position. They don’t want to lead the public to moth sites, but at the same time they want to make people aware of the potential hazard of traveling near high densities of grizzlies and how human presence could impact feeding bears. 

“More people are figuring out ways to go in and take advantage of visible bears,” said Dan Thompson, large carnivore supervisor for the Wyoming Game and Fish Department. “We just don’t want bears to be harassed or anything to happen that can negatively impact the ecology of the bear.”

Pils is seeing similar trends that have him worried: “We’re getting more and more of these commercial filming requests.” 

“When there’s 22 bears on a slope that’s maybe a football field or two [in size] and you can just watch them, it’s just really amazing. It never got old.”

Erika Nunlist, ecologist and former montana state university grad student

Those requests are being denied until the national forest wraps up its moth site management plan, which should be out sometime in the next year. The forthcoming plan emanates from the Shoshone Forest’s 2015 Land Management Plan, which demands it. First, however, there were years of research to better understand moth bear ecology: Nunlist’s study of human interaction, another about what else grizzlies are eating near moth sites and a third study on the moths themselves. 

Army cutworm moths have been known as a grizzly food source in the Greater Yellowstone Ecosystem since the 1980s, but the ecology has been poorly understood, in part because the insects congregate in remote, mountainous country between 11,000 and 13,000 feet in elevation where studies are difficult to pull off. 

“So there’s still many unknowns about the ecology of the moths themselves, how bears are using these sights and how human use could affect these dynamics,” Pils told a crowd at the Draper Natural History Museum last winter. 

Shoshone National Forest wildlife biologist Andy Pils shovels rocks over the base of a warning sign alerting hikers to high densities of grizzly bears in the surrounding mountains. The sign was unspecific and didn’t mention moth sites in order to discourage people from crowding bears. (Mike Koshmrl/WyoFile)

Former Montana State University graduate student Clare Dittemore filled in some of the blanks about the moths, and she upended some traditional assumptions. It was conventionally believed the 1.5-inch-long moths — named for how they move as a fleet from one crop field to the next — migrated east to west, coming by the millions to the Rocky Mountains from the Great Plains. 

“The work that we did illustrates that they’re traveling north to Southwest as well,” Dittemore said, “which was really fascinating.”

Moth movement

Cutworm moths are too small to track with GPS, and so she used stable isotope analysis, which pinpoints the origin of the nutrients that make up the moths, to determine where they traveled from. Most moths, she found, were actually coming from Alberta and British Columbia. Others flew in from Montana, Idaho and Wyoming, while moths that came from the easternmost Great Plains were the least numerous.

“Moths are very capable of dispersing throughout the ecosystem,” Dittemore said. “Because they’re coming from such a wide variety of areas, this particular food source is probably protected against any regional declines of larval populations.”

That’s a good thing, because indications are that army cutworm moths are a vital grizzly food source. Up in the alpine where they feed on wildflower nectar at night, the moths pack on fat. Their body fat percentage can reach 83%, fuel for their own migration back to lower elevations and reproduction. It makes for great bear food. 

“Just the energetic component of this is pretty interesting,” Interagency Grizzly Bear Study Team leader Frank van Manen said. “They are little packets of lipids. There’s not a lot of weight in each insect, but on a per-gram basis, they are one of the highest calorie foods available that we’ve documented in the ecosystem.”

In this aerial image taken in summer 2022, a grizzly bear noses for army cutworm moths on the east side of the Greater Yellowstone Ecosystem. (Mark Gocke/Wyoming Game and Fish Department)

A typical grizzly might consume as many as 40,000 moths — good for 20,000 calories — per day, van Manen said. That’s equivalent to roughly 35 Big Macs, he said. They’re critical calories that grizzly bears need to gain weight and survive winter hibernation. 

Whether it’s through their incredible sense of smell or their memory, the moth bears congregate around the highest-density patches of bugs. 

“They are little packets of lipids.”

Interagency Grizzly Bear Study Team leader Frank van Manen

Even from miles away, the bruins’ game trail travel paths were visible in the talus that August day. 

“If you walk through that stuff, it’s amazing how excavated it is,” Pils said. “It’s like somebody went through it with a plow.” 

More and more grizzlies are catching on.

Federal biologists have spotted more and more grizzly bears using army cutworm moth aggregation areas during observation flights over the decades. (Courtesy/Interagency Grizzly Bear Study Team)

During the study team’s last available survey of moth sites, it logged 324 grizzly observations at 27 of the 35 known moth sites — the second-highest tally to date. Over the decades, as grizzlies have recovered to in excess of 1,000 animals in the Yellowstone ecosystem, more and more bears have visited the alpine moth sites, which generally keeps them away from people and out of harm’s way. That could be because of the decline of whitebark pine — another high-elevation food source — but the precise reasoning is unknown, van Manen said. 

Alpine griz 

Wildlife managers are also gathering new insights into how moth bears move about the landscape. In 2021, a Wyoming Game and Fish-contracted helicopter crew capitalized on grizzlies that dwell above the treeline. When bears were safely away from the steep, hazardous slopes, a number of them were tranquilized and fit with tracking collars.

“That’s a first for anyone in the Lower 48,” Thompson said of the aerial grizzly captures. “We caught 10 bears in three mornings. We didn’t know how well it would work, and it exceeded our expectations. In just over seven hours we caught the same amount of bears that we caught in almost a decade of [ground-based] backcountry trapping.” 

Data from those GPS collars is pouring in, and adding to existing location data from GPS-equipped moth bears that have been incidentally caught in the lowlands over the years. That data has taught van Manen and others that the moth bears typically move up around the middle of July.  

“They stay near those sites for the next two months,” van Manen said. 

Grizzlies of the high Absarokas are mostly filling their guts with gobs of moths, but that’s not all they’re eating. Typically, moth feeding is most productive from daybreak until around 11 a.m., but as the air and talus heats up the insects lose their lethargy and become trickier to catch. 

A grizzly bear hangs on a grassy ridgetop adjacent to an army cutworm moth site on the east side of the Greater Yellowstone Ecosystem. (Mark Gocke/Wyoming Game and Fish Department)

Kate Lozano, another former Montana State University grad student, investigated the moth bears’ summertime diets, finding through scat analyses that their primary alternative food source is a perennial flowering plant called biscuitroot. Her study also pinpointed where biscuitroot tends to grow: At the tops of ridgelines, which are often travel routes for people moving around the backcountry. 

“A lot of those sites that had biscuitroot had [grizzly bear] day beds as well,” Lozano said. 

The research, she said, suggests that the Shoshone Forest should cast a wider net over the landscape when it’s looking at addressing potential areas for human interaction at the most-visited moth sites. 

Forward thinking

Pils now has this data at his disposal as he’s pulling together a moth site management plan for the Shoshone Forest. It’s not likely to call for heavy-handed regulations. Where moths and grizzlies are aggregating deep in the North Absaroka and Washakie designated wilderness areas on seldom-visited slopes, there’s nothing to worry about, he said. 

“Most of these moth sites, all indications are human use is very light and there’s really no issue,” Pils said.

There are a handful of exceptions. 

“This one most prominently,” Pils said the morning of Aug. 8. “We’ll be trying to figure out some criteria, and what our options might be, should we feel compelled to start managing human use.” 

Potential closures, he said, would be a “really big deal for us.” 

“Seasonal restrictions on motorized vehicle use, that’s pretty well established,” Pils said. “But when you’re talking about restricting foot access into areas, that’s very different.” 

The forthcoming plan could also include some regulations for commercial outfitting and guiding around moth sites, he said, as well as prescribe monitoring and information and education efforts. Whatever policies get proposed will be run by the Shoshone Forest’s partners, including Game and Fish, the U.S. Fish and Wildlife Service and van Manen’s Interagency Grizzly Bear Study Team. 

“Ultimately, it’s district rangers and forest supervisors that make decisions,” Pils said. “And so it’s going to be a matter of figuring out what our leadership is comfortable with.” 

Two hikers and a dog hang on the summit of a peak in the Absaroka Range on Aug. 8, 2022. The hikers ascended the mountain via a ridgeline that was in sight of a well-used grizzly bear moth site and sent bears running for safety when they came into view. (Mike Koshmrl/WyoFile)

The hikers spotted by Pils reached their summit shortly after he watched the two bears boogie. The men, Park County locals, according to their pickup truck’s plates, lingered up top for nearly an hour.  

“There’s not many days when you can lounge on top like that,” Pils said. 

It wasn’t until 11:15 a.m. that the Shoshone National Forest biologist observed his first undisturbed moth-eating grizzly. Even through a spotting scope the animal was just a bear-shaped speck in the distance, but its lack of movement told Pils it was lapping up those lipid-filled moths. 

The desire to trek nearer and see the behavior in greater detail was tempered by already witnessing the unintended consequences of getting too close.    

A grizzly bear, faint in the upper left, feeds on moths on a talus-covered mountainside in the Absaroka Range on Aug. 8, 2022. (Mike Koshmrl/WyoFile)

Though he had observed it before, even Pils was entranced by the phenomenon.

“It’s pretty unlikely, isn’t it?” he said. When pressed about what he meant, the biologist clarified. 

“Just that this whole thing happened.” 

Mike Koshmrl

Mike Koshmrl reports from Jackson on state politics and Wyoming's natural resources. Prior to joining WyoFile, he spent nearly a decade covering the Greater Yellowstone Ecosystem’s wild places and creatures...

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  1. Army cutworm moths are certainly important food for Yellowstone area grizzly bears, but they’re also agricultural pests that are vulnerable to changing markets, drought and pesticides. Sadly, no trend data has been collected to indicate whether moths are increasing, decreasing or stable in number, which seems unfortunate given that they are household pests each spring in the Bighorn Basin as they migrate to the high peaks.

    If agency scientists think moth sites are important enough to grizzly bears to consider closing backcountry areas to the public, shouldn’t they also be important enough to collect long-term trend data on moth abundance? Bears have been counted by aerial surveys at moth sites for decades, but moth numbers remain unknown. In contrast, other important grizzly bear foods including whitebark pine, cutthroat trout and elk are monitored annually.

    There are also concerns that pesticides could accumulate in grizzly bears that are transported to the high country by moths. In Australia, bogong moths also migrate to the high country and were found to be transporting arsenic to high elevation wildlife from DDT, which had been banned decades previously in distant agricultural lands.

    I certainly support managing public lands to benefit wildlife conservation, but feel there are a lot of public land issues that deserve attention before closing remote backcountry areas to hikers.

    There are dozens of locations in Alaska where bears have become habituated to public viewing and they’ve proven to be compatible and even supportive of long term bear conservation. The challenge isn’t always keeping bears and humans apart. The challenge is keeping bears from accessing human foods.

    The vast majority of Yellowstone grizzlies that die each year each year are killed by humans and few, if any, of these deaths have occurred at moth sites. If we want to conserve grizzly bears, perhaps we should start by minimizing what is actually killing them currently.

  2. Now have hiked all over the Wyoming Wilderness both up high and down low thru the years. Thru the years of hiking saw much wildlife including many Grizzlies in the Absarokas. At one peak on oneday in the middle of summer saw near 12 Grizzlies doing their thing. Learned along time ago to give all the wildlife the room and space that they deserve. But this begs me to ask this question. What about all of these wildlife biologists and researchers? Sometimes in their research they do more harm then many people. How about protecting the bears from the researchers? The researchers are always talking about saving the wildlife from the common person. But again what about saving the wildlife from all of these wildlife biologists and researchers? Think at times they do more damage and harm then the regular person. All because they are a wildlife biologist or are a researcher does not give them the right to molest the wildlife in the name of science. If they make these areas off-limits to people, may it apply to these wildlife biologists and researchers equally.

  3. This is really easy- Limit access in the area during the appropriate time. This approach has worked well in Montana. High elevation food sources for the bears are diminishing and perhaps that is why we are seeing larger concentrations of bears utilizing this food source. There is plenty of high country to access recreationally. A targeted closure is a simple and effective solution.

  4. Grizzly bears congregating in alpine boulder fields to eat army cutworm moths is not a new phenomenon.
    Grizzlies have been observed congregating in the McDonald Peak area of Montana’s Mission Mountains to eat cutworm moths since the early 1920s. Additionally, closing a grizzly bear moth feeding area to human recreation is also not a new management tool. In 1981 a Special Grizzly Bear Conservation Zone was established around Mount McDonald in the Mission Mountains Tribal Wilderness. This zone was closed to all recreational use from July 15 to October 1 each year unless posted otherwise.
    Increasing documentation of grizzly bear use of alpine areas in the Greater Yellowstone may be a factor of a recent (1950s – 1970s) low grizzly population that was primarily limited to within Yellowstone National Park and a historically-dependable garbage food source. Grizzly feeding areas are largely a learned response passed down from female to cubs. As females with cubs utilize a cutworm resource in the alpine boulder fields, later use by adult cubs and their offspring will naturally result in increased numbers observed in these areas along with expansion into adjacent areas.

  5. A question I have never gotten a good answer for regarding Grizzlies feeding on alpine moth sites : why did we never hear about this phenomenon before the bear was listed?
    Growing up in Cody , I spent a lot of time listening to stories from old time outfitters, especially at my best friend’s dad’s taxidermy shop where outfitters, guides, and hunters congregated to s wap stories ( and have impromptu liar’s contests). From the 1950’s till the early 1970’s we rarely heard stories about any grizzly bears outside of Yellowstone Park, and nary a single word about bears feeding on talus slopes in the high country. Surely the sheep hunters that frequented the Absaroka alpine cirques scouting for rams would have seen this. But there are no anecdotes suggesting it. Before 1973, grizzly bears were legally hunted the same as black bears, without distinction . Still, a grizzly was a rare sight.
    I used to tramp all over the high country in the Shoshone River drainages in the 1970’s and 80’s and never saw bears ( of either specie) on the mothsites then – the same locations that are ‘established’ now and started becoming popular among ardent backpackers and select backcountry horsemen in the 1990’s when the secret got around by word of mouth. Sure, in the early 70’s a grizzly sighting was rare in the alpine away from Yellowstone proper. Then we started seeing more bears with each passing year above timberline, to the point where it was more uncommon to NOT see a griz up high. They also started seeing bears down low , near the developed places.
    Before anyone claims that spike in sightings was entirely due to there being an ever increasing grizzly bear population from being protected and pushed out of Yellowstone Park into surrounding wilderness and forest service /some BLM land, it is way more complicated . Bears outside Yellowstone at all elevations were also due to the loss of Cutthroat trout in Yellowstone Lake by the egregious introduction of Lake Trout ( Mackinaw) . Bears had to replace that easy source of fish protein with something else, which meant browsing new territory further afield. (That same loss of Cutthroat trout in Yellowstone Lake also decimated the Bald Eagle and Osprey numbers roosting on the shoreline) . Bears offset some of the loss of fishing with more time spent up at timberline devouring Whitebark pine nuts.

    All of my anecdotal eidence from the past 60 years points toward an interesting hypothesis. Grizzly bears ” discovered ” they could fatten up on moths , something they did not utilize much before , if at all. If seeing 25-35 different grizzlies on mothsites in the upper greybull River in a single dayw as new and startling to us humans, it was also relatively new to the grizzly bears. My late friend Bob Edgar was a historian and collected a lot of tales of the early white hunters in the Big Horn Basin / Absarokas going back to the early 19th century. From the 1880’s on there were lots of grizzly bear hunting tales told. Like Four Bear Creek being named for the great white hunter and Civil War veteran Col. William D. Pickett who lead Pickett’s Charge at the battle of Gettysburg . Pickett founded a ranch west of Meeteetse on the upper Greybull River. He was an avid trophy hunter. One day in 1883 he killed four grizzlies with his .50 caliber rifle from one stand. The last bear was shot point blank and fell on top of him, breaking one of his legs. Pickett was reputed to have killed more than a hundred bears in his time in Wyoming. Yet he never mentioned grizzlies feeding on moths, nor did anyone else of the era when grizz were plentiful and adamantly pursued for blood sport. The narratives of the old days became fresh again when Bob Edgar and I dug up the skeleton of a frontiersman named Phillip Vetter who had died after being attacked by a grizz along the Greybull River in 1892. Vetter scrawled a death letter describing his demise using his own blood for ink. For all the stories gathered about grizzlies in the past 200 years , all the mothsite stories are from the last 40 years only. Hmmm…

    Bottom Line is the cutthroat trout population has not rebounded; the Whitebark pine are threatened with extinction from climate change and blister rust affliction ; the Army Cutworm moths that migrate hundreds of miles to the summer Absarokas from the prairies , plains, and cultivated fields of the Midwest are also falling dramatically in numbers Probably from the effects of climate cjanmge and flagrant use of pesticides by mechanized farming. So for grizzlies wanting to fatten up on bugs in the Absaroka high country , a perfect storm is brewing. The Cutworm moth population will probably crash in coming years. Which is all the more reason to begin translocating grizzly bears from the greater Yellowstone ecosystem to the millions of acres of unoccupied bear habitat in Idaho, Montana, Colorado, Washington state, and even Utah and California. Wyoming Game & Fish and the US Fish and Wildlife Service need to totally overhaul grizzly management away from killing bears to relocating bears.

    My question stands: does anyone reading this have any anecdotes or evidence from before 1980 of grizzly bears making copious use of Army Cutworm mothsites in the Absarokas? If so, we need to get it into the records.

  6. It is fortunate that most of the grizzly bear moth sites are in the wilderness areas where access is limited to horse back and walking in – and – at high elevations above timberline. The mass of humanity can’t disturb the bears without considerable effort and that’s a good thing.

    1. Lee- you missed the part where the alpine grizzlies see and react to high country visitors on foot or horseback like 5-10 miles off. Further, the number of humans going up to the high country alpine cirques SPECIFICALLY to view grizzlies feed on moths has gone from near zero in the 1980’s to A LOT these days. I’ve seen entire strings of horseback riders heading out on day trips to do just that. Then there’s the weekend warrior backpackers marching to the mothsfields.
      It only takes one guy on horseback or a backpacker brigade to run off the bears long before they even see the bears, because the bears alreadsaw them and skeedaddled. Probably happens sooner if there’s a dog in the mix. They’re really never used to be anybody up there much in August besides early Bighorn sheep hunters scouting , who were discrete to keep their presence minimal.

      It does not take many human incursions at all to rightly say the mothsites are being overrun.

  7. In my 2014 objections to the Shoshone National Forest Plan, which proposed to create visitation management plans for grizzly bear moth feeding sites–apparently, based on the request of commercial film crews–I wrote “given the loss of whitebark pine in The Yellowstone Country, the Army Cutworm Moth is one of the few remaining high-value foods left to the grizzly. It is irresponsible to publicly reveal the location of any moth site, much less permit public access to them. Inevitably, once given access, some members of the public, or more likely commercial outfitters, would seek out and approach other sites, treating grizzlies feeding at the sites as profitable tourist attractions. The potential for disrupting grizzly feeding during this critical hyperphagic time as ignorant people get too close is too great. There is certainly a risk of injury or death for both grizzlies and people. Why extend the conflict problems of fall hunting season to late summer moth sites? It’s insane.”

    My recommendation to the Forest in 2014 was “rather than developing management plans for individual moth sites, which vary from year to year anyway, leaving open the possibility of public access, the Forest should write an absolute prohibition on public access to all moth sites, both known and developing. There is no need for individual site management plans. Let bears manage their own use of the sites. Requests from individuals for access, such as for film crews, should be dealt with on a case by case basis through limited special use permits.”

    Remember, I made these comments to the Forest eight years ago. I knew then where the moth site mentioned in this story is, but I have never broadcast the location because it’s too important for grizzly bears’ winter survival. As far as the bear is concerned, it’s top secret information.

    And yet, the location has become more known. No doubt, other feeding locations have become known as well. And so eight years later, as is always the case, the Forest has unfortunately taken the easy way out, doing nothing to protect the moth sites from ignorant tourists, and now not only do more and more people know where the moth sites are, they’re visiting them and pushing bears off the sites. And the Forest now wants to “manage” this disaster.

    This isn’t just insane, it’s willful negligence.

    So I’ll repeat the recommendation I made to the Forest eight years ago–close down public access to all moth sites. Period. They already do this for the areas where trapping operations are underway. I see the closure signs all the time. Why not do it for the moth grazing sites that are vitally important to the bears’ survival?

    Why not, indeed. Way to go, Forest Service.

    1. I agree…close these sites at the times moths are out and grizzlies are feeding. Montana Fish Wildlife and Parks closes areas in the Mission Wilderness when ladybugs are plenty and grizzlies are eating. Hikers can access these areas other times of the year.

    2. Whilst the grizzly bears are a listed T&E species with designated core habitat it seems they are afforded a certain level of protection within their core habitat which encompasses Yellowstone and the wilderness areas surrounding Yellowstone – the YES. Doesn’t the ESA give the Forest Service the legislated authority they need to enact seasonal closures within the core habitat??? What’s being implied here is that wilderness and national park designated status is not enough to protect the bears in their core habitat – people will violate the seemingly designated protection areas. I’ve commented about grizzlies out migrating onto private land and BLM land outside of their core habitat but this situation is just the opposite – people in migrating into the designated core habitat of the bears. Most of this intrusion is during hunting season when elk hunters pack into the back country at their own risk. So, how much human intrusion should be allowed before limitations are enacted?? It seems ironic that wilderness designation may not be enough in this situation.