Mysterious moth-eating grizzlies have a people problem
August 25, 2022
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.”
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.
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.
“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.”
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.”
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.
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
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.”
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.
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.
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.
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.
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.
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.”
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.
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.”
WyoFile is an independent nonprofit news organization focused on Wyoming people, places and policy.