A grizzly bear forages for army cutworm moths in alpine talus in the mountains east of Yellowstone National Park. Photo by Mark Gocke/Wyoming Game and Fish Department
by Robert Chaney
For decades, biologists have known that grizzly bears climb Greater Yellowstone’s highest mountains to gorge on one of the bears’ more surprising food sources: the army cutworm moth. Now researchers are starting to understand that story from the moths’ perspective.
The grizzlies weigh an average 400 pounds. An army cutworm moth weighs in at 0.003 ounces. But each moth packs half a calorie in body fat. Multiply that by a few billion and concentrate the lot on a few scree-covered peaks, and it becomes clear why the king of the food chain would risk spending weeks of high summer with its worst natural enemies: other grizzlies. It’s the best all-you-can-eat buffet around.
“Of the 266 foods we know bears eat, moths are the most calorie-dense,” said Taylor Kennedy, who just defended her entomology master’s thesis at Montana State University on April 25. “We know the food source is important, but we don’t know how much is available. This is the first time we’ve been able to get this perspective of movements coming into the GYE, opposed to the direct sampling that has been done before.”
Kennedy has a jar of grizzly bear scat near her desk with 6,000 army cutworm moth remains mixed in. That’s the energy equivalent of a half-dozen Quarter Pounders with Cheese, or about one-eighth of what a mature grizzly will eat in a day of moth-munching.
Performing that kind of direct sampling is both dangerous and difficult. It requires setting a base camp around 10,000 feet elevation, getting up at 4 a.m. to climb another 2,000 feet to the feeding sites, collecting the feces of an apex predator that doesn’t like to be disturbed, and then scooting back down before 3 p.m., when the lighting bolts start striking.
That work reveals a lot about what’s going on at ground level. But it’s also a little like studying food in a grocery store, where milk comes in cartons and meat appears on plastic trays. The bigger question is: How did all that food get there?
Six thousand army cutworm moths is the energy equivalent of a half-dozen Quarter Pounders with Cheese, or about one-eighth of what a mature grizzly will eat in a day of moth-munching.
“We got pulled into this about six years ago,” said Bob Peterson, who oversees Kennedy as director of MSU’s Department of Land Resources and Environmental Sciences. “Dan Tyers with the Interagency Grizzly Bear Study Team came to us and said, ‘We have these huge moth aggregation sites in the GYE. We do flyovers, and there’s nine bears on one peak. We know a hell of a lot about bears, but we don’t know what’s going on about moths. When do they come? When do they return? What do they do when they’re here?”
In other words, it’s obvious why the bears go to the mountains. But what’s in it for the moths?
To answer that, Peterson, Kennedy and several other entomologists set up two portable radar stations on the edge of the
The army cutworm moth weighs 0.003 ounces. It's also the grizzly's most calorie-dense food. Photo courtesy Bob Peterson
Rocky Mountains east of Yellowstone National Park. Each radar scanned a 2-square-mile slice of sky. In two sampling seasons, they observed hundreds of millions of moths coming through.
“When you multiply that out, and think about them coming from Canada to Mexico all along the Rocky Mountain Front, we’re talking about billions of calories entering these ecosystems,” Peterson said. “Individuals from the Great Plains are connecting to the Rocky Mountain West. These bioregions are connected — energy is flowing between these two bioregions.”
Over the past six years, Peterson’s labmates have built a picture of the army cutworm moth’s life cycle. Clare Dittemore has located the places the moths flew in from. Katerina Lozano exposed the other food sources attracting grizzlies to the peaks.
Erika Nunlist observed how grizzlies and humans use the feeding areas, and how that might be improved for both species. And Kennedy focused on greater detail about moth flight from the radar scans. They’re presenting all their combined work at the Interagency Grizzly Bear Committee’s Yellowstone Ecosystem meeting on May 1.
Ironically for a creature with such massive prevalence, the moths haven’t attracted much human attention because they don’t do much that people notice. As larval caterpillars in the Great Plains of central North America, they nibble a wide variety of plants off at soil level, hence the name cutworm. But they don’t specialize on any crop, so they aren’t targeted as agricultural pests like locusts or gypsy moths.
When they metamorphose into moths, their diet switches to flower nectar. But prairie flowers have short bloom times and little grassland habitat. And the moths don’t like heat. So around early June, they seek high-altitude meadows.
This migration can cover 1,000 miles. Using stable isotope tracking methods, entomologists have mapped Greater Yellowstone moth origins from Nebraska to Canada’s Northwest Territories. Then the radar revealed new insights to the moths’ capabilities.
Not all mountain meadows are snow-free and wildflower-ready when the new moths have exhausted their prairie nectar. So they can’t simply ride a prevailing wind to their nearest peak. Instead, even though they’re barely a centimeter wide with a 4-centimeter wingspan, they navigate to specific peaks that meet their climate criteria.
U.S. Geological Surveyresearch ecologist Robb Diehl helped Kennedy and Peterson develop the radar scans used to study the moths. A migratory bird specialist, Diehl has been using portable radars to expand the concept of where animals live.
Watch the video below: "Detecting the Migration of Army Cutworm Moths with Radar"
“People like me consider the airspace as a habitat,” Diehl said. “It’s not just empty space. Imagine the open ocean. It’s populated by fish in a 3-D environment. They’re riding currents, hunting for food. The same thing is happening in airspace. It’s just that its properties are mostly invisible to us.”
Those properties include wind channels and magnetic fields, varying temperature and pressure gradients, dirt and seeds and floating creatures (spiders can’t fly but use webs to drift for miles).
Diehl’s radars are similar to those mounted on ships for monitoring weather patterns. They’re sensitive enough to make out details of individual drops of rain so they can easily detect flocks of birds and clouds of dust, or swarms of bugs and the bats chasing them.
Weather forecasters go to great lengths to filter out those “biologicals” from their storm tracks. Diehl reverses the process, pulling back the curtains of rain to reveal the creatures hiding inside. One scientist’s noise is another’s signal.
“Without interdisciplinary experience, you don’t even know what questions to ask,” said Diehl whose own speciality goes by the recently coined term aeroecology. “You need a software developer to capture wing-beat frequency. There’s the physics of radio transmission and statistical analysis. You have to be aware enough to ask the questions and bring on the skill sets.”
The discovery about 40 years ago that army cutworm moths were a crucial part of some bears’ seasonal food supply spurred a scramble to understand how that relationship worked.
One lucky factor for the army cutworm moth is their migration happens when few other things are on the move. The continental flyways of ducks and geese heading to northern breeding grounds clear out by mid-April. So when Kennedy set up two of Diehl’s radars in June near Powell and Meeteetse, Wyoming, they recorded mostly bugs. But the recordings were sensitive enough to differentiate a moth with a 4-centimeter wingspan from a midge barely 4 millimeters wide.
Through Kennedy’s investigation, the radar revealed even more details. It could tease out the difference in wingbeat frequency between moths and midges. It could tell how a moth’s body was oriented relative to the direction it traveled. For example, if it’s bearing southeast but its body is pointing south, it’s using what airplane pilots call “crabbing” to travel sideways while the wind is pushing it forward.
“That’s one of the findings I’m most excited about,” Kennedy said. It indicates the moths may use features such as magnetic fields or air pressure shifts to navigate to the habitat they seek.
A grizzly bear hangs on a grassy ridgetop adjacent to an army cutworm moth site on the east side of the Greater Yellowstone Ecosystem. Photo by Mark Gocke/Wyoming Game and Fish Department
Nailing down such basic science of moth behavior helps build the applied science of grizzly bear recovery. Greater Yellowstone grizzlies have been protected under the Endangered Species Act since 1975. In order to recover them, wildlife managers must ensure the bears have enough habitat producing enough food for them to thrive. The discovery about 40 years ago that army cutworm moths were a crucial part of some bears’ seasonal food supply spurred a scramble to understand how that relationship worked.
One of the first things bear biologists learned was that the moths were so prolific and available that normally solitary grizzlies would share territory with many other bears on the bug-covered scree slopes. That presented an attractive nuisance as increasing numbers of human hikers and climbers began wandering into crowds of grizzly bears. In 2022, a climber on Wyoming’s Francs Peak was severely mauled by a grizzly that was believed to be feeding on moths. In Montana, the Confederated Salish and Kootenai Tribes close some of the most popular parts of their Mission Mountain Tribal Wilderness in July and August to avoid disturbing moth-gorging grizzlies.
Robb Diehl’s radars are sensitive enough to make out details of individual drops of rain so they can easily detect flocks of birds and clouds of dust, or swarms of bugs and the bats chasing them.
It’s one thing to keep hikers out of a bear feeding zone. But what if the food doesn’t show up? As the value of army cutworm moths grew more apparent in bear ecology, wildlife managers realized they needed to know more about the bugs. A 2007 attempt to delist grizzly bears failed because the U.S. Fish and Wildlife Service couldn’t fully account for how the bears adapted to the loss of cutthroat trout and whitebark pine seeds — two other important Greater Yellowstone food sources that crashed in the 1990s and 2000s.
The moth populations appear stable for now. But lots more research remains. In order to gauge how many survive the summer, Peterson hopes eventually to deploy radars in September when the moths fly back to the Great Plains to lay their eggs. That would be more effective, he said only half-jokingly, than trying to count all the moth carcasses in grizzly scat, or worse, attempt before-and-after weighing of grizzlies.
Kennedy did most of her analysis in her Bozeman lab, refining ways of moth wingbeat frequency under different air and temperature conditions. But she did log some time in the mountains, seeing how her basic research transformed to real-world applications. Once, she spent an uneasy night near a sow grizzly with cubs as the family group watched a much bigger male grizzly about a mile away rolling rocks over in search of high-altitude calories.
“I like bears, but I am absolutely an entomologist,” Kennedy said. “People don’t think about bears and moths having anything to do with each other. We’re finding all these different ways they’re connected, from this little tiny moth to this apex predator.”
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About Robert Chaney
Robert Chaney grew up in western Montana and has spent most of his journalism career writing about the Rocky Mountain West, its people, and their environment. His book The Grizzly in the Driveway earned a 2021 Society of Environmental Journalists Rachel Carson Award. In Montana, Chaney has written, photographed, edited and managed for the Hungry Horse News, Bozeman Daily Chronicle, Missoulian and Montana Free Press. He studied political science at Macalester College and has won numerous awards for his writing and photography, including fellowships at the Nieman Foundation for Journalism at Harvard University and the National Evolutionary Science Center at Duke University.
