‘Amazing moths’: Study pinpoints insect habitat that draws grizzlies to Glacier peaks
PULLMAN, Wash. — When grizzly bears clamber onto the talus slopes high in Glacier National Park, they’re searching for an abundant, fatty meal: army cutworm moths.
The inch-long moths hatch on the Great Plains and fly en masse to escape the heat in the stony upper reaches of the Rocky Mountains. Grizzlies hoover them up by the thousands — at about a half-calorie each — as they fatten up for winter.
“The moths are full of fat, and they're also quite nourishing in protein, so they satisfy two macronutrients of a grizzly bear's diet like few natural food resources do,” said Erik Peterson, who led a three-year project studying the moths as a graduate student in Washington State University’s School of the Environment. “Their abdomens are swollen with liquid fat.”
But these moths, which Peterson refers to as “bear butter,” congregate in just 0.3% of the park land, in high, hard-to-reach places. Peterson and his fellow researchers have mapped this habitat to give park managers a tool for protecting grizzly foraging sites as backcountry recreation grows, as part of new research published in the journal Biological Conservation.
From 2019 to 2021, the team studied moth and bear activity in two ways: they conducted ground surveys of potential moth habitat and then surveyed bear activity inside potential moth habitat from helicopter so they could separately model moth habitat and bear foraging sites for comparison. It was arduous, time-consuming work.
“The effort involved in collecting this data cannot be overemphasized,” said Daniel Thornton, an associate professor in the School of the Environment and a co-author of the paper. “Climbing up to talus slopes to survey for moths across Glacier with a team of technicians, going up in helicopters to search for bears ... all happening in a flagship national park. It was a huge effort, and amazing that Erik was able to pull it off and get such interesting and important data.”
The findings also contribute new depth to the understanding of bear-moth dynamics. Researchers identified specific characteristics of the talus environments that draw the moths — from the size of the stones to indices of soil moisture — and demonstrated that the presence of the moths is a key driver of grizzly foraging behavior.
Peterson, who worked for the National Park Service in Glacier for a decade before going to graduate school, conducted the research as a student in Thornton’s lab; Peterson earned his master’s degree at WSU and has moved on to a research position at the University of British Columbia-Okanagan. Other co-authors include John Waller of the National Park Service, Don White Jr. of the University of Arkansas-Monticello, and independent researcher James Pierce.
The project was funded by the Glacier National Park Conservancy and the USDA National Institute of Food and Agriculture.
Compared to iconic grizzly bears, army cutworm moths have a tiny public profile — but their role in Rocky Mountain alpine ecology is large. They hatch on the Great Plains and low-lying areas in the Rockies, and during their caterpillar phase they can be a destructive agricultural pest. Then they pupate into adult moths and fly to the high reaches of Rocky Mountains in the late spring, tucking into cool, shaded spaces in talus fields, where bears sniff them out and devour them in large numbers. In early autumn, the surviving moths fly back to the plains to mate and lay eggs for the next generation, like salmon returning from the ocean to spawn.
“Of all the foods that a grizzly bear eats, the story of this moth might be the most amazing,” Peterson said.
The work can help park managers make decisions about how to protect grizzly foraging habitat. Grizzlies are abundant in Glacier relative to other wild places – but so are people, with some 3 million visitors to the park each year.
While most stay on or near roads and developed areas, backcountry recreation is increasing as well, including climbers on the park’s peaks. The presence of people near foraging areas can interfere with the grizzlies’ most important priority during summer – consuming enough calories to survive the winter.
“There are moth sites in Glacier where there are waves of climbing groups scrambling by each day,” he said. “On a given day, there may be upwards of 20 bears foraging for moths at major sites If a grizzly bear is having to watch people pass by, whether it's alarmed or habituated to people, it’s there to eat calories, not to watch us.”
Journal
Biological Conservation
Method of Research
Observational study
Subject of Research
Animals
Article Title
Broad-scale models of a foundational alpine insect: Implications for grizzly bear ecology and conservation
