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The Odd Phenomenon of Moth-Eating Grizzly Bears

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If you thought bears were all about salmon and berries, think again! In the enchanting wilderness of the Northern Rockies, grizzly bears are embarking on an extraordinary adventure—scaling mountains to feast on as many as 40,000 moths a day. This peculiar phenomenon not only captivates our imagination but also raises questions about the intricate ecological dance between these majestic bears and seemingly ordinary insects. Join us as we dive deep into the world of moth-eating bears, exploring the mysteries and challenges they face in their quest for this unusual delicacy.

Introduction: An Unexpected Journey to the Mountains

Imagine climbing up a mist-shrouded peak, where the air is thin and the rocks are loose underfoot. It’s a journey filled with anticipation, but not for the faint of heart. This is Glacier National Park, and we are on the trail of a fascinating phenomenon that brings grizzly bears to these lofty heights. As we ascend, our guide, photographer, and mountaineer Steven Gnam, points out the presence of ravens circling overhead—a sign that we’re on the right path.

Our adventure begins with the discovery of telltale signs of grizzly bears—deep divots in the scree and talus, piles of scat, and the unmistakable sounds of bears rummaging through stones. But what could possibly entice these mighty bears to navigate treacherous terrain? The answer lies in the mysterious army cutworm moths.

The Moths That Ignite a Bear’s Appetite

Army cutworm moths, silver-winged insects measuring just 1.5 inches in length, migrate to the Northern Rockies every summer. They escape the sweltering plains and flock to the alpine regions in staggering numbers, drawn by the allure of blooming alpine plants. By day, they seek refuge under high-elevation scree, while by night, they feast on the nectar of wildflowers, transforming themselves into plump creatures with up to 75 percent body fat.

Now, enter the grizzly bears. Scaling peaks reaching heights of 13,000 feet, these furry mountaineers excavate tens of thousands of moths daily from their rocky hideaways. This unlikely pairing—a gigantic bear and a seemingly insignificant moth—unveils a captivating tale of survival and interdependence.

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The Moth: A Calorie-Dense Feast

While grizzlies have a varied diet, nothing matches the calorie density of army cutworm moths. These insects are true survivors, spanning a vast range across western North America. Their life cycle begins as worm-like larvae, voracious feeders that consume young plants, including crops. A few weeks later, they metamorphose into moths, embarking on mass migrations that can create “moth blizzards.”

Research by biologist Clare Dittemore and her colleagues at Montana State University indicates that these moths predominantly feed on wild plants and weeds as larvae, rather than fertilized crops. This surprising revelation positions the moths as integral components of the ecosystem, rather than mere agricultural pests. Their diverse origins, including a broad swath of Canada, contribute to their resilience in the face of environmental changes.

Hilary Robinson’s work at Yellowstone National Park further emphasizes the moths’ unique behavior. They choose random mountainous locations for migration and do not return to specific spots, unlike salmon. This site-agnostic behavior, coupled with their widespread distribution, offers a degree of protection against disturbances. Nevertheless, climate change looms as a significant threat, casting a shadow over the future of insect migrations.


In a world filled with stories of awe, moth-eating bears is a reminder of the unexpected marvels in the wilderness. As we reflect on this peculiar ecological phenomenon, may we find inspiration to protect the fragile balance of nature that sustains these remarkable creatures.

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