‘Like butter for bears’: the grizzlies who dine on 40,000 moths a day

‘Like butter for bears’: the grizzlies who dine on 40,000 moths a day

The barren mountain slopes of north-west Wyoming might seem like inhospitable territory for a hungry grizzly bear. Big game animals are few and far between, and blueberries don’t grow from these rocky hillsides, high above the treeline.

But for a couple of months each summer, this stark landscape becomes a high-calorie buffet for hundreds of grizzly bears in the Rocky Mountain west. And it’s all thanks to a small, unassuming insect whose relationship with the grizzly is as essential as it is unexpected.

The army cutworm moth is relatively large, measuring half the size of a thumb, and appears drab from afar. But look a little closer, and a dizzying array of tan and brown geometric designs cover its furry wings. The moths migrate to these mountain peaks by the hundreds of millions each summer, some traveling more than 1,000 miles from as far away as Canada’s Northwest Territories.

Full of fat – some biologists call them “bear butter” – they’ve proved to be an important source of food for the region’s grizzlies.

Biologists estimate about 200 bears each year feast on moths in the eastern portion of the carnivores’ range. Each gram of moth offers bears about eight calories, which means some bears will eat up to 40,000 a day.

“A bear could, in about a month’s time, get one-third of the calories they need to build up fat for hibernation at these moth sites,” says Frank van Manen, leader of the interagency grizzly bear study team with the US Geological Survey.

I witnessed this phenomenon one sunny, late-July day about a decade ago while perched on a grassy, wildflower-filled meadow deep in the Absaroka Mountains. A bear biologist, a retired science teacher and I peered through spotting scopes at a mother grizzly bear chucking rocks between her legs into a ravine below. Her two cubs played next to her, stopping periodically as she grabbed another granite rock from the steep slope, turned it over and munched on what were probably dozens of cutworm moths scurrying underneath.

It may seem like an odd pairing. Grizzly bears are a charismatic carnivore memorialized on state flags and coffee mugs; moths are considered annoying at best, and pests at worst.

But at a time when other food sources, such as whitebark pine nuts and cutthroat trout, have been hit by global heating, disease and invasive species, the army cutworm moth population has remained remarkably stable, making it a critical ingredient in the grizzly bears’ continued recovery in the US.

Now, scientists are on a quest to better understand the moths – their life cycle, their migratory routes and their health – since their fate could be intertwined with that of their much larger predators.

And the unique relationship isn’t guaranteed. Humans are increasingly scrambling up to the most popular and easily accessible moth sites to watch the spectacle, only to scare the bears away, causing them to miss critical calories. And the climate crisis threatens not only lower-elevation fields where caterpillars hatch and feast on young plants but also high-mountain ecosystems where they depend on abundant wildflowers fed by deep snowpack.

The fact that hundreds of millions of moths with inch-and-a-half (40mm) wingspans can migrate so far seems nothing short of miraculous.

“They’re actually sophisticated navigators and flyers and can make use of preferential winds,” says Clare Dittemore, an entomologist who studied the moths at Montana State University in Bozeman.

For the past several decades, moth and grizzly bear researchers assumed the moths originated east of the Rocky Mountains in the Great Plains. And as researchers realized the importance of moths as grizzly bear food, they also began worrying about the moth’s sustainability. If all the moths came from one agricultural region, for example, and that area flooded or was developed, could bears lose an important food source?

So several years ago, researchers with the US Forest Service contacted Bob Peterson, an entomology professor at Montana State, asking him if graduate students would be interested in studying army cutworm moths – also called Miller moths. They wanted to know where the moths originated, how many arrived each year, and how important they were to bears’ diets – a process that required counting moth heads buried in almost 300 piles of bear poop.

Peterson agreed, and Dittemore was enlisted to spend several summers collecting moths from some of the more than 30 sites in north-west Wyoming and analyzing them in a lab.

An army cutworm moth. Photograph: RKD Peterson
An army cutworm moth. Photograph: RKD Peterson

One way to unlock the mystery of their origin was to look at the moth’s own diet. The plants each caterpillar eats hold a unique water signature, known as a stable isotope. Moth wings retain that signature, allowing researchers like Dittemore to compare the stable isotope to regional water information and uncover where these moths were born.

What she found surprised her and Peterson, an entomologist who has been studying moths including the army cutworm for decades since graduate school in Nebraska. Not only did moths fly to the mountains of Wyoming and Montana from the Great Plains, they also flew from as far away as Montana, British Columbia, Alberta and even the southern Northwest Territories.

“For one insect to complete this life cycle in a year is incredible,” Dittemore says.

Their routes are remarkably direct, with few long-ranging detours. They beat their wings about 40 times per second and travel up to 1,500 feet off the ground, says Peterson. They probably navigate by moon and starlight, which is how they end up in houses, attics, garages and anywhere else a light shines at night.

They spend hot summer days under rocks, crawling out as the sun goes down to drink their fill. In the fall, they return to lower elevations, though probably not where they were born.

Another Montana researcher, Taylor Kennedy, will spend the next couple of years using radars pointed into the sky at night to better understand moth abundance.

But for now, researchers believe the army cutworm moth population remains healthy, and because they arrive from many locations as far east as the Missouri River and as far north as the Northwest Territories, no localized issue, such as pesticide use or flooding, can crash their populations.

Frank van Manen has been studying bears for decades. Grizzly bears feasting on moths, however strange it may sound, was no surprise to him.

In fact, Van Manen was part of a team in 2014 that showed the bears eat more than 266 kinds of food, including 175 types of plants, 34 kinds of mammals, seven types of fungi and even dirt. They expertly hunt elk and moose calves and will just as willingly forage on winter-kill carcasses, hornets, thistles, or clover.

Grizzlies have a highly varied diet. Here, they try to catch salmon in Katmai national park, Alaska. Photograph: Design Pics Inc/Alamy
Grizzlies have a highly varied diet. Here, they try to catch salmon in Katmai national park, Alaska. Photograph: Design Pics Inc/Alamy

“When you look at their diets, there are some bears that have an 80%-plus vegetative diet and some bears, like large adult males, where 60% to 80% of their diet is meat,” Van Manen says.

Grizzly bears once roamed the western half of North America from Alaska down to Mexico and California to Oklahoma. They lounged under plum trees along the banks of the Missouri River, followed migratory bison through the Great Plains and ate prickly pear fruit in the deserts of the south-west.

But as Europeans moved in waves across the west in the 1800s, they killed grizzlies for their pelts and claws and to protect livestock. By the mid 1970s, fewer than 800 bears remained in the lower 48 states.

The population has clawed its way black over the last few decades, thanks to growing public awareness and protections gained under the Endangered Species Act. Today, there are about 2,200 grizzlies living primarily in Montana and Wyoming, with smaller populations in Idaho and Washington.

But grizzly advocates argue their future is not assured. In recent years, the bear’s inclusion on the endangered species list has see-sawed, with politicians, hunters and ranchers advocating for their delisting for hunting, and environmentalists saying the bears are far from fully recovered.

What the bears eat has often been at the center of that debate – and monitoring those food sources remains important to understanding the creature’s future. For experts like Van Manen, knowing that bears have a variety of food options provides hope for the future.

And as small as moths may be, they provide important calories at a critical time in bears’ summer feasting. Females pregnant with cubs need at least 20% body fat headed into hibernation to successfully reproduce. Some bears will pack on 30% or even more by winter. They begin building those reserves in early June and by July, army cutworm moths begin arriving in the mountains with their little bodies composed of up to 65% body fat. They’re so fatty, in fact, Van Manen dubbed them “lipid Chiclets”.

“It’s the timing of it that’s so critical,” Van Manen says. “From a bear life-history standpoint, it’s ideal.”

While the odd connection between the country’s largest predator and little brown insect is one of hope, its uniqueness is becoming one of its biggest vulnerabilities.

Andrew Pils, a wildlife biologist with the Shoshone national forest, remembers one of the first commercial films made at a moth site in the Absaroka Range of mountains just east of Yellowstone national park in 2008. At the time, some people knew about grizzly bears eating moths, but not many.

Today, he says, things are very different: “With the advent of social media, there are no secrets any more.”

The grizzlies, Pils and others have found, are bearing the brunt of that exposure. People hike to the mountains to see them, and while some sites can be viewed safely from far away through a spotting scope, others are trickier. On one site in particular, which might have dozens of bears on any given day, viewers often climb over the top of a mountain ridge and are suddenly among the bears.

A deluge of people heading outdoors during the pandemic has only increased the interest, leading Pils and other biologists to worry not just about the bears but people as well.

Bears scared away from the sites may lose a day’s worth of food, which could be up to 20,000 calories, Van Manen says, and if they’re bumped often enough, they may not return at all. Wyoming’s biologist in charge of large carnivores told me plainly, as I considered returning to the site I visited in 2013 for this story, that people should just leave those sites alone. So I did.

But even with concern over human visitation, as long as the climate crisis doesn’t fundamentally alter high mountain slopes by delaying wildflower blooms, Peterson views this relationship between bears and moths as a conservation success.

“It’s such a cool story,” Peterson says. “And a chance to be positive when there’s so much to worry about with respect to the environment.”

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This article by Christine Peterson was first published by The Guardian on 8 October 2023. Lead Image: Grizzly bears can get a third of the calories they need for hibernation from the moths, an expert says. Photograph: Design Pics Inc/Alamy.

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