Grizzly Bears Eat 40,000 Moths a Day In August in Yellowstone


The most common moth in the west is called the miller, and it is the adult of the army cutworm. Researchers are just now starting to understand the intricate relationship between this supposed-pest and bears, but that relationship is very important.

Each summer, moths of the army cutworm fly into tall mountain rocky slides, where they burrow away from the intense mountain sunlight into dark crevices. Hundreds of thousands of them. These moths come from farmland many miles away to these high, remote mountain slopes in Yellowstone.

At these places of slide rock and sunshine, both grizzly and black bears gather each year, climbing high above timberline to feed on the moths. The bears will dig through the slide rock and eat the moths that they uncover. It is estimated that some 40,000 moths per day can end up in the stomach of a hungry bear.

While fat in the diet is not the best thing for humans, it is important to bears. A single moth has a high enough fat content that it accounts for as much as a half a calorie. That means that 20,000 calories of just moths per day can be consumed by a rock-turning grizzly bear.

Researcher Hillary Robison has spent several summers in the high mountains around Yellowstone National Park, observing bears digging for moths.

“It’s kind of like a salmon stream,” says Robison. “We’ve seen bears feeding within several hundred yards of each other and they seem to tolerate each other.”

Usually, it’s grizzly bears that use the high mountain slopes for moths, but once in a while, an occasional black bear will make its way up into the high country above the protection of the forest below.

Robison remembers one of them. “He was digging up moths and they were flying up and landing on his forearms and he was licking them off his forearms,” says Robison. “And then he wandered on and kind of took a nap for most of the day.”

Robison is doing research on the moths in Yellowstone to see how far they come from farmlands to the high mountains. This could provide important information for bear managers, emphasizing the interconnectedness of all things. Poison an ocean and one decimates a salmon stream; spray pesticides on farmland, and one could be destroying a whole crop of bear food.

Since these moth sites are very high and very remote, this means that bears concentrating on these sites during the summer aren’t down in the valleys getting into trouble. As importantly, the moths provide a crucial food source in the face of declines of other bear foods.

“If they are spending a month up in these Yellowstone moth sites in the summer, they could eat close to half their needs for the year,” said Robison.

Comment Feed

6 Responses

  1. I ate a few of these moths to see what the bears found so good about them. They have a slight buttered popcorn flavor and just a little bit of crunch. I don’t think they’ll ever appear on the human diet though. They are thick in the springtime in the high western prairie country.

    Bill FrinkMay 16, 2012 @ 7:41 am
  2. 40k moths a day? I cant see that being possible. if they ate 18 hours a day non stop they would have to eat 37 moths per minute to reach that number. i dont see it.

  3. @JT… “Each moth has enough fat in August to make it worth half a calorie, said White, who completed his doctorate in biology at Montana State University-Bozeman in 1996 and then took a position at the University of Southwestern Missouri. Since grizzly bears eat 20,000 to 40,000 of these “lipid Chiclets” in 24 hours, they’re getting a significant number of their calories from moths. In 30 days alone, the grizzly bears devour at least 300,000 calories from moths, which amounts to about one-fourth to one-third of the total number of calories they need to live for a year.” They did the mouths out of the ground so they get more than one at a time instead they get mouthfulls.

  4. Correction -They dig the moths out of the ground- sorry I was typing fast.

Some HTML is OK

or, reply to this post via trackback.

Continuing the Discussion

  1. […] across Colorado have shed their leafy burdens and bears and other hibernating species are busy stockpiling calories for their annual slumber. Winter is bearing down on us like a frosty locomotive, and the holidays […]

  2. […] and vegetation. There are even grizzlies in the Rocky Mountains that feed on moths for their fat. Yellowstone National Park: Yellowstone Grizzly Bears Eat 40,000 Moths a Day In August Bears, like most predators (and critters in general), will stick with what they know. Humans are […]