Billings, Mont. • Those army cutworm moths are a nuisance to people, but they are an important nutritious snack for grizzly bears in the Greater Yellowstone Ecosystem.

Researchers say more grizzly bears are congregating each summer on steep rocky slopes in the southeastern portion of the 9,210-square-mile ecosystem in search of the moths.

"We've seen an increase in the use of these moth sites in the last three years," said Frank van Manen, of the Interagency Grizzly Bear Study Team in Bozeman, at a recent meeting. "Certainly it's a very intriguing food resource that we want to keep track of."

Concern about what grizzly bears eat is high because once-plentiful natural sources of protein — like cutthroat trout and whitebark pine nuts — have declined.

Aerial surveys of 29 moth sites showed the small bugs, which are packed with fat and protein, can be a powerful attractant. The study team counted 470 grizzly bears at those 29 sites. More detailed analysis has shown 220 unique individuals just at the moth sites this year, including 19 females with cubs from this year.

Grizzly bear researchers didn't document the sites until the mid-1980s. Since then, the team has looked more intensively to find moth areas and has confirmed 37 sites in the ecosystem with another 16 possible locations.

"Almost 80 percent of the sites are being used," van Manen told The Billings Gazette (http://bit.ly/1xsf4ci). "But this is not a resource being used by every bear in the ecosystem."

One study of Glacier National Park grizzlies estimated that a bear could consume up to 40,000 moths a day, equal to an amazing 20,000 calories. So although small — with a wingspan of only 1.5 to 2 inches — the light gray moths pack a huge caloric punch.

A recent study showed that grizzly bears have an incredibly varied diet. The bears, which are still listed as an endangered species in the Greater Yellowstone Ecosystem, chow down on everything from dandelions to ants.

An analysis of grizzly scat in Yellowstone National Park showed 266 different types of plants and animals.

With grizzly bears in the ecosystem proposed for delisting by the U.S. Fish and Wildlife Service, ensuring that the bears have enough to eat is paramount to making that decision, which could come at any time.

Many bear biologists insist that grizzly bears are adaptive enough to find different food sources, even if whitebark and trout numbers are down. Some environmentalists argue just the opposite.