Of Moths and Grizzlies


USGS photo

How does a moth from eastern Montana prairies feed grizzly bears in the Greater Yellowstone ecosystem (GYE)?  When I think of grizzly bear foods, I tend to think of berries, carrion, trout and whitebark pine nuts, but recent research shows that equally important to GYE griz is a moth called the army cutworm moth. These moths gather in the mountains in the GYE above 9000 feet in the late summer/early fall to find food and put on fat to support the development of eggs and power them on their return to the lowlands. That is pretty strange by itself, bears weighing hundreds of pounds depending on moths that might be an ounce or less, but studies show grizzlies can eat 40,000 of these moths a day! That is 20,000 kilocalories per day or the equivalent of 62.5 fast food cheeseburgers. No wonder grizzlies intent on packing on the fat for the winter search out the swarms of these moths in the mountains.

A study just completed found that GYE army cutworm moths fly from agricultural lands spanning Montana, Idaho and Wyoming in search of food. They seek the flowers found at higher elevations later in the season when lowland flowers have withered in the heat. This justifies the energy they spend gaining 8000 plus feet in elevation and traveling hundreds of miles in some cases. The moths gather in talus slopes to hide during the day and emerge to feed and mate at night. Grizzlies seek out their hiding spots and turn over rocks to find concentration of the fat-filled bugs (up to 83% fat). Who knew a bug could hold so many calories!? The surviving adults return to the lowlands to lay eggs that lead to the emergence of caterpillars on wheat and oat crops in eastern Montana.

There are dozens of moth sites in the eastern mountain ranges of the GYE. With so many grizzly foods like whitebark pine nuts and cutthroat trout on the wane, protecting this food source becomes critical. Observations indicate that grizzlies will abandon moth sites if they encounter humans, even hikers. Some guides in GYE are already aware and avoid sites they know about, but keeping these sites available to griz will be all the more important in the coming years.

By Len Broberg

Len photo

Len has been a Montana Sierra Club member since 1995 and has served as the Chair of the Bitterroot-Mission Group and Montana Chapter executive committee. He is currently a member of the Sierra Club Greater Yellowstone Campaign’s Delivery Team and the Wildlands and Wilderness Activist Team. Len is a professor in the Environmental Studies Program at the University of Montana.


One thought on “Of Moths and Grizzlies

  1. Just another amazing e.g. of this interconnectedness of our planet. Next time I’m lost in the high country those moths might be on the menu.
    On an aside, I once found a boulder outcrop in early fall coated in bunched up living lady bugs, (several varieties). And below these rock clinging live hoards were 8-10 inches deep of ladybug carapaces bleached white from years of accumulated winterkill. I assume this black boulder outcrop is used historically as a heat sink to survive the winter? Thanks Len for the post.

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