Grizzlies & Army Cutworm Moths
Bears Binge on Small Insects Found on High Mountain Slopes
During late summer, grizzly bears forage rocky slopes at high elevations, eating massive quantities of army cutworm moths in preparation for winter hibernation.
Where are the Bears?
Grizzly bear, Ursus arctos horribilis, sightings in Yellowstone National Park are a fairly common occurrence if one knows where and when to look. Often, grizzlies can be seen roaming around Hayden Valley and the Dunraven Pass areas at dusk and dawn throughout the spring and summer. As the summer season progresses, getting a view of grizzly bears can become increasingly challenging. The bears are moving higher up in elevation in search of army cutworm moths, Euxoa auxiliaries.
On the Talus Slopes
Army cutworm moths migrate in summer from the warmer climate of the Great Plains to cooler mountain areas. The moths like to stay cool under rocks by day and come out to feed on wildflower nectar at night. Grizzlies travel up rocky talus slopes (large, jagged rock fragments that have broken off the original cliff or peak and are now piled up below) to elevations greater than 10,000 ft. above sea level to find the moths under rocks. The grizzlies dig through the talus and flip over rocks with their paws, consuming tens of thousands of them each day. Anywhere from 10,000 to 40,000 individual moths are consumed per bear daily! During this time, many grizzlies may congregate in the same area.
Grizzlies in other ecosystems tend to follow the same foraging trend, where cutworm moths are available. Researchers have also documented black bears searching for army cutworm moths in the southwestern United States, although this area is not current grizzly habitat. Black bears have not been observed foraging for cutworm moths within any of the current grizzly ecosystems.
6 Grizzly Bear Ecosystems in the United states (not including Alaska):
- Yellowstone
- Northern Continental Divide
- Selkirk
- Cabinet-Yaak
- North Cascades
- Bitterroot
Concern for the Grizzly and the Army Cutworm Moth
One concern is how foraging grizzlies are affected by people climbing through moth aggregations. Aggregations are simply the areas were the moths have congregated. Bears are definitely affected, spending less time eating and becoming more aggressive. The full results of research on this subject can be found in the brief article Potential Energetic Effects of Mountain Climbers on Foraging Grizzly Bears by Don White, Jr., Katherine C. Kendall, and Harold D. Picton, Wildlife Society Bulletin, Vol. 27, No. 1 (Spring, 1999), pp. 146-151.
Since the army cutworm moth plays such a large role in the well-being of grizzly bears, it is vital to assure a continued natural supply of them. The moths and their caterpillars, army cutworms, are considered agricultural pests so pesticides are sprayed to eliminate them. The most damage done to crops occurs during the caterpillar stage. As moth numbers decrease, grizzlies lose out on a prime food source. Research on the issue of pesticide toxicity to grizzlies through eating the moths has shown that levels are too low to be a concern at this time. Current global warming trends are also a cause of concern for both the moths and grizzly bears. A change in temperature will affect cutworm moth distribution. The current range will most likely be altered, therefore, grizzly habitat may need to adapt to that.
