What is an army cutworm moth & why do grizzlies eat them?

Army cutworm moths (Euxoa auxilliaris, ACM), also commonly called miller moths, are one of a number of cutworm moth species found in North America. The moths are migratory, splitting their annual lifecycle between low elevation plains/basin habitats and high elevation mountain talus slopes. Army cutworm moths possess a 1-2″ wing span and are identifiable by two unique spots located on their forewings and cream-colored fringing on their hindwings.

Army cutworm moths were given their common name from their propensity to aggregate in great numbers (like an army) wherever they occur during their annual life cycle. Across the Great Plains and arid basins of the intermountain-west, ACM larvae emerge in the spring. During this period, ACMs feed on a wide range of early growth stage plant species. Often, this includes agricultural fields where they consume alfalfa, canola, and wheat crops. Because they aggregate in great numbers, they often inflict significant economic losses to crop fields when they congregate on agricultural lands. To stem the economic impact, farmers often spray insecticides when ACM density reaches 2 moths per square foot of cropland. More information on the ACM life cycle stage and damage in agricultural areas can be found on websites for agricultural extension offices of universities located in the Great Plains, like Colorado State, Kansas State, and Texas A&M.

Army cutworm moth larvae foraging on wheat – Credit: Jeanne Falk Jones, Kansas State Research & Extension

More recently, ACMs have been documented consuming invasive cheatgrass in the Great Basin. Unlike in agricultural areas, ACM foraging in this habitat could positively function to manage/control cheatgrass infestations, potentially mitigating the negative ecological impact cheatgrass has on food webs for species ranging from golden eagles to pygmy rabbits. For more information: https://www.sciencedirect.com/science/article/pii/S019005281830004X, https://cindysalo.com/

In June, ACMs begin their migration to high mountain areas, primarily across the Rockies. It’s believed ACM’s migrate to the mountains to escape hot, dry summer conditions. If you live along the migration path of ACMs, you may be able to notice when they migrate. Look for them aggregating on screen doors and windows of homes or flying in great numbers in front of car headlights while driving at night (and then splatted on the front end of vehicles the next morning).

Army cutworm moths aggregating on the side of a house – Credit: University of Nebraska-Lincoln Extension Office

Army cutworm moths travel hundreds of miles against prevailing winds during the course of their migration to the Rockies. Where they migrate to Glacier National Park from is not well documented, but migration assuredly occurs from eastern Montana and Alberta. How many other states or Canadian provinces ACM’s migrate to Glacier from is unknown, but the number of different areas could be surprisingly many. Recent research in the Yellowstone National Park area is investigating this using stable isotope analyses to trace ACM origins. This methodology is the same as what’s been used to trace the origins of fabled monarch butterfly migrations, which you can read more about here if you like: https://link.springer.com/article/10.1007/s004420050872. The initial results of this research indicate ACMs travel to site-level talus slopes from an impressive latitudinal range of Great Plains states, and is likely to soon be published.

The army cutworm moth’s annual life cycle

So, why do grizzly bears climb high onto certain mountain talus slopes to feed on army cutworm moths?

Army cutworm moths flying en masse to wildflower nectar resources at night

The answer is rather simple. The individual gross energy content of ACMs (7.9 kcal/g dry weight or about 1/2 calorie each) combined with the moths’ colossal aggregating nature functions to make the talus slopes where ACMs occur a food buffet for grizzly bears, just like a huckleberry patch, where they can feed on up to 40,000 moths per day. For certain individuals, the caloric benefit of travelling to an aggregation site and feeding there outweighs the caloric cost of travel.

A remote camera video sequence of three consecutive events showing a sow grizzly locate and dig for moths at one location before moving on to the next foraging location

Beyond the caloric benefit of consuming ACMs, security considerations may factor into grizzly bear use of aggregation sites. Grizzly bears observed at sites are disproportionally females with young and subadult (less than five years old) age class individuals. The talus slopes where ACMs are found generally possess wide viewsheds, which can aid the security of these subordinate grizzlies while they feed. Larger bodied dominant males incur increased energetic costs travelling to aggregation sites, which reduces the net benefit these individuals receive by feeding at aggregations. This cost may limit their use of these food patches. Instead, it may be advantageous for dominant males to remain at lower elevations, occupying berry and other food patches (where they can be fat and lazy!), leaving ACM aggregations more available for subordinate bears to meet their caloric needs.

A grizzly bear excavated dig site for army cutworm moths – Credit: Steven Gnam

Whatever the exact considerations individual grizzly bears process when feeding at ACM aggregations, their use of these sites occurs from July into September while ACMs are vacationing in their mountain paradise. The timing of ACMs’ departure back to lower elevation agricultural-dominated areas varies each year, but is driven by falling temperatures and those first wintry systems that frost high peaks with the first snowflakes of the season. As the snowflakes start to fly, ACM’s latch onto the prevailing winds and fly home to complete their annual cycle by laying eggs for the next year’s generation of moths. For grizzly bears, their hyperphagic quest for calories continues at lower elevations by feeding on huckleberries and other body-fattening food resources.

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