Army Cutworm Moths


Anyone who spends much time in the woods and pays attention knows that bears eat insects--small animals with a chitinous exoskeleton and three pairs of legs of the class Insecta, most notably ants and hornets of the order Hymenoptera. Most of this consumption occurs in unremarkable environments such as deep woods or mountain meadows (see this paper on consumption of ants, and this one on consumption of hornets). But bears also consume insects of different taxonomic orders in more dramatic environments on the roof of the world--in alpine environments of the Rocky Mountains.

In every recorded instance, this exploitation of alpine invertebrates by bears only occurs when the insects congregate in irresistible masses, whether because of historical artifact or contemporary circumstance. Among these insects are ladybird beetles (Hippodamia caseyi) of the order Coleoptera congregated to hibernate on high mountain peaks and grasshoppers (AKA locusts, Melanoplus sp.) of the order Orthoptera frozen in the deeps of glaciers during past epochs, only to melt in stinking masses during the past century. But my primary focus here is on the phenomenon of bears consuming army cutworm moths (Euxoa auxiliaris) of the order Lepidoptera congregated during summer to feed on the nectar of alpine flowers and prolong their otherwise brief lives.

Briefly: Ladybird beetles & Grasshoppers


Some of the first observations of bears eating insects at high elevations occurred during the 1920s-1950s on McDonald Peak in the Mission Mountains of northwestern Montana. The first impressions to be recorded in a scientific journal were by John Chapman in a paper published in 1955. He speculated that bears were eating aggregations of ladybird beetles, which were known to concentrate in loose rock at high elevations to weather the winter under insulating layers of snow. In fact, his speculations were entirely reasonable given that he observed swarms of ladybird beetles (see the photo to the right), and had studied their aggregating behavior and hibernation for years. But the bear scats he collected only contained remains of army cutworm moths, no ladybird beetles. To this day it remains unclear whether bears do, or did, eat ladybird beetles, either on McDonald Peak or anywhere else.

More certainly, bears have been known to eat grasshoppers melted out of high-elevation glaciers. The entombment of grasshoppers, more specifically Rocky Mountain locusts, is well-documented throughout the northern Rocky Mountains, from northern Colorado north through Wyoming's Wind River Range to the Beartooth Mountains of Montana. Not by chance, there are several Grasshopper Glaciers in this region. The locusts died in their multitudes after being swept into the high mountains by frontal systems that were synchronous with locusts outbreaks on the plains below. For reasons that are still unclear, the Rocky Mountain locust went extinct in the late 1800s, but great masses of these insects were preserved in ice after dying from exposure on the tops and faces of glaciers (see photos to the right). Thereafter, various episodes of warming melted parts of the glaciers and exposed great rotting masses of locusts. Ashley Gurney provided the most definitive account of bears eating these locusts in a report published during 1952 in which he reported authoritative observations made during 1868 and 1934. There have been none published since then, but it is likely that bears have continued to feed on these ancient remains during episodes of melt.

Army cutworm moths

Army cutworm moths are one of the many paradoxical foods eaten by grizzly bears. The moths themselves spend most of their lives in domesticated landscapes of low-elevation grasslands and fields, whereas grizzly bears are associated with extensive wilderness areas. Indeed, grizzlies eat cutworm moths is some of the most remote and rugged mountains to be found in the contiguous United States. This begs the questions of how and why the moths get to the bears, which necessarily entails understanding the moths' life cycle and life-ways.

Army cutworm moths begin their lives as eggs laid by adult moths in the soil of grass- or croplands during August through October. They undergo a short phase of development and then enter a period of dormancy called diapause which takes them through much of the winter. Development resumes during the spring, passing through phases called instars (instars 3 through 7), resulting in vigorous larvae that feed above-ground but seek refuge in the soil (see the photo to the left). These larvae eat vegetation at ground-level (hence "cutworm") and assemble in hordes that march across landscapes in search of fresh food once food in the immediate vicinity has been depleted (hence "army"). These hordes of larvae are considered significant pests by many farmers, who have employed various insecticides to control them. Historically, the favored toxin was Endrin, banned because of its many harmful side-effects, and, more recently, Sevin and Malathion.

By June the larvae have pupated and emerged as winged adults (see the photo left), although this process may require an additional year in northern latitudes. These adults thereafter feed on nectar and, curiously enough, are harmed--if not killed outright--by summer temperatures typical of lower-elevations. The resulting imperative to find food and escape the heat takes army cutworm moths on epic migrations of up to 300 miles to the very highest alpine regions of mid-latitude North America. In the Great Plains, these migrations are invariably westward up into the Rocky Mountains. Once there, the moths settle in great congregations around areas of tundra and feed on the nectar of alpine flowers (including willow catkins!) during cooler parts of the day. During the comparative heat of the day, the lethargic moths concentrate beneath talus rocks.

Which is where the moths meet grizzly bears.

Bears concentrate on rock-fields and talus slopes to feed on army cutworm moths shortly after mid-July, near the culmination of early summer moth migrations, and then abandon these sites synchronous with the fall migration of moths from alpine areas back to the lowlands. Bears almost exclusively feed during earlier and later hours of daylight when moths are either entering or leaving ostensible refuge among the rocks.  

The map at right provides a big-picture view of spatial relations between cutworm moths and bears. Cutworm moths are distributed throughout much of western North America at mid-latitudes (in tan), but with greatest concentrations in semiarid areas (darker brown). Every documented instance where bears have fed on cutworm moths (dusky green blobs) has occurred in talus located in alpine or high subalpine environments (shown as red). All of the sites in the northern Rockies are dominated by grizzly bears, sometimes to the exclusion of black bears. The single site to the south, in the Jemez Mountains of New Mexico, is used by black bears, which are the only surviving ursid in this region.


Several anomalies are worth noting. So far, bears have not been observed consuming cutworm moths in the Cascades, Sierra Nevada, or Canadian Rocky Mountains, despite abundant moths in nearby lowlands. Similarly, no bear feeding on moths has been recorded in the central Rocky Mountains of Colorado and central-southern Wyoming, despite the documented presence of moth aggregations in alpine areas (areas circled in white). The same holds true for Utah's Uinta and Wasatch Mountains. I have no ready explanation for these anomalies other than perhaps (1) lack of effort to document sites with moth aggregations in the Sierras, Cascades, and Canadian Rockies, and (2) the extinction of knowledge about moth sites among most bear populations. Bears that historically consumed moths would have been disproportionately vulnerable to episodes of slaughter during the 1800s because the occurrence of moth aggregations is predictable in both time and space. Moreover, this vulnerability is likely perpetuated by contemporary sport hunting regimes, with related on-going attrition of knowledge about moth sites among surviving bears.


For more detail on spatial and temporal aspects of bear foraging on cutworm moths, see the pages linked to the buttons below: