Natal origin, migratory patterns, and abundance of the army cutworm moth, Euxoa auxiliaris

Abstract

The army cutworm moth, Euxoa auxiliaris, is a migratory noctuid that migrates from and returns to the Great Plains. At their Rocky Mountain summering range, it is an important food for the Greater Yellowstone Ecosystem (GYE) grizzly bears, Ursus arctos horribilis. However, a limited understanding of moth migratory patterns, abundance at summering ranges, and the associated vulnerability and variability of these populations, is a shortfall in agency grizzly bear conservation strategies. Therefore, the objectives of our study were to assess the natal origin of moths collected from two mountain ranges and within the Great Plains, characterize larval feeding habits of migrants collected from the Absaroka Range, and assess a monitoring protocol to determine moth abundance at a GYE aggregation site. Using stable isotopes, we estimated the natal origin of migrants collected from the Absaroka and Lewis Ranges, examined migratory patterns within the Great Plains, and assessed the larval feeding habits of migrants collected in the Absaroka Range during 2017-2021. To estimate abundance, we sampled the airspace with a radar stationed within 1 km of an aggregation site in the GYE during 2020 and 2021. There was strong evidence that moths collected in the Great Plains and both mountain ranges were migrating north-south, in addition to previously established east-west movement. Although their origins were varied, moths of the Absaroka Range had the highest probability of origin in Alberta and British Columbia, and moderate probability in Montana, Idaho, and Wyoming across all collection years. Lewis Range moths had the highest probability of origin almost exclusively within the lower third of Canada. As larvae, the moths collected from the Absaroka Range fed almost exclusively on C3 plants. We estimate that 5 million moths passed through our radar's sampling plane (160-750 m above ground level and 2600 m across) over the course of 20 hours (5 nights of movement). Overall, our findings suggest that army cutworm moths at aggregation sites are 'supplied' by various source locations, and thus insulated against regional declines within their natal origins. Radar should be used at moth aggregation sites to continue monitoring army cutworm moths.