Following the flight: Tracking the migration of the army cutworm moth

Seasonal migrations of insects play a vital role in maintaining ecosystem connectivity yet remain understudied at broad spatial and temporal scales. In the Greater Yellowstone Ecosystem (GYE), the army cutworm moth (Euxoa auxiliaris, Grote) provides a major calorie influx to alpine food webs, particularly serving as a critical food source for grizzly bears (Ursus arctos horribilis (L., Carnivora: Ursidae). Female grizzly bears, in particular, rely heavily on moth aggregations to meet the high energetic demands of reproduction and cub rearing. However, the migratory dynamics of E. auxiliaris have remained speculative until recently, when preliminary application of radar to their migration demonstrated its potential for identifying and tracking these movements. In this study, we present the first large-scale quantification of E. auxiliaris migration into the GYE. Using radar technology, we characterized the timing, duration, direction, and magnitude of this movement over two years and across two sites within a 4.46 km2 front at each radar site. Results showed consistent migratory direction and orientation behavior that compensated for wind drift. These observed flight trajectories led millions of moths into alpine zones identified as moth aggregation sites, representing a substantial calorie flux into the GYE ecosystem. Together, these results demonstrate the ecological significance of E. auxiliaris as a nutrient transporter and emphasize the need for continued real-time monitoring to anticipate the effects of climate and land use change along with habitat fragmentation on insect migration and in turn the reproductive success and survival of the grizzly bears that depend on them.