Connecting bioregions: migration of the army cutworm moth (Euxoa auxiliaris, Lepidoptera: Noctuidae)

Abstract

Each summer, Euxoa auxiliaris (Grote) migrate from the agricultural lowlands of North America to high-elevation habitats in the Rocky Mountains, where they aggregate in talus fields during the day and forage on alpine flowers by night until returning to natal ranges at the end of the season. During this period, these moths provide a high-calorie food source for the grizzly bear, Ursus arctos horribilis (Linnaeus, Carnivora: Ursidae), which relies on this resource to build essential fat stores for hibernation. Despite the ecological importance of E. auxiliaris within these mountain ecosystems, limitations in ground-based observations have restricted our understanding of their role, leaving key knowledge gaps in both grizzly bear management and broader alpine ecosystem dynamics. Improving our understanding of E. auxiliaris migration is essential not only for the conservation of grizzly bears in the Rocky Mountains, but also for recognizing the functional linkages between geographically disparate ecosystems. To address this, we investigated the physiological and behavioral characteristics of E. auxiliaris, including critical thermal limits and wingbeat frequency under environmental stress, and used radar to track migratory patterns in the Greater Yellowstone Ecosystem (GYE), aiming to predict the timing and magnitude of moth arrivals into seasonally occupied alpine habitats. We determined the critical thermal limits (CTL max and CTL min) of both lab-reared and wild-caught moths, finding a CTL max typical for a temperate lepidopteran species, but a CTL min that reflected an extraordinary ability to remain active and feed under cold conditions. Wingbeat frequency was measured under controlled combinations of temperature and barometric pressure, and our results show that temperature significantly affected wingbeat frequency, but barometric pressure did not. Radar monitoring over two years and across two sites revealed consistent migratory trajectories that delivered millions of moths and thus millions of calories into alpine zones of the GYE. These findings demonstrate the value of radar for characterizing the movement ecology of E. auxiliaris, highlights the physiological resistance that enables migration, and underscores the ecological importance of this insect to the persistence of grizzly bears in the Rocky Mountains.