Abstract 61 – Modeling prey & predator: Using army cutworm moth seasonal occurrence to inform grizzly bear foraging

Erik Peterson, John S. Waller, Don White Jr., James R. Pierce, Daniel H. Thornton

Migratory army cutworm moths (Euxoa auxiliaris; ACM) are one of the most calorie-rich food
resources for grizzly bears (Ursus arctos horribilis) in the Rocky Mountains. Due to the
inaccessible nature of the alpine talus slopes where ACMs reside in summer, the magnitude of
the grizzly bear-ACM relationship is unclear. We undertook a two-part study to better
understand the seasonal significance of this fat and protein-packed food resource for grizzly
bears within 4000 km2 Glacier National Park, Montana, USA. We first conducted ground
surveys of potential ACM habitat to develop presence-absence models of ACM occurrence. We
found that, in particular, geological characteristics (namely dominant geological formation,
surficial talus rock size and talus depth) influenced ACM occurrence. Our model predicted ACM
occurrence across a total area of 12 km2, just 0.3% of the park. Second, we conducted aerial
surveys to map grizzly bear use within potential ACM habitat and develop presence models of
grizzly bear foraging suitability. Using predicted ACM occurrence as a single variable bear
foraging model, we found ACM occurrence drove predicted patterns of grizzly bear foraging.
Spatially, our models showed the overlap of ACM occurrence and grizzly bear foraging for ACMs
was 69%. And temporally, grizzly bear moth consumption aligns with their crucial period of
overeating to sustain winter hibernation. A grizzly has the capacity to consume ~20,000 kcal in
ACMs per day, so individuals could potentially acquire ≥ 50% of their seasonal energy needs by
foraging for moths. Given the significance of this predator-prey relationship and increasing off-
trail human recreation, we recommend Glacier National Park implement site-level access
management strategies at ACM occupied talus slopes to limit grizzly bear disturbance and
minimize impacts to the energy budgets of individual bears utilizing this seasonal, concentrated
food resource.