Abstract 336 – Assessing Dietary Importance of Army Cutworm Moths to Grizzly Bears in the Greater Yellowstone Ecosystem

Jennifer Fortin-Noreus, Dan Tyers, Andy Pils, Justin E. Teisberg, Joy Erlenbach,
Hilary Cooley

Grizzly bears in the Greater Yellowstone Ecosystem (GYE) are opportunistic omnivores that
move seasonally within their home ranges depending on food abundance. While grizzly bears
supplement their diet with lower-caloric foods that tend to be widely distributed and readily
available (vegetation, insects, fungi, berries, small mammals), the availability and distribution of
high-caloric foods in the GYE (e.g., ungulates, army cutworm moths, whitebark pine seeds) has
the potential to influence seasonal movements of grizzly bears. Previous studies have
investigated grizzly bear use of whitebark pine seeds and ungulates, yet little is known about
grizzly bear use and availability of army cutworm moths. Some grizzly bears whose home
ranges overlap with army cutworm moth aggregation sites appear to forage extensively on
moths during mid-to-late summer when the moths aggregate on remote, high-elevation talus
slopes, but many questions remain regarding the importance of this food source to the GYE
grizzly bear population. The Interagency Grizzly Bear Study Team annually monitors trends in
grizzly bear presence at confirmed moth sites through aerial observations, providing an indirect
measure of the importance of moths in a given year. Using stable isotope analysis (δ15N, δ13C,
δ2H), we present a method to instead directly estimate the proportion of assimilated diet
composed of army cutworm moths. Likely because of their migratory life history, the hydrogen
isotopic ratio (δ2H) of army cutworm moths at confirmed aggregation sites is unique when
compared to other food items in the GYE. Additionally, hair samples were collected and
analyzed from 1) bears at or near confirmed moth sites and 2) collared bears with no GPS
locations at confirmed moth sites. Moths, other dietary items, and hair samples will be
collected from additional peaks during 2020 for expanded analysis.