Grizzly bears (brown bears) and black bears engage in a specialized seasonal feeding strategy involving the consumption of vast quantities of moths in high-altitude environments. This energy-rich food source becomes a major part of the bears’ diet during the critical late-summer and early-fall feeding period. This behavior is tied to specific, high-elevation locations, such as the Rocky Mountains and Sierra Nevada, where the insects aggregate. This unusual insect consumption demonstrates how bears adapt their omnivorous diet to exploit the most nutrient-dense food sources available.
The Specific Moth and Its Migration
The specific insect involved is the Army Cutworm Moth (Euxoa auxiliaris). These migratory moths emerge from their larval stage in agricultural lowlands before undertaking a mass migration to high-elevation mountain ranges in early summer. They seek out alpine talus slopes for a period of summer rest called aestivation. During the day, the moths aggregate in large numbers, sheltering in cool spaces beneath the rocks. They emerge at night to feed on alpine wildflowers, building fat reserves before returning to the lowlands.
The Nutritional Imperative
Bears seek out this food source due to the exceptional caloric density of the Army Cutworm Moth. As summer progresses, the moths accumulate significant fat reserves to fuel their return migration. At the peak of aggregation, the moths’ bodies can be composed of up to 70% fat by dry weight, making them one of the most energy-rich foods a bear can find. Each single moth contains approximately 0.5 kcal, which is a substantial energy return for its size. This high-fat content makes the moths a highly efficient source of rapid weight gain compared to other seasonal foods like berries or grasses, and the energy benefit far outweighs the cost of travel and foraging to these remote, high-altitude locations.
Daily and Seasonal Consumption Rates
Scientific analysis estimates that a single grizzly bear can consume between 20,000 and 40,000 Army Cutworm Moths per day during the peak feeding period in August. Bears forage by turning over rocks and scooping up dozens of aggregated insects at a time. This daily intake provides a massive caloric boost, equating to approximately 10,000 to 20,000 calories from moths alone. Researchers confirm these numbers by analyzing bear scat, which contains moth exoskeletons. Over the aggregation period (July through September), a single bear can gain at least 300,000 calories from moths, satisfying over one-third of the nutritional requirements needed for winter.
Impact on Bear Ecology
Moth consumption is intrinsically linked to the bear’s annual life cycle, particularly the period of hyperphagia. Hyperphagia is the intense feeding phase bears enter during late summer and fall to accumulate body fat for hibernation. The rapid, high-fat intake from the moths is crucial for building these necessary reserves. Female bears, especially pregnant ones, need to achieve at least 20% body fat to successfully carry cubs to term during winter denning. Disruption to aggregation sites, such as from climate change or human activity, could reduce access to this food, potentially affecting the health and survival of bear populations.