Bumble bees alter diet breadth but maintain pollen macronutrient levels in response to varying floral availability in managed timberlands

Bumble bees ( Bombus spp.) are keystone pollinators in early seral managed conifer plantations, yet little is known about how floral availability influences their foraging behaviour, diet breadth and nutritional intake.

In a 2‐year observational study of bumble bee communities in 21 regenerating Douglas‐fir ( Pseudotsuga menziesii ) stands in western Oregon, USA, we quantified relationships between floral availability and diet breadth, resource partitioning and protein:lipid ratio of collected pollen. We also assessed bee floral preferences across time and between native and exotic plants.

We sampled 2,220 individuals representing 12 bumble bee species visiting 34 flowering plant species. Neither pollen diversity nor macronutrient composition varied with floral availability. Although most foragers collected pure pollen loads, visitation diversity at species and community scales increased with local floral abundance and diversity, while resource partitioning remained consistent. Floral preferences varied seasonally with no bias towards native or exotic plants.

Our results indicate that bumble bees exploit a wider range of floral resources when available while maintaining a similar macronutrient balance, suggesting that selective foraging across varying resource conditions regulates intake. Pollen protein:lipid ratios were lower than in other bumble bee studies, potentially indicating nutrient limitation.

The diverse bumble bee communities we observed in regenerating timber stands suggest that these areas provide important forage in otherwise resource‐limited forest landscapes. Additionally, bees' lack of preference for exotic plants suggests that invasive plant management should not conflict with bumble bee conservation if weeds are replaced with native species that provide comparable rewards.