Pollen Foraging by Bumble Bee Queens During a Critical Nesting Period Revealed by DNA Metabarcoding

The nest-founding stage represents an especially vulnerable period of the bumble bee (Bombus) life cycle, during which solitary queens must locate and collect sufficient foraging resources to sustain themselves and their brood. Yet, we lack contemporary information about floral foraging resources used by queens in early spring. Here, we use next-generation sequencing to characterize the floral species used by queens for pollen provisions during early nest establishment. We collected pollen loads from over 100 wild bumble bee queens at working farms, rural and city parks, and nature preserves across the Piedmont region of Virginia, USA. Using metabarcoding of two universal DNA barcodes for plants, ITS2 and rbcL, we determined the taxonomic composition of pollen used by queens. Pollen loads contained native and non-native woody (e.g., Cercis: Fabaceae, Prunus: Rosaceae, Salix: Salicaceae), herbaceous (e.g., Lamium: Lamiaceae, Viola: Violaceae), and vine (e.g., Lonicera: Caprifoliaceae) taxa. The non-native Lamium and Elaeagnus (Elaeagnaceae) most frequently hosted foraging queens, owing in part to their abundance across sites and the season. Pollen composition varied more over time than among bumble bee species or across sites, but land cover predicted a small amount of variation in pollen composition. Specifically, the percentage of crop land within 1 km increased the representation of Lamium in queen pollen loads, likely reflecting the abundance of the disturbance-adapted flower in fallow cornfields. Finally, the pollen communities detected by rbcL were twice as diverse as those by ITS2, perhaps owing to the better taxonomic resolution afforded by the fast-evolving rbcL marker. This study demonstrates that queens are flexible foragers and that among the most common Bombus species, plant phenology drives pollen use more than species identity. Further, this study highlights the importance of monitoring pollen diets to inform regional management strategies and considerations about metabarcoding techniques.