Hungry for quality—individual bumblebees forage flexibly to collect high-quality pollen

Abstract: Nutritional deficits may be one factor contributing to the ongoing decline of wild and managed bees. As a consequence, interest in understanding the effect of floral resource availability on nutritional intake - and subsequently bee health and performance - has increased. However, the proximate mechanisms underlying bee foraging choices are still poorly understood. We do, for instance, not yet know whether and how bees assess the nutritional quality of pollen or whether they use information on pollen quality to adjust foraging patterns. In a previous study, we showed that Bombus terrestris workers use their sense of taste to discriminate pollen differing in concentration and, thus, nutrient content. We now investigated whether they translate this ability into foraging behavior by observing microcolonies offered the choice between two feeders containing different pollen types and concentrations and, thus, qualities. We examined whether bumblebees used direct (chemotactile) or indirect (olfactory) cues to assess pollen quality and whether they individually assessed pollen quality or relied on larval feedback. By changing pollen quality after 21 days, we further tested whether information on pollen quality was subsequently associated with other (faster assessable) cues (e.g., odor) or continuously re-assessed. We found that bees always preferred pollen of higher quality and individually assessed quality based on chemotactile cues. Moreover, changing pollen quality resulted in rapid equivalent changes in foraging behavior, even without feedback from larvae. Consequently, individual bumblebee foragers continuously re-assess pollen quality to provide their colonies with high-quality food. Significance statement: The ongoing decline of wild and managed bees is likely driven by several factors with nutritional deficits being one of them. Because bees are amongst the most important pollinators, interest in understanding the effect of floral resource availability on nutritional intake and, subsequently, bee health and performance has increased. However, the proximate mechanisms underlying bee foraging choices are still poorly understood. In this study, we investigated how bumblebees differentiate between different pollen qualities by observing microcolonies offered the choice between different diets. We found that bees always preferred pollen of higher quality and individually assessed quality based on chemotactile cues. This finding increases our understanding of how social bees assess nutritional quality and make appropriate foraging choices.