Pollinators forage in a multidimensional floral marketplace: rather than collecting a single reward, floral visitors often select among flowers that differ in the composition, quality, and chemistry of multiple rewards. For example, the foraging decisions of bees are contingent not only on the nutritional quality of pollen (primarily protein and lipids) and nectar (primarily carbohydrates), but also their secondary chemistry. Though nectar secondary chemistry has clearly-demonstrated impacts on bee foraging decisions and plant fitness, surprisingly little is known regarding the role of pollen chemistry in these interactions. This bias is especially notable considering theoretical support for the idea that plants might defend pollen to mitigate consumptive emasculation. Here, we asked two questions about the consequences of chemically defended pollen: 1) Do bees discriminate against flowers with chemically defended pollen at an intra-patch scale, and 2) Can flowers overcome this discrimination by providing high quality nectar? We tested the foraging decisions of 149 captive bumblebee (Bombus impatiens) foragers on two-‘species’ arrays of artificial flowers capable of offering pollen and/or nectar. Across four treatments, we varied whether co-flowering competitors offered putatively defended pollen (adulterated with quinine, a defensive alkaloid, or neutral cellulose), alongside nectar quality (nectarless, 15%, or 50% sucrose).
Results/Conclusions
We found that bees developed a strong aversion to flowers with chemically defended pollen, but that this was dependent upon the quality of nectar offered by flowers. When co-flowering species offered identical nectar rewards (15%), bees developed significant preferences for plants with undefended pollen (LRT, Χ2=7.9482, p<0.01). Alternatively, when flowers with defended pollen offered higher quality nectar (50%) than coflowering competitors (15%), bees foraged as if pollen was undefended, developing a significant preference for defended-pollen plants (LRT, Χ2=23.867, p<0.0001). Additionally, we found that bees spent significantly less time foraging on arrays where one species offered defended pollen (LRT; Χ2=356.11; p < 0.0001), raising the intriguing possibility of reverse-magnet species effects. These findings raise questions about how bees might forage in real landscapes where flowers differ dramatically in nutritional composition and chemical defense, and suggest one path to solving the “pollen paradox”.