The impact of yeast presence in nectar on bumble bee behavior and fitness

The presence of yeasts in pollen and floral nectar is rather the norm than the exception. Due to the metabolic activities of yeasts, sugar and amino acid composition of nectar often drastically change and may negatively impact the nutritional value of nectar for pollinators and hence insect fitness. On the other hand, the presence of yeasts in floral nectar may also increase its nutritional value due to yeast's probiotic effect and the release of yeast's metabolites. In this study, we investigated whether the presence of defined flower‐ and insect‐associated yeasts affected individual and colony fitness of the bumble bee pollinator Bombus terrestris. Specifically, we tested whether the presence of yeasts in nectar affected bumble bee foraging behavior and nectar consumption, individual growth and colony development, larval and queen mortality, and mating success. Quantitative analyses of sugar and amino acid profiles showed that nectar yeasts significantly affected the chemical composition of nectar. However, dual‐choice experiments indicated that yeast inoculation did not significantly affect foraging behavior or consumption rates. Nest development, on the other hand, was significantly affected by the presence of yeasts, but effects largely depended on species identity, with Candida bombiphila, Metschnikowia gruessii, and Rhodotorula mucilaginosa having the largest positive impact on colony growth. Interestingly, the effects at the colony level were more pronounced than at the individual level. In vitro growth tests further showed that yeasts have the potential to suppress the growth of the bumble bee gut pathogen Crithidia bombi. Overall, these results demonstrate that nectar‐inhabiting yeasts can have diverse effects on bumble bee fitness and therefore may mediate plant–pollinator mutualisms. That which is not good for the bee‐hive cannot be good for the bees.—Marcus Aurelius About 90% of all plant species are pollinated by animals (Ollerton et al. 2011). Most insect pollinators are able to cover relatively large distances and therefore are efficient vectors of plant gametophytes (pollen). However, flying large distances requires high amounts of carbohydrates (Nicolson 2007). Floral nectar is the main reward plants offer to pollinators and consists of an aqueous solution that is mainly composed of sugars (sucrose and its monomers, glucose and fructose) and, to a lesser extent, amino acids, lipids, alkaloids, organic acids, proteins, and phenolics (Baker and Baker 1983, Simpson and Neff 1983, Nicolson 2007). Besides carbohydrate quantity, different