Pollen and yeast change nectar aroma and nutritional content alone and together, but honey bee foraging reflects only the avoidance of yeast

Authors: Rering, Caitlin; Rudolph, Arthur; Beck, John

Submitted to: Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2021
Publication Date: 4/19/2021
Citation: Rering, C.C., Rudolph, A.B., Beck, J.J. 2021. Pollen and yeast change nectar aroma and nutritional content alone and together, but honey bee foraging reflects only the avoidance of yeast. Environmental Microbiology. 23(8):4141–4150. https://doi.org/10.1111/1462-2920.15528.
DOI: https://doi.org/10.1111/1462-2920.15528

Interpretive Summary: Many crops require pollination (the transfer of pollen between flowers) in order to produce fruit or seed. In some plants, pollen is very light and can be moved via wind. For others, insect or animal visitors such as bees are needed to move pollen. Pollinators visit flowers to collect rewards such as sugar-rich nectar, which is the chief source of energy for adult bees. Bees display preferences for certain types of nectar based in part on its chemical composition. Pollen and microorganisms like yeast and bacteria are often found in floral nectar and can lead to chemical changes within the nectar, which in turn may change the taste and scent of nectar, as well as affect bee’s preference for a nectar. ARS Scientists at the Center for Medical, Agricultural, and Veterinary Entomology, in Gainesville, FL investigated how sunflower pollen and a nectar-dwelling yeast changed nectar chemistry and honey bee preference. Bees avoided nectar containing yeast, but foraged from yeast-free nectar regardless of pollen content. Knowledge of the changes common nectar contaminants confer on nectar and honey bee responses to those changes will help ARS scientists develop effective strategies to increase the health of beneficial insects that are important to agricultural commodities.

Technical Abstract: Floral nectar often contains pollen and microorganisms, introduced via interactions with floral visitors, the local host-flower and plant, and the wider environment. The presence of pollen and/or microbes within nectar can change its chemical composition, which in turn may impact pollinator affinity. However, these effects, particularly in combination, remain understudied. Here, in a factorial experimental design, we examine the impacts of the presence/absence of the nectar specialist yeast, Metschnikowia reukaufii, and the addition of sunflower (Hellianthus annus) pollen at three concentrations (0, 10, and 100 grains/µL) to a synthetic nectar. Changes to nectar chemistry and yeast growth were monitored, and the foraging preferences of honey bees evaluated in an apiary-based assay. Yeast cell densities increased substantially with greater pollen content: peak population densities in high-pollen samples exceeded the no-pollen samples by over 30 million cells/µL on average. Both pollen and yeast introduced volatiles to nectar. The emission of yeast-derived volatiles was positively correlated with pollen concentration, likely as a result of the higher yeast cell densities achieved with pollen addition. Conversely, the emission of many pollen-derived volatiles was reduced when nectar yeast were present. Despite reaching high cell densities, nectar yeast did not significantly impact sugar content. Pollen supplementation however reduced sucrose concentration by ~2/3 in high-pollen vs. no-pollen samples, and increased the concentration of its monomer component sugars, fructose and glucose. Honey bees avoided foraging on yeast-inoculated nectar, and foraged equally among un-inoculated nectars regardless of pollen content, underscoring the importance of microbial metabolites in mediating pollinator foraging.