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Caffeine and ethanol in nectar interact with flower color impacting bumblebee behavior

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Abstract

Caffeine and ethanol are naturally occurring compounds in floral nectar. We examined how these compounds influenced pollinator behaviors including floral preference, floral constancy, and social behavior using bumblebees, Bombus impatiens, which were given prior experience foraging on either human blue or human white (hereafter blue and white) artificial flowers. Because flower color influenced bee behavior, with strong preferences for blue, we focused on the interaction between nectar chemistry and flower color. Bees that had experience with blue flowers preferred blue regardless of nectar chemistry. In contrast, for bees that had prior experience with white flowers, only the control treatment preferred white, while bees exposed to caffeine and ethanol showed no preference. The effects of nectar compounds may therefore only occur when bees are already foraging on a less-preferred color. We also examined the impact of nectar chemistry on the social behavior of joining other bees at flowers. In the same treatments for which bees showed a preference for previously experienced flower colors (all of the blue treatments and only the white control), bees also preferentially visited unoccupied flowers. In the treatments where bees showed no color preference, however (the white caffeine and ethanol treatments), bees showed no preference for unoccupied flowers. We show that the impacts of field-realistic levels of caffeine and ethanol in nectar on pollinator behavior depend on flower color, highlighting that the potential costs and benefits of nectar chemistry to plants are likely to be dependent on bee behavioral biases for other floral traits.

Significance statement

Flower nectar often contains toxic compounds hypothesized to impact pollination, but little research has shown their effects on the behavioral decisions of free-flying bees. Caffeine and alcohol occur in the nectar of some flowers. We found that bee response to these nectar compounds depends on the flower color. Bees preferentially visited blue flowers regardless of nectar chemistry, but the presence of caffeine or alcohol reduced bee color preference when bees had experience foraging on white flowers. The bumblebee’s social behavior of joining other bees at flowers showed related effects; in treatments where bees showed a preference for flower type, they also preferred to forage alone. This research highlights that bees make decisions based on the interaction between multimodal cues including nectar chemistry, and therefore the strength of selection on nectar chemistry is dependent on bee behavioral biases for other floral traits.

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Acknowledgements

Bowdoin undergraduate students Ayana Harscoet, Danielle Horne, Shawn Bayrd, Jonothan Harris, and Susannah Lawhorn conducted video analysis. Illustrations of bees used in figures were created by Damond Kyllo. Thanks to Claire Hemmingway for the helpful comments on the manuscript.

Funding

This research was funded by Bowdoin College and the National Science Foundation (NSF IOS-1907491 to [AAA and PLJ]).

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Correspondence to Patricia Jones.

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Jones, P., Agrawal, A.A. Caffeine and ethanol in nectar interact with flower color impacting bumblebee behavior. Behav Ecol Sociobiol 76, 103 (2022). https://doi.org/10.1007/s00265-022-03208-3

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