Bumblebees can detect floral humidity

Abstract

Floral humidity, a region of elevated humidity in the headspace of the flower, occurs in many plant species and may add to their multimodal floral displays. So far, the ability to detect and respond to floral humidity cues has been only established for hawkmoths when they locate and extract nectar while hovering in front of some moth-pollinated flowers. To test whether floral humidity can be used by other more widespread generalist pollinators, we designed artificial flowers that presented biologically-relevant levels of humidity similar to those shown by flowering plants. Bumblebees showed a spontaneous preference for flowers which produced higher floral humidity. Furthermore, learning experiments showed that bumblebees are able to use differences in floral humidity to distinguish between rewarding and nonrewarding flowers. Our results indicate that bumblebees are sensitive to different levels of floral humidity. In this way floral humidity can add to the information provided by flowers and could impact pollinator behaviour more significantly than previously thought.

Approach Direction Prior to Landing Explains Patterns of Colour Learning in Bees

“…The bees would typically extend the proboscis while they landed, therefore we did not analyse any landing or drinking responses. In the final part of the approach, just prior to landing, the bees’ antennae came close or even in contact with the nib, and we must assume that they possibly see it and sense the humidity from sucrose solution in the small opening ( Harrap et al, 2021 ). We therefore excluded any data points closer than 0.5 cm from the analysis of the trajectory data.…”

The role of petal transpiration in floral humidity generation

“…and California poppy Eschscholzia californica Cham.. These species are appropriate choices for demonstrating the role of transpiration in floral humidity generation, as both produce larger amounts of floral humidity compared to other flower species (Harrap et al 2020a ) comparable to levels which have been demonstrated to be able to influence pollinator foraging decisions (von Arx et al 2012 ; von Arx 2013 ; Wolfin et al 2018 ; Harrap et al 2021 ). E. californica has been identified (along with other poppies) to produce little in the way of nectar rewards (Hicks et al 2016 ), and so nectar evaporation is unlikely to explain E. californica humidity production.…”

“…Floral humidity, as part of this multimodal floral display, can influence pollinator foraging decisions. Hawkmoths (von Arx et al 2012 ), bumblebees (Harrap et al 2021 ), and flies (Nordström et al 2017 ) have innate preferences for flowers that produce higher floral humidity intensities when they are able to choose between flowers producing differing intensities. Furthermore, floral humidity differences between flowers, regardless of whether elevated rewards are associated with higher floral humidity production or not, can aid bumblebee learning of rewarding flowers (Harrap et al 2021 ).…”

“…Hawkmoths (von Arx et al 2012 ), bumblebees (Harrap et al 2021 ), and flies (Nordström et al 2017 ) have innate preferences for flowers that produce higher floral humidity intensities when they are able to choose between flowers producing differing intensities. Furthermore, floral humidity differences between flowers, regardless of whether elevated rewards are associated with higher floral humidity production or not, can aid bumblebee learning of rewarding flowers (Harrap et al 2021 ). By influencing floral preferences and learning, floral humidity may affect foraging success of naïve and experienced pollinators (Raine and Chittka 2008 ) and will in turn influence visitation rates of pollinators and thus pollen receipt and export (Ashman et al 2004 ; Schiestl and Johnson 2013 ).…”

“…learning of rewarding flowers (Harrap et al 2021). By influencing floral preferences and learning, floral humidity may affect foraging success of naïve and experienced pollinators (Raine and Chittka 2008) and will in turn influence visitation rates of pollinators and thus pollen receipt and export (Ashman et al 2004;Schiestl and Johnson 2013).…”