Bumblebees can detect floral humidity

Abstract

Floral humidity, a region of elevated humidity proximal to 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.Summary statementWe demonstrate for the first time that bumblebees show a preference to elevated floral humidity and can learn to distinguish flowers that differ in floral humidity levels.

The ability of bumblebees Bombus terrestris (Hymenoptera: Apidae) to detect floral humidity is dependent upon environmental humidity

“…Our observations in high humidity support this view as bees used humidity differences to determine rewards, but were unable to learn a positive association to humid variants. Observations by Harrap et al (2020, 2021) that floral humidity is produced at varying and sometimes negligible levels, and that bumblebees learnt to associate rewards with both dry and artificial flowers led to the suggestion that floral humidity may in some cases be used instead to confer the identity of a flower to pollinators instead of reward status. The fact that bees learned to associate humidity with both non-rewarding and rewarding flowers supports the interpretation that in some cases floral humidity may be used to confer identity instead of reward status (Harrap et al 2021), although this is less likely to be the case in extreme conditions, and more likely in conditions of intermediate humidity where osmotic demands are not strongly weighted in either direction.…”

“…The artificial flowers used were constructed according to the design of the ‘passive artificial flower’ used by Harrap et al (2021). The flowers were identical except for their internal components, which were controlled to create a humid variant with elevated floral headspace humidity, and a dry variant.…”

“…In order to check that the humid variants created at least a 2% rise in relative humidity compared to background levels, a hand-operated hygrometer was used to read the relative humidity above each flower. Harrap et al (2021) report that a more accurate reading of the relative humidity difference when the hand held hygrometer read 2% could be taken using DHT-22 humidity probes (Aosong Electronics Co., Ltd., Huangpu) attached to a 6-axis articulated Staublie RX 160 robot arm (Pfäffikon, Zurich), and was roughly 1.36%. This value reflects the differences in relative humidity observed by the same authors between various flower species and between flowers and background humidity, and is therefore a similar value to the change in relative humidity that might be experienced by a foraging bee in the natural environment.…”

“…Evidence for floral humidity acting as a signal currently comes from two pollinator species. The hawkmoth Hyles lineata was demonstrated (von Arx et al 2012, von Arx 2013) to show a preference to artificial flowers producing floral humidity comparable to that produced by Oenothera caespitosa (which is naturally pollinated by H. lineata ), and the bumblebee Bombus terrestris is capable of learning and differentiating between artificial flowers that mimic natural patterns (Harrap et al 2021). Other existing evidence of the capacity of pollinators to respond to floral humidity is limited (von Arx 2013), with non-experimental observations that flies may use floral humidity in addition to other floral display traits produced within Indian alpine environments (Nordström et al 2017).…”

“…In this study, we use differential conditioning techniques to explore the detectability of an elevated floral humidity signal comparable to natural flowers when presented to bumblebees Bombus terrestris audax (Harris) against different levels of signalling noise comparable with native conditions. This species is suitable for these experiments, as B. terrestris foragers have been used in similar differential conditioning experiments to those described here, showing a capacity to detect floral humidity and associate nectar with both humid and dry artificial flowers (Harrap et al 2021). If there is no effect of noise on detectability, making floral humidity a robust signal, we would expect that the bees’ learning responses would not be affected by the environmental conditions.…”