Bumblebees Can Detect Humidity of Flowers, Biologists Say | Biology

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Buff-tailed bumblebees (Bombus terrestris) are able to use differences in floral humidity to distinguish between rewarding and non-rewarding flowers, according to new research led by biologists from the University of Bristol and the University of Exeter.

By experimentally varying the levels of floral humidity in artificial flowers within a range that is biologically relevant, Harrap et al. show here that buff-tailed bumblebees (Bombus terrestris) are able to detect and utilize humidity differences in a flower foraging context. Image credit: Myriam.

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.

It is created by a combination of nectar evaporation and floral transpiration although the contribution of these two influences may vary between species.

Whether variations in floral humidity can be used as a foraging cue is poorly understood, and has only been demonstrated in a single pollinator species, the white-lined sphinx (Hyles lineate), a hawkmoth species frequently pollinating tufted evening primroses (Oenothera caespitosa).

We know that different species of plants produce flowers that have distinct patterns of humidity, which differ from the surrounding air, said lead author Dr. Michael Harrap, a researcher in the School of Biological Sciences at the University of Bristol and the Centre for Research in Animal Behaviour at the University of Exeter.

Knowing that bees might use these patterns to help them find food shows that flowers have evolved a huge variety of different ways of attracting pollinators that make use of all the pollinators senses.

To test whether floral humidity can be used by other more widespread generalist pollinators, Dr. Harrap and colleagues designed artificial flowers that presented biologically-relevant levels of humidity similar to those shown by flowering plants.

They also built a robotic sensor that was able to accurately measure the shape of the humidity patterning.

Buff-tailed bumblebees showed a spontaneous preference for flowers which produced higher floral humidity.

Furthermore, learning experiments showed that the bumblebees are able to use differences in floral humidity to distinguish between rewarding and non-rewarding flowers.

Our study shows that bumblebees not only use this sensory information to make choices about how they behave, but are also capable of learning to distinguish between humidity patterns in a similar way to how they learn to recognize the color or smell of a flower, said co-author Dr. Natalie Hempel de Ibarra, a researcher in the Centre for Research in Animal Behaviour at the University of Exeter.

If humidity patterns are important for attracting pollinators, they are likely to be one of several different signals (such as color, scent and pattern) that a flower is using at the same time, and could help the bee to identify and handle the flower more efficiently, added co-author Dr. Sean Rands, a researcher in the School of Biological Sciences at the University of Bristol.

The effectiveness of humidity patterns may depend upon the humidity of the environment around the flower; climate change may affect this environmental humidity, which in turn could have a negative effect on a visiting bee because the effectiveness of the humidity pattern will be altered.

The results were published in the Journal of Experimental Biology.

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Michael J.M. Harrap et al. Bumblebees can detect floral humidity. J. Exp. Biol, published online May 21, 2021; doi: 10.1242/jeb.240861

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