In this post, Daniel Kenna from Imperial College London’s Silwood Park Campus, explores how bumblebee flight responds to temperature change, discusses what this implies about the effects of climate change on our pollinators, and recounts his experiences in the lab.

About the paper

Bees’ flight performance affects their ability to pollinate plants, which is a crucial service for many of our crops and garden plants. Considering current climate change predictions, there is a need to understand how flight behaviour responds to thermal variation, but there has been surprisingly little work on this.

This might be due to the challenges involved. For example, in field experiments, how do you isolate the effect of temperature on behaviour, when so many other factors such as wind, rain, and humidity are at play? And in lab experiments, how do you persuade bees to fly? To overcome these challenges, we used a tethered flight mill setup under controlled lab conditions to measure the flight ability of bumblebees. From this we produced a thermal performance curve to predict flight performance over a 10-35^oC temperature range.

The flight mills, which are essentially a carousel for bees, were built and tested by an innovative team of technicians and scientists at Imperial College. We temporarily attached bumblebees to the mills, which allowed them to fly in circles whilst the distance and speed of flight was recorded.

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We found that flight performance rose rapidly as temperatures increased, peaking between 24-27^oC. Beyond this, however, performance started to decline. Our results suggest that European and North American bumblebee populations in cooler northern ranges may see benefits to their flight performance under climate warming. But populations in warmer southern ranges, where temperatures above 27^oC are more readily exceeded, may not be so lucky. Worryingly, extreme weather events, such as the increasingly frequent and intense heatwaves we’ve experienced in recent years, could consistently push temperatures beyond the comfortable flight range for certain species of bumblebees.

About the research

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I was most surprised by how detrimental the coldest temperatures were to bumblebee workers, because they are generally considered a group of insect pollinator that can cope well with cool conditions due to their large size and hairy bodies. Our flight mill setup cannot fully replicate real-world settings, where factors like wind and solar radiation may help bees fly. Nevertheless, the bees in our study struggled to initiate and sustain flight between 12 and 15^oC. Here in the UK, we experience these cooler temperatures a lot during spring and even summer months. Also, with climate change comes more erratic weather patterns, and our work suggests prolonged cold spells could be extremely difficult for bumblebees to cope with. This shows just how important it is to have an abundance of flowering plants in the environment, especially early in the year, so bees do not need to travel far to find pollen and nectar.

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Interestingly, bumblebee workers within a single colony can vary substantially in body size, with some workers being six times larger than others! Our work showed that at low temperatures, it was only really the largest workers that were able to sustain flight. These results can help us to understand how different sized flying insects may respond to future climate change. But this is not just limited to pollinators. For example, flight capacity also underpins the spread of insect-borne diseases and pest outbreaks. Applying our experimental setup and findings to other species could help us anticipate the effects of warming on these global issues.

About the author

I became interested in social insects whilst studying hornets and ants during degree projects, and I’m fascinated by the cooperative traits that have allowed them to conquer most terrestrial habitats. During my studies, I was also struck by the growing number of reports of an ‘insect apocalypse’, and more generally on how human induced changes are impacting biodiversity loss. Together, this drove me to undertake my PhD, where I now look at how bumblebees respond to multiple anthropogenic stressors such as climate change and pesticide exposure (https://doi.org/10.1002/ece3.5143).

Throughout my PhD my most common colleagues have been bumblebees, but as co-workers go, they are pretty impressive. I am continually amazed by how they work as a collective, and I have witnessed some impressive feats, with one bee in our experiment flying for over three hours without stopping! Bees were flown on the flight mills under white light to try and mimic normal foraging conditions. However, most other tasks in the lab, such as handling colonies or attaching tags to bees, are done under red light, which is like being in a dark room when developing photographs. Bees generally cannot see deep red light, so it doesn’t disturb their behaviour, and this allows us to work with them without them becoming too agitated or aggressive. Similar to when you walk out the cinema on a sunny day, coming out of a red-light room after a long stint can knock your senses for six. And after years of lab work, I don’t think I’ll ever stop hearing bees buzzing in my sleep! But on the plus side, bee themed presents seem to be all the rage at the moment, and over the last four years I’ve built up an impressive collection!

I’ve been lucky to have been part of a brilliant lab group during my PhD (www.gillinsectresearch.com), for which I’m very grateful. I’m excited to see what the future holds in store for the group, which is now looking to expand this current research to explore how warming can affect pollination delivery across different types of landscapes.

Read the article in full here