Temperature extremes on both ends impair bees’ flight, raising new concerns about climate change

Rising mean temperatures could help bees in colder areas fly better. Overall, however, climate change is going to impair the insects’ ability to fly, mainly through the increase in freak and extreme weather events that it promotes.

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In order to do their job (pollination), bees need to be able to fly. And we definitely need pollinators to do their job. But, according to researchers from Imperial College London, rising temperatures all over the world are likely to impair bees’ flight performance. While colonies in areas closer to the poles (which are naturally colder) might actually see an improvement in their flight performance, as their ranges shift closer to the bees’ ideal temperatures, the increase in extreme weather brought about by higher temperatures means that, overall, bees worldwide will have a harder time flying around.

According to the findings, bee flight performance peaks at around 25-27°C but declines rapidly in both lower and hotter temperatures.

Too hot for comfort

“Climate change is often thought of as being negative for bumblebee species, but depending on where in the world they are, our work suggests it is possible bumblebees will see benefits to aspects of an important behavior,” explains first author Daniel Kenna from the Department of Life Sciences at Imperial. “However, more extreme weather events, such as cold snaps and the unprecedented heatwaves experienced in recent years, could consistently push temperatures beyond the comfortable flight range for certain species of bumblebees”.

“These risks are particularly pertinent for ‘fixed colony’ pollinators like bumblebees, which cannot shift their position within a season if conditions become unfavorable, and potentially provide a further explanation as to why losses have been observed at species’ southern range limits.”

Air temperature has a direct effect on the body temperature of flying insects, including bees, the team explains — and body temperature has an impact on their ability to fly. Temperatures that are too low impair muscle activity, making them function too slowly to support flight. In too warm temperatures, the insects overheat.

In order to measure the impact of air temperature on bees’ ability to fly, the team temporarily attached bumblebees to ‘flight mills’ — devices in which they fly in circles like a carousel while their speed and flown distance were recorded. Bumblebees of several body sizes were tested at temperatures from 12-30°C, and the results were used to construct a thermal performance curve (TPC). This TPC predicts that while bumblebees can fly around 3km at their thermal optimum, this distance would fall to under 1kmThis TPC predicts that whilst bumblebees can fly around 3km at their thermal optimum, this average flight distance could be reduced to under 1km when temperatures rise to 35°C. At 10°C, this distance could drop to as little as a few hundred meters.

Observationally, the team found that temperatures of 15°C and below would frequently limit their flights to under 100m. Larger bees were the only ones that managed to fly in these conditions, too, which suggests that smaller individuals might be more affected by cold days but stand to benefit more from warmer conditions.

At temperatures of 15°C and below, the team observed that bees were demotivated to fly and frequently would not fly past 100m. Moreover, it was only the larger bees that successfully flew at these low temperatures, suggesting smaller individuals dislike cold days but may benefit more from climate warming.

Lead researcher Dr. Richard Gill, from the Department of Life Sciences (Silwood Park) at Imperial, said:

“While we still need to understand how these findings translate to factors like foraging return to colonies and pollination provision, as well as applicability to other bumblebee species, the results can help us understand how smaller versus larger flying insects will respond to future climate change,” says co-author Dr. Richard Gill, also from Imperial.

“It’s not just pollination: how different flying insects respond to warming temperatures could also affect the spread of insect-borne diseases and agricultural pest outbreaks that threaten food systems. Applying our experimental setup and findings to other species can help us to understand future insect trends important for managing service delivery or pest control methods.”

For now, the team’s focus was on how climate change impacts flying efficiency exclusively, but they plan to expand their work to include its effects on other stressors such as pesticide exposure. Furthermore, they’re also looking to examine how climate change stands to impact pollination efficiency across different landscapes.

The paper “Thermal flight performance reveals impact of warming on bumblebee foraging potential” has been published in the journal Functional Ecology.

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