Recent findings by the Bumblebee Conservation Trust have revealed a another challenge that bumblebees face - HEAT.  The bumblebee walk (their annual survey of bumblebee numbers) of 2022, when three heatwaves were recorded noted that the number of 

• Buff tailed bees fell by 25%
• Carder bumblebees fell by 35%
• Red tailed bumblebees fell by 20%

The numbers of queens was also lower in the following year.  There are reasons for this reduction in numbers of the bumblebees.

Historically speaking, UK bumblebees have adapted / evolved to suit our cooler climate.  They have ‘fat’, hairy bodies that help them retain heat, and their flight muscles generate significant heat. These features are good when it is chilly outside.  However, once the ambient air temperature exceeds 28+, they struggle to fly as their flight muscles generate so much heat that their bodies begin to overheat.  They can no longer forage and they may die.

Buff-Tailed Bumblebee

Also, as temperatures go up, plants too begin to suffer.  They may wilt and experience water stress.  As plants struggle to obtain water, so nectar [and pollen] production will fall.  This means that the bees will need to forage over greater distances.  The energy expenditure may not be proportional to the nectar collected.  If the bees cannot collect enough nectar and pollen to take back to the colony, so the growth of the colony / larvae will be affected. It is also the case that bumblebees get their water from the nectar collected. Dwindling food supplies inevitably lead to fewer bees.  Research has also shown that high temperatures reduce bumblebees ability to detect scent, which again reduce the effectiveness of their foraging.  The loss of scent detection continues for a day or so after the heat has dissipated.

Just as heat waves lead to an increase in the number of ‘excess deaths’ in human populations, so they affecting our bumblebee species.

Article on the effects of heat on bumblebees : https://www.bumblebeeconservation.org/news/heatwaves-and-bumblebees-lessons-from-beewalk/

Article on scent detection in bumblebees : https://royalsocietypublishing.org/rspb/article/291/2029/20240352/116502/The-heat-is-on-reduced-detection-of-floral-scents