Bumblebees in heatwaves

The gentle buzzing of bumblebees is one of the iconic sounds of the British summer. But how will our favourite furry bees respond to the UK’s unprecedented temperatures?
Bumblebees are built for cooler climes. Unlike most insect groups, they actually prefer the milder temperatures of the temperate regions – that’s why Britain has almost 10% of the world’s bumblebee species, a much higher proportion than for most other insects. The reasons for this date back around 35 million years, to the proto-bumblebees of northern India.
At that point the Himalayas were a relatively low mountain range, with some fairly average bees. But as the Indian sub-continent pushed into Asia, the climate changed to become cooler and damper, and the bees had to adapt to survive. They already generated a lot of heat through flapping their wing muscles, but they became furrier, to keep that heat in, and rounder and larger to reduce surface area to stay warmer for longer. Eventually they became our familiar bumblebees.
But their adaptations limited their options. They were so good at remaining warm, that if they tried to fly in hot climates they couldn’t vent heat quickly enough. In trials, the American Yellow-faced bumblebee Bombus vosnesenskii started to overheat after just three minutes of flight at 42°C. The bees started trying to cool their heads by regurgitating a droplet of nectar and stirring it round with their proboscis, increasing the surface area and cooling their heads down by around 2°C. When temperatures reached around 44-45°C, the bees did this even without flying.
That temperature intolerance meant that instead of dispersing south from the Himalayas into the tropics of Northern India, they had to spread northwards, into the cooler temperate regions of Asia, Europe and North America. It’s also the reason why, in the tropics, bumblebees are generally found at high altitudes, where it’s that bit cooler.
Britain, then, was more or less ideal for the group with our mild summers. Our general lack of drought suited them too – warm and wet is good plant-growing weather, which means lots of pollen and nectar available all summer. We know that drought stresses plants, causing them to produce fewer flowers, smaller flowers, and less nectar and pollen within flowers, and that droughted flowers attract fewer bees. Hot, dry landscapes are worse for bees than warm, damp areas.
British bumblebees aren’t as temperature-tolerant as Bombus vosnesenskii. In the UK, bumblebee activity starts to drop off at around 28°C. Cuckoos and males tend to tuck themselves away in the shade and wait it out: workers keep foraging, but species gradually give up as they reach their thresholds. Buff-tailed and Red-tailed seem to be the last to go, along with some of the rarer southern species. Once temperatures reach about 32°C, virtually all outside activity ceases, though nest-fanning (workers using their wings to blow air through the nest) will continue.
Bumblebees are still able to forage on hot days – they’re out first thing in the morning and last thing at night, they just need more of a siesta in the middle of the day. But that cuts into their foraging time, and drought-hit plants are less rewarding to visit – the day’s foraging suddenly brings in a lot less food for the colony. For bumblebees that are already struggling, that isn’t great news.
Dr Richard Comont, Science Manager
Twitter @RichardComont
With the world talking climate, our science team take a look at what a changing world means for the UK bumblebees.