Bumblebees are literally cooking themselves from the inside out as heatwaves become more frequent, new data reveals — with some of the UK’s most common species declining by up to 35%.
The warning, shared exclusively with the BBC by the Bumblebee Conservation Trust, draws on the BeeWalk national monitoring scheme. The data shows sharp falls after the record-breaking heat of 2022, when the UK had three official summer heatwaves and temperatures reached 40.3C. Britain is now emerging from a third heatwave in as many months, with temperatures in the range where bumblebees struggle to fly without overheating.
“UK bumblebees decline up to 35% as heatwaves cause overheating, with data showing lasting population impacts.”
Unlike honeybees, UK bumblebees are adapted to cold — their fat, furry bodies retain heat and powerful flight muscles warm them as they fly. But in a heatwave, those same adaptations become a liability. Dr Richard Comont, the Trust’s science manager, explains that once air temperatures reach 28C to 32C, bumblebees can start to struggle to fly without overheating. “They actually cook themselves from the inside out,” he says. “Once their internal body temperature is much above 40C, they literally start dying, they start keeling over.” If they cannot fly, they cannot forage.
In 2022, the effects were stark. Red-tailed, buff-tailed and common carder bumblebees — three of the country’s most common species — peaked well below normal levels: down 20%, 25% and 35% respectively. The declines deepened as autumn arrived and colonies continued to struggle. The impact persisted beyond that summer: BeeWalk data showed overall queen numbers were below average in March, April and May 2023.
Scientists stress that warm, sunny weather is not automatically bad for bumblebees. Previous warm years have been good for some species, particularly those with more southerly ranges. The problem is extreme heat and drought. As well as overheating bodies, the other issue is dwindling food supplies. Colonies depend on workers bringing back a steady supply of nectar and pollen through the summer. If flowering plants begin to dry up, they produce less.
Dr Paul Williams, a bumblebee expert at the Natural History Museum, notes that foraging is an energy calculation for bumblebees. The nectar return has to make the expenditure of energy worthwhile. With heatwaves expected to intensify, the future looks precarious for these vital pollinators, which sustain wildflowers and crops the UK relies on. The question is how much more they can take.