A heatwave may not have to kill a bee to harm it. New research has found that extreme heat during their larval development stage can leave solitary bees with lasting damage to their reproductive health, raising fresh concerns about how climate change could affect wild pollinators around the world.
The study, led by researchers at the University of Hull and published in the Journal of Thermal Biology, focused on red mason bees (Osmia bicornis), a solitary species often seen nesting in hollow stems, crevices and bee hotels.
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To test what happens during extreme weather, the research team recreated the three-day heatwave that affected the UK in July 2022. Developing red mason bee larvae were exposed to temperatures that peaked at 40°C each day, while a control group experienced more typical July temperatures for Hull, peaking at about 25°C.
At first, the heat-exposed bees seemed to have escaped unharmed. They completed their development, spun cocoons, overwintered and emerged as adults nine months later. The hidden damage only became clear when the researchers examined their reproductive organs.
In males that had experienced the simulated heatwave, sperm activity was about half that of the control group, while sperm counts were reduced by roughly one third. Females were harmed too, with both the size and number of developing eggs reduced by about 15 per cent.
“The heatwave had wrecked their fertility, especially in males,” says James Gilbert, senior lecturer in zoology at the University of Hull and a co-author of the research.
While social bees that live in large colonies, such as honeybees and bumblebees, have some protection against temperature extremes because workers can help regulate conditions inside the nest, solitary bees have no such back-up system. Their young develop in nest cells that are much more exposed to the surrounding environment – and are therefore more vulnerable to heatwaves.
The research also suggests that the effects of a heatwave may not be visible straight away. “A heatwave one year could lead to a drastic drop in the number of bees the following year, and therefore less efficient pollination for key crops like apples, cherries and oilseed rape,” Gilbert explains.
As most bees are solitary, the consequences for farmers and ecosystems experiencing extreme heat in many parts of the world could be catastrophic, particularly in regions already facing pressure from habitat loss, pesticides, disease and poor nutrition.
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