Lasting Impacts of Sublethal Prepupal Heatwaves on Fertility-related Traits of the Solitary Bee Osmia bicornis

Climate change is increasing the frequency and intensity of heatwaves, which can have severe consequences for insect physiology, development, and reproduction. Solitary bees, key ecosystem pollinators with commercial value, may be particularly vulnerable due to their lack of social thermoregulation and limited ability to buffer nest temperatures. Despite growing awareness of temperature effects on insect reproduction, less is known about how sublethal heatwaves during development impact adult solitary bee reproductive traits. We investigated the effects of a field-realistic 3-day fluctuating heatwave regime (<40 °C), below the CT max for this species, during prepupal development on reproductive traits of the red mason bee, Osmia bicornis . This simulated heatwave regime replicated temperatures experienced in the UK during July 2022. After overwintering following juvenile exposure, we assessed male sperm and female oocytes to examine effects on reproductive potential. Survival to adulthood was not affected, but experiencing the 3-day heatwave regime as a juvenile led to significant sublethal reproductive costs. Male sperm counts were reduced by 33 % and motility lowered by 53 %, whilst sperm length was shortened by 17 %. In females, production of oocytes was reduced by 15 %, as well as a reduction in terminal oocyte volume of 17 %. These results show that heatwaves during solitary bee development can have lasting effects on fertility-related traits, with potential implications for population dynamics and pollination services if these disruptions translate into reduced lifetime reproductive output. Deepening our understanding of how these extreme weather events affect insect reproduction is crucial for better predicting population vulnerability and informing more effective conservation strategies in a warming world. • Simulated sublethal prepupal heatwaves alter adult solitary bee fertility-related traits • 3-day heatwaves reduce sperm number, sperm motility and sperm length in male bees • Heat-exposed females produced fewer and smaller oocytes • Sublethal heat stress may threaten pollination services under climate change