Short-term heat exposure at sublethal temperatures reduces sperm quality in males of a solitary bee species, Osmia cornifrons

Increased temperature variability and extreme weather events associated with climate change can be detrimental to bees and lead to their population declines. While there is some research on the effects of heatwaves on insect biology and reproduction, impacts on male insect reproduction are not well described. Solitary bees may be more exposed to temperature variation than social bees, since there is no mitigation by group thermoregulation. Here, we evaluated the effects of sublethal heat exposure (at 30 °C and 38 °C) on reproductive fitness of adult males of a solitary bee species, Osmia cornifrons. After 4 h of heat exposure, bees were maintained at 24 °C for 48 h in laboratory cages to reach sexual maturity and later analyzed for sperm abundance (quantity) and proportion of viable sperm (quality). Bees that were reared and maintained at 24 °C served as controls. Despite rehabilitation after heat exposure, both sperm quantity and quality of male O. cornifrons bees were significantly reduced in heat-treated groups compared to controls. These results demonstrate that the exposure to elevated non-lethal temperatures, even for short periods and allowing time for recovery, may negatively affect the reproductive capacity of male solitary bees. Our findings reveal a possible mechanistic explanation for wild bee population declines. Given the importance of solitary bee species (such as O. cornifrons) in natural and agricultural landscapes, our study provides valuable insights into the potential consequences of climate change on these insect populations and the ecosystems they inhabit.