Immediate and delayed impact of heat stress on sperm quality in a key pollinator, the buff-tailed bumblebee (Bombus terrestris).

Bumblebee populations are declining at an alarming rate due to anthropic-driven global changes. Increasing temperatures and more frequent extreme heat events impose substantial physiological stress on organisms, reducing fitness and contributing to population declines and biodiversity loss. While the immediate impact of heat stress (HS) on bumblebee behaviour and physiology has been largely documented, the delayed effect on individual fertility remains more elusive. We quantified the effect of HS on the viability of sperm stored in the male accessory testes of buff-tailed bumblebees (Bombus terrestris) following exposure to 25°C (control), 38°C (mild HS) or 42°C (severe HS) for 60 min, 1 day or 15 days after exposure. Sperm quality was assessed by the percentage of viable sperm (sperm viability) and the relative changes in total number of viable sperm cells (Δ viable sperm count) between controls and treatments. Sperm quality was significantly impaired following exposure to 38°C and 42°C compared to control males. However, reproductive impairment was driven by HS intensity, with severe stress causing an acute and immediate loss of the percentage of viable sperm, and mild stress eliciting delayed effects on the number of viable sperm cells. By inducing both immediate and delayed damages to male reproductive function, HS may have potential consequences for fertility under warming conditions.