Oviposited eggs are sensitive to experimental heatwaves

Extreme, variable heatwaves are predicted to increase in frequency under global warming, leading to thermal stress that can be damaging to biodiversity. Recently, it has been shown that exposure to heatwaves is detrimental for adult insect reproduction, reducing population viability. However, relatively little is known about how oviposited insect eggs, one of the early stages post‐fertilisation, are affected by heatwaves. Here, we explore the impact of short‐ and longer‐term thermal stress from experimental heatwaves in an economically important insect model, the red flour beetle Tribolium castaneum . We uncovered that even short‐term exposures (of 2 and 5 h at 30°C, 40°C, 42°C, 44°C, 46°C, 48°C and 50°C) resulted in a drastic decline in the survival of oviposited eggs, a trend that was also observed when eggs experienced longer durations (1, 3 and 5 days at 42°C). Experiments were run independently across two labs, with results found to be reproducible, reinforcing confidence in the findings. Collectively, our results suggest that oviposited eggs (early embryos) may be particularly vulnerable to heatwaves, even when the exposures are brief.