Heatwaves increase larval mortality and delay development of a solitary bee

Abstract

Heatwaves are expected to increase in frequency, intensity and duration due to climate change. For organisms like insects with discrete development, sensitivity may differ among life stages. Thermal sensitivity is of particular concern for species like bees that provide critical ecosystem services. Although social bees moderate nest temperatures through worker behaviour, solitary bees do not thermoregulate their nests, making immobile developing offspring especially vulnerable to such extreme events. We studied the effects of heatwaves on larval development in the solitary bee, Osmia lignaria, an important orchard pollinator and model species for solitary bee biology. We used a factorial design to assess the impacts of heatwave temperature and duration on larval mortality and development rate. Larvae were exposed to heatwaves under realistic diel temperature regimes, with daytime maxima of 31 or 37°C for 4 or 7 days at the beginning of development. Heatwave temperature strongly affected larval mortality. Exposure to 37°C heatwaves increased larval mortality by 130%, but the cooler 31°C heatwaves did not significantly impact mortality. Heatwave duration did not impact larval mortality. Larval development time also was affected by heatwave exposure. Compared with the no-heatwave-control, bees in the 31°C heatwave developed faster, and bees in the 37°C heatwave developed slower. Our study reveals the importance of stage-specific effects of extreme events and suggests that the timing and maximum temperature of projected heatwaves may be more detrimental to populations than heatwave duration.