Experimental short-term heatwaves negatively impact body weight gain and survival during larval development in <i>Bombus terrestris</i> L. (Hymenoptera: Apidae)

Climate change-induced heatwaves threaten global biodiversity, including crucial pollinators like bumblebees. In particular, the increasing frequency, duration and intensity of heatwaves is alarming. Despite these projections, little is known about the effects of short-term heatwaves on insect larval development. Hence, we investigated the impact of simulated heatwaves on the development of 4th instar larvae (L4) of Bombus terrestris L. (Hymenoptera: Apidae) using an in vitro rearing method. Individual larvae were incubated at 37°C and 38°C for a period of 4 days, with a constant rearing temperature of 34°C as the control. We examined body weight gain, developmental duration, survival to adult stage, and adult body size (i.e. dry mass, intertegular distance, and head width). A simulated heatwave of 37°C did not significantly affect larval development, but 38°C impaired larval body mass gain. While developmental duration and adult body size were unaffected, an acute heat stress of 38°C during the L4 stage reduced the probability of pupae reaching adulthood. These findings highlight the potential for heatwaves to negatively affect bee populations by impairing larval growth and reducing survival to the adult stage, which may have severe implications for colony fitness.