Climate plays a key role in shaping population trends and determining the geographic distribution of species because of limits in species' thermal tolerance. An evaluation of species tolerance to temperature change can therefore help predict their potential spatial shifts and population trends triggered by ongoing global warming. We assessed inter-and intraspecific variations in heat resistance in relation to body mass, local mean temperatures, and evolutionary relationships in 39 bumblebee species, a major group of pollinators in temperate and cold ecosystems, across 3 continents, 6 biomes, and 20 regions (2386 male specimens). Based on experimental bioassays, we measured the time before heat stupor of bumblebee males at a heatwave temperature of 40°C. Interspecific variability was significant, in contrast to interpopulational variability, which was consistent with heat resistance being a species-specific trait. Moreover, cold-adapted species are much more sensitive to heat stress than temperate and Mediterranean species. Relative to their sensitivity to extreme temperatures, our results help explain recent population declines and range shifts in bumblebees following climate change.

Climate change is related to an increase in frequency and intensity of extreme events such as heatwaves. It is well established that such events may worsen the current world‐wide biodiversity decline. In many organisms, heat stress is associated with direct physiological perturbations and could lead to a decrease of fitness. In contrast to endotherms, heat stress resistance has been poorly investigated in heterotherms; especially in insects, in which the internal physiological mechanisms available to regulate body temperature are almost negligible making them sensitive to extreme temperature variations. Wild bees are crucial pollinators for wild plants and crops. Among them, bumblebees are experiencing a strong decline across the world. Therefore, the ongoing global decline of these insect pollinators partly due to climate change could cause major economic issues. Here we assess how simulated heatwaves impact fertility and attractiveness (key parameters of sustainability) of bumblebee males. We used three model species: Bombus terrestris, a widespread and warm‐adapted species, B. magnus and B. jonellus, two declining and cold‐adapted species. We highlight that heat shock (40°C) negatively affects sperm viability and sperm DNA integrity only in the two cold‐adapted species. Heat shock can also impact the structure of cephalic labial glands and the production of pheromones only in the declining species. The specific disruption in key reproductive traits we identify following simulated heatwave conditions could provide one important mechanistic explanation for why some pollinators are in decline through climate change. A free Plain Language Summary can be found within the Supporting Information of this article.

Thermotolerance has often been linked to species distribution for a diverse range of organisms. In the context of climate change, assessing heat resistance ability is useful for understanding potential future range shifts and the physiological response of populations. As bumblebee (Bombus ) populations have been declining for several decades with several documented range shifts, an assessment of the hyperthermic resistance of species is urgently needed. In this study, we measure in males the heat resistance of ten bumblebee species living in temperate regions (northwestern Europe) with a static temperature methodology to evaluate the time before heat stupor (THS) which corresponds to a chill coma. Our results on heat stress resistance show that not all species are affected in the same way to heat stress. The most widespread species, B. terrestris (median THS 395 min) and B. lucorum (median THS 257 min) are the least sensitive to hyperthermic stress. The resistance time of bumblebee males is up to 10 times longer than the THS for declining species such as B. jonellus (median THS 48 min) and B. magnus (median THS 58 min). We highlight the high interspecific variability of heat resistance in a morphologically homogeneous genus such as bumblebees. From a conservation point of view, our research highlights the urgency for assessing the heat resistance of different species since each one can display a species-specific thermal sensitivity that is likely linked to a risk of decline in the case of heat waves.

Climate change is related to an increase in the frequency and intensity of extreme events such as heatwaves. In insect pollinators, heat exposure is associated with direct physiological perturbations, and in several species, could lead to a decrease of fitness related to a decrease in fertility. Here we developed a new experimental protocol in controlled conditions to assess if the exposure to high temperatures could modify the attractiveness and fertility of Bombus terrestris males. Our results show that virgin queens of B. terrestris do not have preferences between the pheromonal secretions of heat-exposed and control males. Moreover, mating with a heat-exposed male has no impact on the copulation behavior and the development of the nest (brood composition). We advise to extend trials to cover a range of wild and heat-sensitive species on multiple generations to better understand the impact of heat waves on the bumblebee communities.

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