Hyperthermic stress resistance of bumblebee males: test case of Belgian species

Abstract

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.

Thermal flight performance reveals impact of warming on bumblebee foraging potential

“…Our results suggest that climate warming may favour aspects of bumblebee flight performance, such as motivation and endurance, across large parts of their geographical range. However, sensitivity to climate change can vary widely both between bumblebee species and across insect groups more generally (Arribas et al, 2012;Hamblin et al, 2017;Sunday et al, 2014;Zambra et al, 2020), and our results suggest that even within the same species, responses to temperature vary with body mass and thus likely affect smallerand larger-bodied taxa differently. It is therefore important that we quantify and compare flight and other key behavioural thermal response curves for species across the insect phylogeny to understand spatiotemporal risks and identify conservation priorities of similar and different functional groups.…”

Impact of intraspecific variation on measurements of thermal tolerance in bumble bees

“…Although B. t. terrestris queens were bigger, critical thermal limits did not differ among the subspecies. No significant correlation with body size was also found in studying heat resistance in males of 10 bumble bee species (Zambra et al, 2020). Furthermore, other selective pressures influence body size in bumble bees, as has been discussed in Gérard et al (2020), who found that queens of four bumble bee species became bigger over the last 100 year.…”

“…Recent evidence on thermal tolerances in terrestrial ectotherms accumulates in favor of a variant of this climate variability hypothesis, the climate extremes hypothesis, which predicts that selection for thermal tolerances strongly depends on rare but extreme thermal events (see Sundays et al, 2019 and refs therein). Although the pattern of higher heat resistance in males of widespread bumble bee species compared to those of more restricted, declining, or alpine species (Martinet et al, 2015;Zambra et al, 2020) might fit to this rule, more research is necessary to confirm and reveal to which extent periods of such extreme weather conditions impacts thermal tolerance breadths of B. terrestris or bumble bees in general.…”

“…In Hamblin et al (2017), the authors showed high interspecific variability of CT max among 15 bee species, with lowest CT max -values in solitary bee and bumble bee species. By measuring hyperthermic resistance in males of 10 bumble bee species, Zambra et al (2020) found a species-specific thermal sensitivity in bumble bees, with widespread species, such as B. terrestris and B. lucorum, being least sensitive, while declining species, such as B. jonellus and B. magnus, being most vulnerable to the induced thermal stress. Furthermore, Oyen et al (2016) showed that bumble bee species inhabiting higher altitude have a larger thermal tolerance breadth (where TTB = CT max -CT min ) than species from the valley, in correlation with the climatic amplitude of their habitat.…”

Response mechanisms to heat stress in bees

“…The difference of heat resistance in different bumblebee species is also presented (Martinet et al 2015;Oyen et al 2016;Zambra et al 2020). The heat resistance threshold temperature of high-elevation species (Bombus sylvicola and B. bifarius ) is nearly 5 °C lower than for lowelevation species (B. huntii ) (Oyen et al 2016).…”