Heatwave-like events affect drone production and brood-care behaviour in bumblebees

Introduction

Global climate change is currently considered one of the major threats to biodiversity (Leadley et al., 2010), causing distributional shifts (Kerr et al., 2015), population declines (Parmesan & Yohe, 2003), and even pushing some species to the brink of extinction (Thomas et al., 2004). This complex phenomenon, however, is not limited to an increase in mean global temperatures, which are predicted to rise between 1.2 °C and 4.5 °C under very low and very high emissions scenarios (Intergovernmental Panel on Climate Change (IPCC), 2023), but is also associated with increased climatic variability and extreme weather events, such as heatwaves (Meehl & Tebaldi, 2004; Christidis, Jones & Stott, 2015). Although the definition of a heatwave varies, the United Kingdom’s Meteorological Office defines it as a period of at least three consecutive days of hot weather based on local climatological conditions, with thermal conditions recorded above given thresholds (McCarthy, Armstrong & Armstrong, 2019). These extreme weather events are predicted to become more frequent, intense, and long-lasting (Meehl & Tebaldi, 2004), potentially creating acutely stressful conditions that are too short to allow organisms to adapt. These temperature extremes can exert considerable stress on plant (Breshears et al., 2021) and insect populations, pushing them to and even beyond their adaptive limits (Harvey et al., 2020). This stress may result in phenological disruptions and the structural reorganisation of food webs (Harvey et al., 2020) and could lead to a potential disruption in the performance and survival of ecologically and economically important organisms, such as insect pollinators (Bridle & Vines, 2007; Huntley et al., 2010).

Among insect pollinators, bumblebees (genus Bombus, family Apidae) are considered extremely valuable for their services to pollination (Gunnarsson & Federsel, 2014), particularly in the temperate northern hemisphere (Woodard, 2017). However, as cold-adapted insects, bumblebees might be affected by increasing temperatures associated to climate change (Maebe et al., 2021), particularly in areas where climatic conditions exceed species thermal tolerances (Soroye, Newbold & Kerr, 2020) or during prolonged heat events (Rasmont & Iserbyt, 2012). Previous research indicates that climate change is negatively affecting several bumblebee species, for example, by causing declines in some species because of their inability to shift their range (Kerr et al., 2015) or by forcing other species to move to higher altitudes in response to warming (Pyke et al., 2016). Heatwaves can affect bumblebees individually at the physiological level, for example when temperatures exceed their thermal threshold, and/or at a colony level, for example when extreme temperatures force workers to switch from foraging to colony thermoregulation (Rasmont & Iserbyt, 2012). The latter is particularly concerning because maintaining the nest at an optimal temperature is crucial for brood production, the development of the larvae and pupae, and the overall growth of the colony (Weidenmüller et al., 2022). Although, in general, bumblebees are capable of regulating the temperature of the colony and maintaining the nest temperature between 28 and 32 °C (Vogt, 1986; Weidenmüller, Kleineidam & Tautz, 2002), this comes at an energetic cost and unusually elevated ambient temperatures might cause thermal distress (Vogt, 1986) and affect bees’ foraging performance and colony development (Guiraud et al., 2021).

Heatwaves generally occur during the summer, a period when some bumblebee species are actively producing drones (males) and gynes (Goulson, 2003). During one of these events, the brood might experience unusually elevated temperatures for a portion of their development (Perl et al., 2022), which can accelerate growth rate and decrease body size (Davidowitz & Nijhout, 2004), cause morphological changes, such as wing shape alterations (Gerard et al., 2018), and/or an impairment in adult behaviour (Perl et al., 2022). For example, Gerard et al. (2018) found that exposure to heatwave-like events (33 °C) during development can lead to smaller body sizes and wing asymmetry in Bombus terrestris, which could lead to a reduction in pollination effectiveness (Jauker, Speckmann & Wolters, 2016). Bumblebees may also be exposed to heat stress during adulthood, which can impact essential behaviours, such as colony maintenance when bees increase fanning intensity (Vogt, 1986) and brood production, with more workers and queens produced at higher temperatures (Holland & Bourke, 2015). This exposure to high temperatures may also affect the number of sexuals produced and their reproductive capacity (Guiraud et al., 2021), which is essential for colony and population success.

Recent studies show that elevated temperatures can affect the reproductive fitness of bumblebees (Campion, Rajamohan & Dillon, 2023). In Bombus ignitus, an important pollinator in China and Japan, colony foundation, initiation, and male production has been shown to be negatively affected by temperature, with bees reared at 27 °C performing better than those at 30 °C (Yoon, Kim & Kim, 2002). In male bumblebees, high temperatures can have negative effects on fertility (Martinet et al., 2021b). For example, males of the cold-adapted species B. jonellus and B. magnus exhibited a significant decrease in sperm viability after being exposed to a simulated heatwave of 40 °C (Martinet et al., 2021b). These heatwave events need only occur once to affect an individual’s responses, reduce fitness (Kingsolver, Diamond & Buckley, 2013; Truebano et al., 2018), and can lead to mortality in extreme cases (Parmesan, 2006; Rasmont & Iserbyt, 2012). Rasmont & Iserbyt (2012) reported that, after a record summer temperature of 33 °C in Lapland, Finland, bumblebee densities were abnormally low. How these extreme weather events affect colony survival and fitness, for example through increased worker mortality or sub-lethal effects on behaviour, requires more detailed investigation, both to increase our understanding of how heatwaves may contribute to bumblebee population declines, and how best to mitigate the effects of such events.

Here we investigated the impacts of simulated (5-day) heatwaves of two different magnitudes (30–32 and 34–36 °C) on the performance of queenless microcolonies of the bumblebee subspecies Bombus terrestris audax. Despite its heat tolerance and thermoregulatory capabilities, the colony performance of B. terrestris, particularly colony growth, has been reported to be affected by high temperatures (33 °C) (Vanderplanck et al., 2019). However, few studies have focused on assessing colony performance at heatwave temperatures equivalent to those that have been reported recently. In addition, microcolonies have been widely used to assess the impact of other stressors, such as pesticides (Laycock et al., 2014; Dance, Botías & Goulson, 2017) and are considered reliable predictors of the response of queenright colonies (Tasei & Aupinel, 2008). Queenless microcolonies are also an ecologically relevant scenario because queen mortality often occurs in field conditions (Samuelson et al., 2018) and can result in workers switching to egg laying to produce drones (Duchateau, 1989). To assess colony performance under simulated heatwaves, we measured and analysed parameters including (a) mortality of workers, (b) microcolony growth (production of drones), (c) discarded larvae, (d) food consumption, (e) worker weight changes, (f) drone body size (intertegular distance) and (g) dry body mass of drones. Furthermore, to determine whether heatwave events affect behaviours associated with colony maintenance and thermoregulation, we recorded (h) the number of workers engaged in wing fanning (flapping wings to cool the colony) and (i) brood incubation (positioned on top of brood to keep them warm) during the 5-day heatwaves. We hypothesise that heatwaves of a higher magnitude would lead to a lower performance of bumblebee microcolonies, likely leading to higher mortality rates and decreased colony growth.

Materials and Methods

Results

Mortality of workers

There were no statistically significant differences in bumblebee mortality across treatments (GLMM; χ² = 2.95, df = 2, p = 0.22). However, bumblebees exposed to the extreme heatwave exhibited a slightly higher mortality in relation to those from the control group (24 °C) (GLMM with emmeans post-hoc test; p = 0.24) and the mild heatwave (32–34 °C) (GLMM with emmeans post-hoc test; p = 0.38) (Fig. 1). This led to the loss of two whole microcolonies from the extreme heatwave by the end of the study period.

Colony growth

Drone production

The production of males (drones) differed among treatments (GLMM; χ² = 8.20, df = 2, p = 0.01). Microcolonies exposed to the mild (30–32 °C) heatwave produced significantly more drones in comparison to the control group (24 °C) (GLMM with emmeans post-hoc test; p = 0.01), but not to the extreme heatwave (34–36 °C) (GLMM with emmeans post-hoc test; p = 0.72). Microcolonies subjected to the extreme heatwave exhibited higher drone production compared to the control group, although this difference was not statistically significant (GLMM with emmeans post-hoc test; p = 0.09) (Fig. 2).

Discarded larvae

During the study period, bumblebee workers from all treatments were observed to remove and discard larvae. However, there were no significant effects of either treatment (GLMM; χ² = 5.52, df = 2, p = 0.06) or the number of drones produced per microcolony (GLMM; χ² = 0.51, df = 1, p = 0.47). Figure is available in Supplemental Material (Fig. S1).

Brood-care behaviours: wing fanning and brood incubation

Wing fanning

The incidence of wing fanning varied significantly between treatments (GLMM; χ² = 34.49, df = 2, p < 0.01). Microcolonies exposed to the extreme heatwave exhibited a higher proportion of workers engaged in wing fanning in comparison to those exposed to the mild heatwave (GLMM with emmeans post-hoc test; p < 0.01) and the control group (24 °C) (GLMM with emmeans post-hoc test; p < 0.01) (Fig. 3). Workers exposed to the mild heatwave exhibited a slight increase in wing fanning behaviour; however, there were no significant differences when compared to the control group (GLMM with emmeans post-hoc test; p = 0.35).

Brood incubation

There were no significant differences in the incidence of brood incubation between temperature treatments (GLMM; χ² = 4.42, df = 2, p = 0.10).

Discussion

This study investigated the effects of heatwaves of different magnitude on the performance of bumblebee (B. terrestris audax) microcolonies. We found that exposure to a mild 5-day heatwave of 30–32 °C resulted in increased offspring production compared to those exposed to an extreme heatwave of 34–36 °C and to the control group (24 °C). Brood-care behaviours were also impacted by the magnitude of the heatwave. Wing fanning occurred occasionally at temperatures of 30–32 °C, whereas at 34–36 °C the proportion of workers engaged in this thermoregulatory behaviour increased significantly. However, we did not find evidence indicating that exposure to heatwaves affects the survival of workers or the body size and mass of drones.

Although our analyses did not reveal a significant effect of heatwave exposure on bumblebee mortality, microcolonies exposed to the extreme heatwave exhibited the loss of several workers and two whole microcolonies. This finding contrasts with previous research that has documented the effects of prolonged exposure to sublethal temperatures on bumblebee survival. Several studies have shown that sublethal temperatures can become lethal if they persist for an extended period of time (Rasmont & Iserbyt, 2012). For example, Bombus impatiens workers exposed to 36 °C for a prolonged period of time (5 h) experienced greater worker mortality compared to those exposed to 26 °C (Quinlan et al., 2023); while in B. terrestris audax workers, repetitive exposure to sub-lethal temperatures of ~50 °C led to high mortality and irreversible loss of coordination despite their high heat tolerance (Sepúlveda & Goulson, 2023).

High mortality levels have also been observed in other heat-tolerant bees, such as Tetragonula stingless bees, which experienced 95% mortality at temperatures below their upper thermal limits after longer periods of heat exposure (Nacko et al., 2023). However, most of these studies use higher experimental temperatures, potentially accounting for the observed differences in mortality rates. For example, Martinet et al. (2021a) showed that B. terrestris can reach heat stupor when exposed to temperatures of 40 °C for a prolonged time. The temperatures used in our study are also lower than those experienced during recent heatwave events. B. terrestris inhabits regions ranging from Europe to North Africa, where ambient temperatures exceeding these levels have already been recorded. For instance, during the 2022 summer heatwave in the UK, some regions recorded temperatures exceeding 40 °C (Kendon et al., 2023). Similarly, in 2021, the Mediterranean region (Italy) registered record-breaking temperatures of 48.8 °C, while 50.3 °C was recorded in North Africa (Tunisia) (United Nations Climate Change, 2021). It is essential to note, however, that temperatures within the nest, especially for species that establish nests underground, may not reach such extremes. While nests that are located above-ground can experience high thermal extremes, those located below-ground may exhibit relatively stable temperatures due to insulation (Mullan, 2022). Thus, it remains necessary to investigate the nest temperatures during extreme weather events. Finally, it is also plausible that our sample sizes, i.e., number of colonies, might have influenced the detection of a significant difference in mortality levels. Future studies focused on the effect of elevated temperatures on bumblebees and other insects should include larger sample sizes to enhance the statistical power and robustness of the findings.

Interestingly, bumblebees exposed to the mild heatwave produced significantly more drones in comparison to the extreme heatwave and the control group. Previous research indicates that B. terrestris need to keep the nest at constant temperatures between 28 °C and 32 °C (Vogt, 1986), which could explain the higher drone production on those exposed to temperatures of 30–32 °C. Temperatures of this range are considered optimal for the brood in bumblebee colonies and microcolonies (Livesey et al., 2019) under both field (Weidenmüller, Kleineidam & Tautz, 2002) and laboratory conditions (Vogt, 1986). Thus, nests kept at this temperature range might therefore result in a more efficient colony development due to a decrease in the energy spent on thermoregulation (Vollet-Neto, Menezes & Imperatriz-Fonseca, 2011; Wynants et al., 2021). Recent studies (Guiraud et al., 2021) have shown that both worker and drone production can remain constant even at high, stressful temperatures (33 °C). For example, Amin, Suh & Kwon (2008) found that B. terrestris queens that had hibernated for 4 months and had been activated (termination of diapause) at 36 °C produced the highest number of drones in comparison to those activated at 24 °C, 28 °C, and 32 °C. This low sensitivity to elevated temperatures could explain the species’ capacity to thrive in a broader range of climatic zones (Martinet et al., 2021a). On the other hand, recent studies focused on bumblebees and honeybees suggest that higher queen and/or worker production may represent an emergency-state that leads to more investment in reproduction for the ultimate success of the colony during stressful conditions (Schott et al., 2021; Guiraud et al., 2021). However, further experimentation would be required to comprehend whether higher drone production may be a stress response that enables bumblebees to ensure future colony foundation through future queen fertilisation and subsequent nest foundress activity by new queens (Belsky, Camp & Lehmann, 2020).

Previous work on insects has shown that high developmental temperature can lead to decreased body size (Atkinson, 1994). For example, the body size of high-Arctic butterflies has decreased as a response to high summer temperatures (Bowden et al., 2015), while the decrease in size of some large-bodied beetles, such as Scaphinotus angusticollis, has been correlated with increased temperatures (Tseng et al., 2018). However, we did not find any significant differences in the body size or body mass of bumblebee drones across treatments. This is consistent with Guiraud et al. (2021), who found that colonies of B. t. audax produced smaller workers, but not drones, when reared at temperatures of 33 °C. This indicates that effects of temperature on body size might be sex-dependent, as it is suggested that the body size of each sex may respond differently to temperature or have no response at all (Fenberg et al., 2016). Furthermore, it is important to note that other factors, such as nutrition, can play a role in body size in bees (Nicholls, Rossi & Niven, 2021). For example, honeybee workers reared on high protein diets were smaller and lighter on emergence compared with those reared on high carbohydrate larval diets (Nicholls, Rossi & Niven, 2021); while in B. terrestris, queens produced smaller drones in poor environments with reduced food availability (Schmid-Hempel & Schmid-Hempel, 1998). Similarly, tolerance to heat stress can also be influenced by nutrition. For example, stingless bees (Melipona subnitida) with previous access to food showed higher critical and lethal temperatures in comparison to unfed bees (Maia-Silva et al., 2021). In our study, microcolonies received a constant supply of syrup and pollen and there were no significant differences in syrup consumption among treatments. As this may not accurately reflect field conditions where bumblebees need to actively gather food resources, further research is necessary to explore how bumblebee behaviours and responses to heat stress are affected during natural heatwaves. Thus, it remains important to continue exploring the interactive effects of nutrition and elevated temperature on body size and other morphological traits, as any changes could directly affect foraging (Greenleaf et al., 2007) and, consequently, colony development (Gérard et al., 2022).

We also found that brood-care behaviours, particularly wing fanning, were impacted by the magnitude of the heatwave. Wing fanning occurred occasionally at temperatures of 30–32 °C, whereas at 34–36 °C the proportion of workers engaged in this thermoregulatory behaviour increased significantly. This is consistent with Vogt (1986), who observed a higher occurrence of wing fanning and low levels of brood maintenance in colonies of B. impatiens and B. affinus exposed to 33–39 °C in comparison to 28–32 °C. Similarly, an increase in fanning accompanied by a reduction in incubation as a response to increased temperatures was observed in B. bifarius nearcticus workers (O’Donnell & Foster, 2001). Wing fanning is an effective strategy to cool the colony and reduce the levels of respiratory gases (CO₂) within a nest (Weidenmüller, 2004). However, increasing the recruitment of workers for thermoregulation at the expense of other essential behaviours in response to extreme temperatures may affect the growth of colonies (Vogt, 1986). For example, at temperatures of ~32 °C, B. terrestris workers decrease their foraging activity by almost 70% compared to ~25°C (Kwon & Saeed, 2003). Reductions in foraging and other essential behaviours can result in significant losses at a larger scale and even population collapses (Zeaiter & Myerscough, 2020). Therefore, investigating the effects of heatwaves in queenright colonies when foraging is required remains essential. Finally, when assessing impacts of temperature on colony performance, the number of workers in a colony is important to consider as the size of a colony can help buffer environmental stress (Vanderplanck et al., 2019). Small colonies, such as those from our experiments, might be more sensitive to extreme temperatures (Weidenmüller, Kleineidam & Tautz, 2002), since they cannot recruit more workers to carry out essential tasks and generally require the presence of other members to initiate fanning (Cook & Breed, 2013). While studies have indicated that small microcolonies may be more successful at brood incubation given that additional workers consume resources without a corresponding increase in performance (Livesey et al., 2019), it is crucial to investigate whether this phenomenon extends to wing fanning and wild small colonies as well. Therefore, decreases in colony size as a result of heat exposure and/or its interaction with other stressors should be considered when predicting responses to extreme weather events (Vanderplanck et al., 2019).

Conclusions

In the present study, we demonstrate that heatwaves can have significant impacts on the performance of Bombus terrestris audax. Unexpectedly, bumblebees exposed to the mild heatwave produced more drones in comparison to those exposed to the extreme heatwave and the control group. We also found that brood-care behaviours were impacted by the magnitude of the heatwave, with higher temperatures leading to more wing fanning. Changes in behavioural responses to elevated temperatures can affect colony growth and performance, especially in small colonies with limited number of workers that can invest in other essential tasks.

Our findings add to an existing body of work on the impact of extreme weather events on bees. However, further experiments are necessary to understand the impact of elevated temperatures on different developmental stages and to assess the long-term effects of heat stress and the potential behavioural responses that might enable them to avoid and/or cope with extreme temperatures. It is important to highlight that, in our study, we provided a constant supply of syrup and pollen. Therefore, further research is necessary to explore how bumblebee behaviours are affected when foraging is required during these events. Moreover, even though B. terrestris is considered a heat-resistant species, the negative impacts of extreme temperatures have been documented. Thus, we strongly encourage exploring the effects of heatwaves of comparable magnitude to those observed in recent years and to include larger sample sizes to enhance power and robustness. Finally, we suggest carrying out more research to comprehend the impact of heatwaves on other bee species that might be more sensitive to temperature extremes to predict and prevent population declines of pollinators in a warming world.

Supplemental Information