Bumblebee Workers Research Articles

Sperm can’t take the heat: Short-term temperature exposures compromise fertility of male bumble bees (Bombus impatiens)

Bumble bee (genus Bombus) populations are increasingly under threat from habitat fragmentation, pesticides, pathogens, and climate change. Climate change is likely a prime driver of bumble bee declines but the mechanisms by which changing climates alter local abundance, leading to shifts in geographic range are unclear. Heat tolerance is quite high in worker bumble bees (CTmax ∼ 48–55 °C), making it unlikely for them to experience these high temperatures, even with climate warming. However, the thermal tolerance of whole organisms often exceeds that of their gametes; many insects can be sterilized by exposure to temperatures well below their upper thermal tolerance. Male bumble bees are independent from the colony and may encounter more frequent temperature extremes, but whether these exposures compromise spermatozoa is still unclear. Using commercially-reared Bombus impatiens colonies, males were reared in the lab and spermatozoa were exposed (in vivo and isolated in vitro) to sublethal temperatures near lower and upper thermal tolerance (CTmin and CTmax, respectively). Heat exposure (45 °C for up to 85 min) reduced spermatozoa viability both for whole males (in vivo; control = 79.5 %, heat exposed = 58 %, heat stupor = 57.7 %) and isolated seminal vesicles (in vitro; control = 85.5 %, heat exposed = 62.9 %). Whole males exposed to 4 °C for 85 min (in vivo; control = 79.2 %, cold = 72.4 %), isolated seminal vesicles exposed to 4 °C for 85 min (in vitro; control = 85.5 %, cold = 85.1 %), and whole males exposed to for 4 °C for 48 h (in vivo; control = 88.7 %, cold = 84.3 %) did not differ significantly in spermatozoa viability. After<85 min at 45 °C, males had significantly reduced spermatozoa viability, suggesting that short-term heat waves below CTmax could strongly reduce the fertility of male bumble bees with potential population-level impacts.

Bumblebee cognitive abilities are robust to changes in colony size

Eusocial insect colonies act as a superorganism, which can improve their ability to buffer the negative impact of some anthropogenic stressors. However, this buffering effect can be affected by anthropogenic factors that reduce their colony size. A reduction in colony size is known to negatively affect several parameters like brood maintenance or thermoregulation, but the effects on behaviour and cognition have been largely overlooked. It remains unclear how a sudden change in group size, such as that which might be caused by anthropogenic stressors, affects individual behaviour within a colony. In this study, the bumblebee Bombus terrestris was used to study the effect of social group size on behaviour by comparing the associative learning capabilities of individuals from colonies that were unmanipulated, reduced to a normal size (a colony of 100 workers) or reduced to a critically low but functional size (a colony of 20 workers). The results demonstrated that workers from the different treatments performed equally well in associative learning tasks, which also included no significant differences in the learning capacity of workers that had fully developed after the colony size manipulation. Furthermore, we found that the size of workers had no impact on associative learning ability. The learning abilities of bumblebee workers were thus resilient to the colony reduction they encountered. Our study is a first step towards understanding how eusocial insect cognition can be impacted by drastic reductions in colony size.Significance statementWhile anthropogenic stressors can reduce the colony size of eusocial insects, the impact of this reduction is poorly studied, particularly among bumblebees. We hypothesised that colony size reduction would affect the cognitive capacity of worker bumblebees as a result of fewer social interactions or potential undernourishment. Using differential conditioning, we showed that drastic reductions in colony size have no effect on the associative learning capabilities of the bumblebee Bombus terrestris and that this was the same for individuals that were tested just after the colony reduction and individuals that fully developed under the colony size reduction. We also showed that body size did not affect learning capabilities. This resilience could be an efficient buffer against the ongoing impacts of global change.

Prevalence and Distribution of Three Bumblebee Pathogens from the Czech Republic.

Bumblebees are significant pollinators for both wild plants and economically important crops. Due to the worldwide decrease in pollinators, it is crucial to monitor the prevalence and distribution of bumblebee pathogens. Field-caught bumblebee workers and males were examined for the presence of three pathogens during the summer months of the years 2015-2020 (Bombus terrestris/lucorum) and 2015-2017 (Bombus lapidarius). The greatest prevalence was in the case of Crithidia bombi, where significantly more workers (57%) of B. terrestris/lucorum were infected than males (41%). Infection was also confirmed in 37% of B. lapidarius workers. The average prevalence was very similar in the case of Nosema bombi in workers (25%) and males (22%) of B. terrestris/lucorum. In the case of B. lapidarius, 17% of the workers were infected. The lowest number of infected individuals was for Apicystis bombi and the prevalence of infection was significantly higher for males (22%) than workers (8%) of B. terrestris/lucorum. Only 3% of workers and 4% of males of B. terrestris/lucorum were simultaneously infected with three types of pathogens, but no worker was infected with only a combination of N. bombi and A. bombi. The greatest prevalence of C. bombi was found in urban or woodland areas.

Microbiome assembly and maintenance across the lifespan of bumble bee workers.

How a host's microbiome changes over its lifespan can influence development and ageing. As these temporal patterns have only been described in detail for a handful of hosts, an important next step is to compare microbiome succession more broadly and investigate why it varies. Here we characterize the temporal dynamics and stability of the bumble bee worker gut microbiome. Bumble bees have simple and host-specific gut microbiomes, and their microbial dynamics may influence health and pollination services. We used 16S rRNA gene sequencing, quantitative PCR and metagenomics to characterize gut microbiomes over the lifespan of Bombus impatiens workers. We also sequenced gut transcriptomes to examine host factors that may control the microbiome. At the community level, microbiome assembly is highly predictable and similar to patterns of primary succession observed in the human gut. However, at the strain level, partitioning of bacterial variants among colonies suggests stochastic colonization events similar to those observed in flies and nematodes. We also find strong differences in temporal dynamics among symbiont species, suggesting ecological differences among microbiome members in colonization and persistence. Finally, we show that both the gut microbiome and host transcriptome-including expression of key immunity genes-stabilize, as opposed to senesce, with age. We suggest that in highly social groups such as bumble bees, maintenance of both microbiomes and immunity contribute to inclusive fitness, and thus remain under selection even in old age. Our findings provide a foundation for exploring the mechanisms and functional outcomes of bee microbiome succession.

Experimental viral spillover can harmBombus terrestrisworkers under field conditions

AbstractDeformed wing virus (DWV), notorious for its virulence in the western honey bee (Apis mellifera)when vectored by the ectoparasitic miteVarroa destructor, is also widespread among wild bumble bee species, presumably through spillover from honey bees. Experimental studies on the virulence of DWV inBombusspp. have provided equivocal results and have until now been confined to bumble bees under laboratory conditions.Here, we inoculated commercially rearedBombus terrestrisworkers with DWV‐A through feeding or injection and introduced them into experimental colonies placed in the field, thus exposing them to the environment and associated stressors. We monitored the survival of inoculated worker bumble bees and quantified their viral load at 10 days post inoculation.Bombus terrestrisworkers injected with DWV‐A supported high viral loads and exhibited significantly reduced median survival compared to controls. Bumble bees inoculated by feeding had low or zero detectable viral loads while their mortality did not differ from the control group.Our results demonstrate that, although DWV‐A is pathogenic for commercialB. terrestris, the risks for individual fitness from spillover of DWV‐A during foraging on shared flowers appear limited.The findings of this experiment also highlight the necessity to address the potential context‐dependence of virulence when evaluating the impact of a pathogen in an alternative host.

How foraging preference and activity level of bumble bees contribute to colony flexibility under resource demand

Groups of cooperative organisms, including humans, benefit from the contributions of both specialist and generalist individuals. Group responses to resource or task demand emerge from the specific actions of individuals within the group. Bumble bee colonies need to respond to resource demands for both pollen and nectar for colony growth and reproduction. Bumble bee workers have been shown to display differences in foraging preferences for these essential resources and in activity level. This study investigated the responses of Bombus terrestris individuals to resource demand by subjecting colonies to different pollen and sucrose storage scenarios, considering their individual foraging preferences and activity levels. The sucrose sensitivity and olfactory learning ability of individuals were then compared along a spectrum of foraging preference (between pollen and sucrose foraging) using the proboscis extension reflex paradigm. When sucrose was in demand, individuals with a preference for sucrose showed the largest upregulation in foraging bouts. Individual preference for pollen and activity level correlated with the upregulation of foraging bouts when pollen was in demand. This study detected no relationship between body size, sucrose preference and worker sucrose sensitivity or olfactory learning ability, tested with proboscis extension reflex conditioning. This study highlights the potential value of foraging preferences in bumble bee colonies but also the flexibility these colonies have with a small foraging workforce. • Bumble bees with high sucrose preference upregulate to fill sucrose demands. • Pollen preference and individual activity level predict upregulation in pollen demand. • Body size and sucrose preference do not predict worker sucrose sensitivity. • Body size and sucrose preference do not predict odour learning performance. • Workload is not evenly split between individuals.

Decreased efficiency of pollen collection due to Sulfoxaflor exposure leads to a reduction in the size of bumble bee workers in late European summer

Bumble bees (Bombus terrestris) are important pollinators of wild and crop plants. Despite their importance in the process of fruit and seed production on crop sites, their activity may be impaired due to exposure to pesticides. This species has a yearly life cycle and colony success may rely on effective foraging of workers on ruderal plants late in summer when most crops are no longer flowering. In the current study, we investigated the effect of chronic exposure to Sulfoxaflor on aspects of the foraging behavior of bumble bees and whether Sulfoxaflor influences the body size of workers of B. terrestris in a crop landscape. We found that 2 weeks of continuous exposure to Sulfoxaflor influenced workers’ foraging dynamics and collection of resources. However, there was no evidence that the 5 ppb dose of the pesticide impacted the ability of bees to handle flowers with different traits. Workers from colonies exposed to Sulfoxaflor were smaller. The effect on worker size may be explained as a consequence of the reduced pollen income per unit of worker foraging. Thus, if the effects of Sulfoxaflor applied directly to crops had the same effect as that observed on commercial bumble bees after our chronic exposure, it might negatively impact colony success due to the impact on pollen collection and the reduction in the size of workers.

No effect of dual exposure to sulfoxaflor and a trypanosome parasite on bumblebee olfactory learning

Bees are important pollinators in wild and agricultural ecosystems, and understanding the factors driving their global declines is key to maintaining these pollination services. Learning, which has been a focus of previous ecotoxicological studies in bees, may play a key role in driving colony fitness. Here we move beyond the standard single-stressor approach to ask how multiple stressors, an agrochemical (sulfoxaflor, a relatively new insecticide) and a parasite (Crithidia bombi, a prevalent gut parasite of bumblebees), impact learning in the bumblebee Bombus terrestris. We developed a modified version of the classic proboscis extension reflex assay to assess the combined effects of acute oral sulfoxaflor exposure and infection by C. bombi on olfactory learning of bumblebee workers. We found no evidence that either sulfoxaflor, C. bombi, or their combination had any significant effect on bumblebee olfactory learning, despite their known negative impacts on other aspects of bumblebee health. This suggests that losses in cognitive ability, as measured here, are unlikely to explain the impacts of sulfoxaflor and its interactions with other stressors on bumblebees. Our novel methodology provides a model system within which to test interactive effects of other key stressors on bee health.

Local Weather Conditions Affect Forager Size and Visitation Rate on Bramble Flowers (Rubus fruticosus) in Bumble Bees (Bombus spp)

Bumble bees (Bombus; Hymenoptera) are crucial pollinators for many agricultural crops, but their numbers have declined - with climate change as a possible driver. Different aspects of local weather conditions can have substantial effects on bumble bees’ ability to forage. Here we made 501 observations of bumblebee workers on bramble flowers (Rubus fruticosus) to explore how the characteristic sizes of foragers active and their individual rate of movement between flowers were correlated with aspects of weather. Firstly, we found that increased ambient temperature is correlated to reduced activity in larger workers. Moreover, while higher humidity was linked to an increase in smaller workers, an increase in cloud cover led their numbers to decrease. Visitation rate and temperature showed a significant negative relationship whereas no other weather aspect affected visitation rate significantly. Our results suggest that predicted rises in ambient temperatures will adversely affect bumble bee foraging, and particularly so for larger-bodied workers. We anticipate that this study can be used as a foundation for longer-term and more detailed studies on bumble bee foraging and colony performance. Such work is vital to identify management measures to mitigate pollinator declines and preserve food security.

The effect of the brood and the queen on early gene expression in bumble bee workers' brains

Worker reproduction in social insects is often regulated by the queen, but can be regulated by the brood and nestmates, who may use different mechanisms to induce the same outcomes in subordinates. Analysis of brain gene expression patterns in bumble bee workers (Bombus impatiens) in response to the presence of the queen, the brood, both or neither, identified 18 differentially expressed genes, 17 of them are regulated by the queen and none are regulated by the brood. Overall, brain gene expression differences in workers were driven by the queen’s presence, despite recent studies showing that brood reduces worker egg laying and provides context to the queen pheromones. The queen affected important regulators of reproduction and brood care across insects, such as neuroparsin and vitellogenin, and a comparison with similar datasets in the honey bee and the clonal raider ant revealed that neuroparsin is differentially expressed in all species. These data emphasize the prominent role of the queen in regulating worker physiology and behavior. Genes that serve as key regulators of workers’ reproduction are likely to play an important role in the evolution of sociality.

Sensitivity of Buff-Tailed Bumblebee (Bombus terrestris L.) to Insecticides with Different Mode of Action.

Simple SummarySeveral neonicotinoid insecticides that were once widely used for pest control are currently banned for outdoor use in the European Union (EU) because they pose a risk to bees. This restriction meant that farmers had to look for alternatives for pest management and use known insecticides or new substances with supposedly more bee-friendly characteristics. We evaluated the toxicity of six insecticides on buff-tailed bumblebee workers (Bombus terrestris): two banned neonicotinoids (imidacloprid, thiacloprid), two pyrethroids (deltamethrin, esfenvalerate), one sulfoximine (sulfoxaflor) and a microbial insecticide based on Bacillus thuringiensis toxins, which are present in genetically modified (Bt) maize. The results obtained show that certain insecticides in use have higher acute toxicity to B. terrestris than some of the banned neonicotinoids.Systemic insecticides are recognized as one of the drivers of the worldwide bee decline as they are exposed to them through multiple pathways. Specifically, neonicotinoids, some of which are banned for outdoor use in the European Union (EU), have been pointed out as a major cause of bee collapse. Thus, farmers have had to look for alternatives for pest control and use known insecticides or new substances reportedly less harmful to bees. We evaluated the oral acute toxicity of six insecticides (three of them systemic: imidacloprid, thiacloprid and sulfoxaflor) with four different modes of action on buff-tailed bumblebee workers (Bombus terrestris): two banned neonicotinoids (imidacloprid, thiacloprid), two pyrethroids (deltamethrin, esfenvalerate), one sulfoximine (sulfoxaflor) and a microbial insecticide based on Bacillus thuringiensis toxins, present in genetically modified (Bt) maize. The microbial insecticide only caused mortality to bumblebee workers at extremely high concentrations, so it is expected that Bt maize does not pose a risk to them. The toxicity of the other five insecticides on bumblebees was, from highest to lowest: imidacloprid, sulfoxaflor, deltamethrin, esfenvalerate and thiacloprid. This outcome suggests that certain insecticides in use are more toxic to B. terrestris than some banned neonicotinoids. Further chronic toxicity studies, under realistic conditions, are necessary for a proper risk assessment.

Selection on size variation: more variation in bumble bee workers and in the wild

Selection on size variation: more variation in bumble bee workers and in the wild

Heatwave-Like Events During Development Are Sufficient to Impair Bumblebee Worker Responses to Sensory Stimuli

Heatwaves are increasingly common globally and are known to have detrimental impacts on animal morphology and behaviour. These impacts can be severe, especially if heatwaves occur during development, even on animals that can regulate the temperature of their developing young. The onset and duration of heatwaves are stochastic and therefore may affect all or only part of development. In the heterothermic bumblebee Bombus terrestris, elevated temperatures over the entirety of development cause morphological changes in adults, despite their capability to regulate brood temperature. However, the effects of heatwaves that occur during a short period of development are unclear. We test the impact of elevated developmental temperature during the latter fraction of development on the behaviour and morphology of adult worker B. terrestris. We show that exposure to elevated temperature over a portion of late development is sufficient to impair the initial behavioural responses of workers to various sensory stimuli. Despite this, exposure to elevated temperatures during a period of development did not have any significant impact on body or organ size. The negative effect of elevated developmental temperatures was independent of the exposure time, which lasted from 11–20 days at the end of the workers’ developmental period. Thus, heat stress in bumblebees can manifest without morphological indicators and impair critical behavioural responses to relevant sensory stimuli, even if only present for a short period of time at the end of development. This has important implications for our understanding of deleterious climactic events and how we measure indicators of stress in pollinators.

Effect of a 110 ppb mercury exposition on neotropical bumble bee workers, Bombus atratus: in situ localization of Hsp70 and Hsp90 and general morphological changes of hepato-nephrocitic cells

We analyzed the effect of mercury (Hg) on the hepato-nephrocitic system (HNS) of Bombus atratus workers exposed to an estimated concentration similar to that found in honey stores of neotropical bees (110 ppb). The bees were divided into control and experimental groups. A solid mixture of honey, pollen and organic soy was offered to both experimental groups, ad libitum. The control group received distilled water and the exposed group received a 110 µg.L−1 (110 ppb) Hg solution. After 48 h of exposure, the bees were cryo-anesthetized, and the dorsal vessel was dissected directly in 4% paraformaldehyde. The samples were prepared for routine morphological analysis (HE), fluorescent histochemical staining (Acridine Orange and F-actin + DAPI), and in situ immunohistochemical labeling (Hsp70 and Hsp90). Our results showed that both the trophocytes and oenocytes of bees exposed to mercury exhibited chromatin damages. The Hg exposure also induced trophocytes deactivation of the nucleus-cytoplasm exchange as a result of branched contour loss of the nuclei. The pericardial cells were predominantly found at stage IV with pyknotic nuclei. Although the fluorescence intensity of both Hsp70 and Hsp90 was reduced in the exposed group compared to the control group, there was an indication of misfolded proteins. In conclusion, our results showed that the concentration of 110 µg.L−1Hg, which can be found in the honey stores of B. atratus colonies, has severely damaged the HNS of B. atratus workers. These effects can trigger major damage to their populations, contributing to bee declines in natural environments worldwide.

Within-Year Effects of Prescribed Fire on Bumble Bees (Hymenoptera: Apidae) and Floral Resources.

Despite the importance of bumble bees (genus Bombus Latreille) for their services to natural and agricultural environments, we know little about the relationship between grassland management practices and bumble bee conservation. Prescribed fire is a common grassland maintenance tool, including in areas where endangered and threatened bumble bees are present. Thus, knowledge of the effects of prescribed fire on bumble bees is essential for designing management schemes that protect and bolster their populations. Using nonlethal surveys to record bumble bee species richness, abundance, and community composition, we evaluated the effects of spring controlled burns on summer bumble bee gynes and workers across five sites in southern Wisconsin. In addition, we explored the effects of fire on floral resources by measuring floral genus richness, abundance, ground cover, and proportion of transects containing blooming flowers in adjacent burned and unburned parcels. Prescribed fire had no measurable effects on bumble bee gyne or worker community composition, species richness, or abundance. However, consistent with previous studies prescribed fire increased floral genus richness and ground cover. The disconnect between bumble bee and floral responses to fire highlights some opportunities for improving our understanding of fire’s effects on bumble bee diapause, nest site choice, and foraging.

Cuckoo male bumblebees perform slower and longer flower visits than free-living male and worker bumblebees

Cuckoo bumblebees are a monophyletic group within the genus Bombus and social parasites of free-living bumblebees, upon which they rely to rear their offspring. Cuckoo bumblebees lack the worker caste and visit flowers primarily for their own sustenance and do not collect pollen. Although different flower-visiting behaviours can be expected between cuckoo and free-living bumblebees due to different biological constraints, no study has yet quantified such differences. Here, we provide the first empirical evidence of different flower-visiting behaviours between cuckoo and free-living bumblebees. We recorded the flower-visiting behaviour of 350 individual bumblebees over two years in a wild population of the entomophilous plant Gentiana lutea, of which they are among the main pollinators. In cuckoo bumblebees (28.9% of the total), we only found males, while we found both workers and males in free-living bumblebees. Cuckoo bumblebees visited significantly more flowers for longer time periods than both free-living bumblebee workers and males within whorls, while differences at the whole-plant level were less marked. Free-living bumblebee males visited more flowers and performed slightly longer flower visits than workers. Behavioural differences between cuckoo male bumblebees and free-living bumblebee workers are likely related to different foraging needs, while differences between cuckoo and free-living bumblebee males may be caused by differences in colony development and a delayed mating period of free-living bumblebees. The longer visits made by cuckoo male bumblebees will likely negatively affect plant reproductive success through increased within-plant pollen flow.

Cuckoo bumblebee males might reduce plant fitness

A recommendation of: Alessandro Fisogni, Gherardo Bogo, François Massol, Laura Bortolotti, Marta Galloni Cuckoo male bumblebees perform slower and longer flower visits than free-living male and worker bumblebees https://doi.org/10.5281/zenodo.4489066

Juvenile hormone functions as a metabolic rate accelerator in bumble bees (Bombus terrestris)

Juvenile hormone (JH) is a modulator of many physiological transitions in insects, including molting, metamorphosis, diapause, and reproduction. These processes often include metabolic changes. Here we show that JH accelerates metabolic rate in bumble bees (Bombus terrestris). We reduced JH levels in worker bumble bees by removing their corpora allata (allatectomy) and elevated JH levels in queens through a topical application of JH-III. Natural and high JH levels increased the metabolic rate in both workers and queens and triggered an increased protein turnover rate. Following the treatments, JH also caused an increase in food consumption and a reduction in lipid levels and flight muscle mass of queens, and a reduction in lipids levels in workers. Furthermore, the topical application of a JH analog to queens prior to their diapause caused a decline in their survival of diapause. These findings support the hypothesis that JH acts as a metabolic rate accelerator, initiating a resource shift in bumble bees, and thereby reducing diapause survival in queens. Based on previous studies on JH we suggest that, additional to its behavioral or physiological effects, JH's function as an accelerator of metabolic processes is conserved across different life stages and insect species.

Bumble bee queen pheromones are context-dependent

Queen pheromones have long been studied as a major factor regulating reproductive division of labor in social insects. Hitherto, only a handful of queen pheromones were identified and their effects on workers have mostly been studied in isolation from the social context in which they operate. Our study examined the importance of behavioral and social context for the perception of queen semiochemicals by bumble bee workers. Our results indicate that a mature queen’s cuticular semiochemicals are capable of inhibiting worker reproduction only when accompanied by the queen’s visual presence and the offspring she produces, thus, when presented in realistic context. Queen’s chemistry, queen’s visual presence and presence of offspring all act to regulate worker reproduction, but none of these elements produces an inhibitory effect on its own. Our findings highlight the necessity to reconsider what constitutes a queen pheromone and suggest a new approach to the study of chemical ecology in social insects.

Size constrains oxygen delivery capacity within but not between bumble bee castes

Bumble bees are eusocial, with distinct worker and queen castes that vary strikingly in size and life-history. The smaller workers rely on energetically-demanding foraging flights to collect resources for rearing brood. Queens can be 3 to 4 times larger than workers, flying only for short periods in fall and again in spring after overwintering underground. These differences between castes in size and life history may be reflected in hypoxia tolerance. When oxygen demand exceeds supply, oxygen delivery to the tissues can be compromised. Previous work revealed hypermetric scaling of tracheal system volume of worker bumble bees (Bombus impatiens); larger workers had much larger tracheal volumes, likely to facilitate oxygen delivery over longer distances. Despite their much larger size, queens had relatively small tracheal volumes, potentially limiting their ability to deliver oxygen and reducing their ability to respond to hypoxia. However, these morphological measurements only indirectly point to differences in respiratory capacity. To directly assess size- and caste-related differences in tolerance to low oxygen, we measured critical PO2 (Pcrit; the ambient oxygen level below which metabolism cannot be maintained) during both rest and flight of worker and queen bumble bees. Queens and workers had similar Pcrit values during both rest and flight. However, during flight in oxygen levels near the Pcrit, mass-specific metabolic rates declined precipitously with mass both across and within castes, suggesting strong size limitations on oxygen delivery, but only during extreme conditions, when demand is high and supply is low. Together, these data suggest that the comparatively small tracheal systems of queen bumble bees do not limit their ability to deliver oxygen except in extreme conditions; they pay little cost for filling body space with eggs rather than tracheal structures.