Biogeography, climate, and land use create a mosaic of parasite risk in native bumble bees.

Host-parasite interactions are crucial to the regulation of host population growth, as they often impact both long-term population stability and ecological functioning. Animal hosts navigate a number of environmental conditions, including local climate, anthropogenic land use, and varying degrees of spatial isolation, all of which can mediate parasitism exposure. Despite this, we know little about the potential for these environmental conditions to impact pathogen prevalence at biogeographic scales, especially for key ecosystem service-providing animals. Bees are essential pollination providers that may be particularly sensitive to biogeography, climate, and land-use as these factors are known to limit bee dispersal and contribute to underlying population genetic variation, which may also impact host-parasite interactions. Importantly, many native bumble bee species have recently shown geographic range contractions, reduced genetic diversity, and increased parasitism rates, highlighting the potential importance of interacting and synergistic stressors. In this study, we incorporate spatially explicit environmental, biogeographic, and land-use data in combination with genetically derived host population data to conduct a large-scale epidemiological assessment of the drivers of pathogen prevalence across >1000 km for a keystone western US pollinator, the bumble bee Bombus vosnesenskii. We found high rates of infection from Crithidia bombi and C. expoekii, which show strong spatial autocorrelation and which were more prevalent in northern latitudes. We also show that land use barriers best explained differences in parasite prevalence and parasite community composition, while precipitation, elevation, and B. vosnesenskii nesting density were important drivers of parasite prevalence. Overall, our results demonstrate that human land use can impact critical host-parasite interactions for native bees at massive spatial scales. Further, our work indicates that disease-related survey and conservation measures should take into account the independent and interacting influences of climate, biogeography, land use, and local population dynamics.

Consuming sunflower pollen reduced pathogen infection but did not alter measures of immunity in bumblebees.

Certain diets can benefit bee health by reducing pathogens, but the mechanism(s) driving these medicinal effects are largely unexplored. Recent research found that sunflower (Helianthus annuus) pollen reduces the gut pathogen Crithidia bombi in the common eastern bumblebee (Bombus impatiens). Here, we tested the effects of sunflower pollen and infection on two bee immune metrics to determine whether sunflower pollen diet drives changes in host immunity that can explain this medicinal effect. Bees were infected with C. bombi or not and given either sunflower or wildflower pollen. Subsequently, bees received a benign immune challenge or were left naive to test the induced and constitutive immune responses, respectively. We measured haemolymph phenoloxidase activity, involved in the melanization cascade, and antibacterial activity. Sunflower pollen reduced C. bombi infection, but we found no significant pollen diet effect on either immune measure. Phenoloxidase activity was also not affected by C. bombi infection status; however, uninfected bees were more likely to have measurable constitutive antibacterial activity, while infected bees had higher induced antibacterial activity. Overall, we found that sunflower pollen does not significantly affect the immune responses we measured, suggesting that the mechanisms underlying its medicinal effect do not involve these bee immune parameters. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Interacting Antagonisms: Parasite Infection Alters Bombus impatiens (Hymenoptera: Apidae) Responses to Herbivory on Tomato Plants.

Little is known about how simultaneous antagonistic interactions on plants and pollinators affect pollination services, even though herbivory can alter floral traits and parasites can change pollinator learning, perception, or behavior. We investigated how a common herbivore and bumble bee (Bombus spp.) parasite impact pollination in tomatoes (Solanum lycopersicum L.) (Solanales: Solanaceae). We exposed half the plants to low-intensity herbivory by the specialist Manduca sexta L. (Lepidoptera: Sphigidae), and observed bumble bee visits and time spent on flowers of damaged and control plants. Following observations, we caught the foraging bees and assessed infection by the common gut parasite, Crithidia bombi Lipa & Triggiani (Trypanosomatida: Trypanosomatidae). Interestingly, we found an interactive effect between herbivory and Crithidia infection; bees with higher parasite loads spent less time foraging on damaged plants compared to control plants. However, bees did not visit higher proportions of flowers on damaged or control plants, regardless of infection status. Our study demonstrates that multiple antagonists can have synergistic negative effects on the duration of pollinator visits, such that the consequences of herbivory may depend on the infection status of pollinators. If pollinator parasites indeed exacerbate the negative effects of herbivory on pollination services, this suggests the importance of incorporating bee health management practices to maximize crop production.

Sunflower pollen reduces a gut pathogen in the model bee species, Bombus impatiens, but has weaker effects in three wild congeners.

Commercial bumblebees have become popular models to understand stressors and solutions for pollinator health, but few studies test whether results translate to other pollinators. Consuming sunflower pollen dramatically reduces infection by the gut parasite Crithidia bombi in commercially reared Bombus impatiens. We assessed the effect of sunflower pollen on infection in wild B. impatiens, Bombus griseocollis, Bombus bimaculatus and Bombus vagans. We also asked how pollen diet (50% sunflower pollen versus wildflower pollen) and infection (yes/no) affected performance in wild B. impatiens microcolonies. Compared to controls, sunflower pollen dramatically reduced Crithidia infection in commercial and wild B. impatiens, had similar but less dramatic effects in B. bimaculatus and B. vagans, and no effect in B. griseocollis. Bombus impatiens, B. bimaculatus and B. vagans are in the same subgenus, suggesting that responses to sunflower pollen may be phylogenetically conserved. In microcolonies, 50% sunflower pollen reduced infection compared to wildflower pollen, but also reduced reproduction. Sunflower pollen could control Crithidia infections in B. impatiens and potentially close relatives, but may hinder reproduction if other resources are scarce. We caution that research using managed bee species, such as B. impatiens, be interpreted carefully as findings may not relate to all bee species.

Sunflower pollen induces rapid excretion in bumble bees: Implications for host-pathogen interactions.

Host diet can have a profound effect on host-pathogen interactions, including indirect effects on pathogens mediated through host physiology. In bumble bees (Bombus impatiens), the consumption of sunflower (Helianthus annuus) pollen dramatically reduces infection by the gut protozoan pathogen Crithidia bombi. One hypothesis for the medicinal effect of sunflower pollen is that consumption changes host gut physiological function, causing rapid excretion that flushes C. bombi from the system. We tested the effect of pollen diet and C. bombi infection on gut transit properties using a 2x2 factorial experiment in which bees were infected with C. bombi or not and fed sunflower or wildflower pollen diet. We measured several non-mutually exclusive physiological processes that underlie the insect excretory system, including gut transit time, bi-hourly excretion rate, the total number of excretion events and the total volume of excrement. Sunflower pollen significantly reduced gut transit time in uninfected bees, and increased the total number of excretion events and volume of excrement by 66 % and 68 %, respectively, in both infected and uninfected bees. Here we show that a sunflower pollen diet can affect host physiology gut function, causing more rapid and greater excretion. These results provide important insight into a mechanism that could underlie the medicinal effect of sunflower pollen for bumble bees.

Eristalis flower flies can be mechanical vectors of the common trypanosome bee parasite, Crithidia bombi.

Flowers can be transmission platforms for parasites that impact bee health, yet bees share floral resources with other pollinator taxa, such as flies, that may be hosts or non-host vectors (i.e., mechanical vectors) of parasites. Here, we assessed whether the fecal-orally transmitted gut parasite of bees, Crithidia bombi, can infect Eristalis tenax flower flies. We also investigated the potential for two confirmed solitary bee hosts of C. bombi, Osmia lignaria and Megachile rotundata, as well as two flower fly species, Eristalis arbustorum and E. tenax, to transmit the parasite at flowers. We found that C. bombi did not replicate (i.e., cause an active infection) in E. tenax flies. However, 93% of inoculated flies defecated live C. bombi in their first fecal event, and all contaminated fecal events contained C. bombi at concentrations sufficient to infect bumble bees. Flies and bees defecated inside the corolla (flower) more frequently than other plant locations, and flies defecated at volumes comparable to or greater than bees. Our results demonstrate that Eristalis flower flies are not hosts of C. bombi, but they may be mechanical vectors of this parasite at flowers. Thus, flower flies may amplify or dilute C. bombi in bee communities, though current theoretical work suggests that unless present in large populations, the effects of mechanical vectors will be smaller than hosts.

Managed bumble bees acquire parasites from their foraging environment: A case study on parasite spillback.

The use of commercially reared bumble bees in agricultural environments has been recognized as a potential threat to wild pollinators due to competition, genetic contamination, and most notably, disease transmission. Higher parasite prevalence near greenhouses where managed bumble bees are used has been linked to parasite spillover from managed to wild bees. However, pathogen transmission is not unidirectional, and can also flow from wild to managed bees. These newly infected managed bees can subsequently re-infect (other) wild bees, in a process known as spillback, which is an alternative explanation for the increased parasite prevalence near greenhouses. Reducing parasite prevalence in managed bees is key to controlling host-parasite dynamics in cases of spillover; in spillback, producing managed bees that are resilient to infection is important. Here we establish that the managed bumble bee Bombus terrestris can acquire parasites from their foraging environment, which is the major infection route for Apicystis spp. and Crithidia spp., but not for Nosema spp.. Managed B. terrestris were found to have a higher prevalence of Crithdia and a higher load of Apicystis than local wild conspecifics, showing that for these parasites, spillback is a possible risk scenario.

Crithidia bombi can infect two solitary bee species while host survivorship depends on diet.

Pathogens and lack of floral resources interactively impair global pollinator health. However, epidemiological and nutritional studies aimed at understanding bee declines have historically focused on social species, with limited evaluations of solitary bees. Here, we asked whether Crithidia bombi, a trypanosomatid gut pathogen known to infect bumble bees, could infect the solitary bees Osmia lignaria (females) and Megachile rotundata (males), and whether nutritional stress influenced infection patterns and bee survival. We found that C. bombi was able to infect both solitary bee species, with 59% of O. lignaria and 29% of M. rotundata bees experiencing pathogen replication 5­11 days following inoculation. Moreover, access to pollen resulted in O. lignaria living longer, although it did not influence M. rotundata survival. Access to pollen did not affect infection probability or resulting pathogen load in either species. Similarly, inoculating with the pathogen did not drive survival patterns in either species during the 5­11-day laboratory assays. Our results demonstrate that solitary bees can be hosts of a known bumble bee pathogen, and that access to pollen is an important contributing factor for bee survival, thus expanding our understanding of factors contributing to solitary bee health.

Herbivory and Time Since Flowering Shape Floral Rewards and Pollinator-Pathogen Interactions.

Herbivory can induce chemical changes throughout plant tissues including flowers, which could affect pollinator-pathogen interactions. Pollen is highly defended compared to nectar, but no study has examined whether herbivory affects pollen chemistry. We assessed the effects of leaf herbivory on nectar and pollen alkaloids in Nicotiana tabacum, and how herbivory-induced changes in nectar and pollen affect pollinator-pathogen interactions. We damaged leaves of Nicotiana tabacum using the specialist herbivore Manduca sexta and compared nicotine and anabasine concentrations in nectar and pollen. We then pooled nectar and pollen by collection periods (within and after one month of flowering), fed them in separate experiments to bumble bees (Bombus impatiens) infected with the gut pathogen Crithidia bombi, and assessed infections after seven days. We did not detect alkaloids in nectar, and leaf damage did not alter the effect of nectar on Crithidia counts. In pollen, herbivory induced higher concentrations of anabasine but not nicotine, and alkaloid concentrations rose and then fell as a function of days since flowering. Bees fed pollen from damaged plants had Crithidia counts 15 times higher than bees fed pollen from undamaged plants, but only when pollen was collected after one month of flowering, indicating that both damage and time since flowering affected interaction outcomes. Within undamaged treatments, bees fed late-collected pollen had Crithidia counts 10 times lower than bees fed early-collected pollen, also indicating the importance of time since flowering. Our results emphasize the role of herbivores in shaping pollen chemistry, with consequences for interactions between pollinators and their pathogens.

Assessing Chemical Mechanisms Underlying the Effects of Sunflower Pollen on a Gut Pathogen in Bumble Bees.

Many pollinator species are declining due to a variety of interacting stressors including pathogens, sparking interest in understanding factors that could mitigate these outcomes. Diet can affect host-pathogen interactions by changing nutritional reserves or providing bioactive secondary chemicals. Recent work found that sunflower pollen (Helianthus annuus) dramatically reduced cell counts of the gut pathogen Crithidia bombi in bumble bee workers (Bombus impatiens), but the mechanism underlying this effect is unknown. Here we analyzed methanolic extracts of sunflower pollen by LC-MS and identified triscoumaroyl spermidines as the major secondary metabolite components, along with a flavonoid quercetin-3-O-hexoside and a quercetin-3-O-(6-O-malonyl)-hexoside. We then tested the effect of triscoumaroyl spermidine and rutin (as a proxy for quercetin glycosides) on Crithidia infection in B. impatiens, compared to buckwheat pollen (Fagopyrum esculentum) as a negative control and sunflower pollen as a positive control. In addition, we tested the effect of nine fatty acids from sunflower pollen individually and in combination using similar methods. Although sunflower pollen consistently reduced Crithidia relative to control pollen, none of the compounds we tested had significant effects. In addition, diet treatments did not affect mortality, or sucrose or pollen consumption. Thus, the mechanisms underlying the medicinal effect of sunflower are still unknown; future work could use bioactivity-guided fractionation to more efficiently target compounds of interest, and explore non-chemical mechanisms. Ultimately, identifying the mechanism underlying the effect of sunflower pollen on pathogens will open up new avenues for managing bee health.

Host density drives viral, but not trypanosome, transmission in a key pollinator.

Supplemental feeding of wildlife populations can locally increase the density of individuals, which may in turn impact disease dynamics. Flower strips are a widely used intervention in intensive agricultural systems to nutritionally support pollinators such as bees. Using a controlled experimental semi-field design, we asked how density impacts transmission of a virus and a trypanosome parasite in bumblebees. We manipulated bumblebee density by using different numbers of colonies within the same area of floral resource. In high-density compartments, slow bee paralysis virus was transmitted more quickly, resulting in higher prevalence and level of infection in bumblebee hosts. By contrast, there was no impact of density on the transmission of the trypanosome Crithidia bombi, which may reflect the ease with which this parasite is transmitted. These results suggest that agri-environment schemes such as flower strips, which are known to enhance the nutrition and survival of bumblebees, may also have negative impacts on pollinators through enhanced disease transmission. Future studies should assess how changing the design of these schemes could minimize disease transmission and thus maximise their health benefits to wild pollinators.

Temperature dependence of parasitic infection and gut bacterial communities in bumble bees.

High temperatures (e.g., fever) and gut microbiota can both influence host resistance to infection. However, effects of temperature-driven changes in gut microbiota on resistance to parasites remain unexplored. We examined the temperature dependence of infection and gut bacterial communities in bumble bees infected with the trypanosomatid parasite Crithidia bombi. Infection intensity decreased by over 80% between 21 and 37°C. Temperatures of peak infection were lower than predicted based on parasite growth in vitro, consistent with mismatches in thermal performance curves of hosts, parasites and gut symbionts. Gut bacterial community size and composition exhibited slight but significant, non-linear, and taxon-specific responses to temperature. Abundance of total gut bacteria and of Orbaceae, both negatively correlated with infection in previous studies, were positively correlated with infection here. Prevalence of the bee pathogen-containing family Enterobacteriaceae declined with temperature, suggesting that high temperature may confer protection against diverse gut pathogens. Our results indicate that resistance to infection reflects not only the temperature dependence of host and parasite performance, but also temperature-dependent activity of gut bacteria. The thermal ecology of gut parasite-symbiont interactions may be broadly relevant to infectious disease, both in ectothermic organisms that inhabit changing climates, and in endotherms that exhibit fever-based immunity.

Flagellum Removal by a Nectar Metabolite Inhibits Infectivity of a Bumblebee Parasite.

Plant phytochemicals can act as natural "medicines" for animals against parasites [1-3]. Some nectar metabolites, for example, reduce parasite infections in bees [4-7]. Declining plant diversity through anthropogenic landscape change [8-11] could reduce the availability of medicinal nectar plants for pollinators, exacerbating their decline [12]. Existing studies are, however, limited by (1) a lack of mechanistic insights into how phytochemicals affect pollinator diseases and (2) the restriction to few, commercially available chemicals, thereby potentially neglecting plants with the biggest antiparasitic effects. To rapidly identify plants with the greatest potential as natural bee medicines, we developed a bioactivity-directed fractionation assay for nectar metabolites. We evaluated 17 important nectar plants against the bumblebee pathogen Crithidia bombi (Trypanosomatidae) [13-17]. The most bioactive species was heather (Calluna vulgaris), the second most productive UK nectar plant [10]. We identified 4-(3-oxobut-1-enylidene)-3,5,5-trimethylcyclohex-2-en-1-one (callunene) from heather nectar as a potent inhibitor of C. bombi. Wild bumblebees (Bombus terrestris) foraging on heather ingest callunene at concentrations causing complete C. bombi inhibition. Feeding on callunene was prophylactic against infections. We show that C. bombi establishes infections by flagellar anchoring to the ileum epithelium. Short-term callunene exposure induced flagellum loss in C. bombi choanomastigotes, resulting in a loss of infectivity. We conclude that plant secondary metabolites can disrupt parasite flagellum attachment, revealing a mechanism behind their prophylactic effects. The decline of heathlands [18-21] reduces the availability of natural bee "medicine" and could exacerbate the contribution of diseases to pollinator declines. VIDEO ABSTRACT.

Trypanosomatid parasites infecting managed honeybees and wild solitary bees.

The parasite Crithidia mellificae (Kinetoplastea: Trypanosomatidae) infects honeybees, Apis mellifera. No pathogenic effects have been found in individual hosts, despite positive correlations between infections and colony mortalities. The solitary bee Osmia cornuta might constitute a host, but controlled infections are lacking to date. Here, we challenged male and female O. cornuta and honeybee workers in laboratory cages with C. mellificae. No parasite cells were found in any control. Parasite numbers increased 6.6 fold in honeybees between days 6 and 19 p.i. and significantly reduced survival. In O. cornuta, C. mellificae numbers increased 2-3.6 fold within cages and significantly reduced survival of males, but not females. The proportion of infected hosts increased in O. cornuta cages with faeces, but not in honeybee cages without faeces, suggesting faecal - oral transmission. The data show that O. cornuta is a host of C. mellificae and suggest that males are more susceptible. The higher mortality of infected honeybees proposes a mechanism for correlations between C. mellificae infections and colony mortalities.

Infection Outcomes are Robust to Thermal Variability in a Bumble Bee Host-Parasite System.

Climate change-related increases in thermal variability and rapid temperature shifts will affect organisms in multiple ways, including imposing physiological stress. Furthermore, the effects of temperature may alter the outcome of biotic interactions, such as those with pathogens and parasites. In the context of host-parasite interactions, the beneficial acclimation hypothesis posits that shifts away from acclimation or optimum performance temperatures will impose physiological stress on hosts and will affect their ability to resist parasite infection. We investigated the beneficial acclimation hypothesis in a bumble bee-trypanosome parasite system. Freshly emerged adult worker bumble bees, Bombus impatiens, were acclimated to 21, 26, or 29°C. They were subsequently experimentally exposed to the parasite, Crithidia bombi, and placed in a performance temperature that was the same as the acclimation temperature (constant) or one of the other temperatures (mismatched). Prevalence of parasite transmission was checked 4 and 6 days post-parasite exposure, and infection intensity in the gut was quantified at 8 days post-exposure. Parasite strain, host colony, and host size had significant effects on transmission prevalence and infection load. However, neither transmission nor infection intensity were significantly different between constant and mismatched thermal regimes. Furthermore, acclimation temperature, performance temperature, and the interaction of acclimation and performance temperatures had no significant effects on infection outcomes. These results, counter to predictions of the beneficial acclimation hypothesis, suggest that infection outcomes in this host-parasite system are robust to thermal variation within typically experienced ranges. This could be a consequence of adaptation to commonly experienced natural thermal regimes or a result of individual and colony level heterothermy in bumble bees. However, thermal variability may still have a detrimental effect on more sensitive stages or species, or when extreme climatic events push temperatures outside of the normally experienced range.

Bee pathogen transmission dynamics: deposition, persistence and acquisition on flowers.

Infectious diseases are a primary driver of bee decline worldwide, but limited understanding of how pathogens are transmitted hampers effective management. Flowers have been implicated as hubs of bee disease transmission, but we know little about how interspecific floral variation affects transmission dynamics. Using bumblebees ( Bombus impatiens), a trypanosomatid pathogen ( Crithidia bombi) and three plant species varying in floral morphology, we assessed how host infection and plant species affect pathogen deposition on flowers, and plant species and flower parts impact pathogen survival and acquisition at flowers. We found that host infection with Crithidia increased defaecation rates on flowers, and that bees deposited faeces onto bracts of Lobelia siphilitica and Lythrum salicaria more frequently than onto Monarda didyma bracts . Among flower parts, bracts were associated with the lowest pathogen survival but highest resulting infection intensity in bee hosts. Additionally, we found that Crithidia survival across flower parts was reduced with sun exposure. These results suggest that efficiency of pathogen transmission depends on where deposition occurs and the timing and place of acquisition, which varies among plant species and environmental conditions. This information could be used for development of wildflower mixes that maximize forage while minimizing disease spread.

pH-mediated inhibition of a bumble bee parasite by an intestinal symbiont.

Gut symbionts can augment resistance to pathogens by stimulating host-immune responses, competing for space and nutrients, or producing antimicrobial metabolites. Gut microbiota of social bees, which pollinate many crops and wildflowers, protect hosts against diverse infections and might counteract pathogen-related bee declines. Bumble bee gut microbiota, and specifically abundance of Lactobacillus 'Firm-5' bacteria, can enhance resistance to the trypanosomatid parasite Crithidia bombi. However, the mechanism underlying this effect remains unknown. We hypothesized that the Firm-5 bacterium Lactobacillus bombicola, which produces lactic acid, inhibits C. bombi via pH-mediated effects. Consistent with our hypothesis, L. bombicola spent medium inhibited C. bombi growth via reduction in pH that was both necessary and sufficient for inhibition. Inhibition of all parasite strains occurred within the pH range documented in honey bees, though sensitivity to acidity varied among strains. Spent medium was slightly more potent than HCl, d- and l-lactic acids for a given pH, suggesting that other metabolites also contribute to inhibition. Results implicate symbiont-mediated reduction in gut pH as a key determinant of trypanosomatid infection in bees. Future investigation into in vivo effects of gut microbiota on pH and infection intensity would test the relevance of these findings for bees threatened by trypanosomatids.

High Gut Microbiota Diversity Provides Lower Resistance against Infection by an Intestinal Parasite in Bumblebees.

The microbiome, especially the gut flora, is known to affect the interaction between parasites and their hosts. In this context, a parasitic infection can be viewed as an invasion into the preexisting microbial ecological community. Hence, in addition to the intrinsic defense mechanisms of the host itself, infection success depends on the colonization resistance of the microbiota. In the bumblebee Bombus terrestris, the microbiota provides resistance to the intestinal parasite Crithidia bombi, yet which properties actually provide protection remains largely unknown. Here, we show that the community structure of the gut microbiota-in terms of bacterial operational taxonomic units (OTUs) of 16S ribosomal RNA gene sequences-before parasite exposure can be informative of the eventual infection outcome. Specifically, higher microbiota OTU diversity is associated with less resistance. However, the microbial community structure does not differ between infected and noninfected individuals or between infected individuals of varying susceptibility. This suggests that parasite infection success depends on the microbiota composition but that subsequent changes occur, although the exact alteration that occurs remains elusive. In fact, the bumblebee microbiota is surprisingly unaffected by parasite exposure and infection. Rather, the microbiota-host interaction before parasite exposure seems to be a key mechanism regulating resistance to infection.

Microbiome Structure Influences Infection by the Parasite Crithidia bombi in Bumble Bees.

Recent declines in bumble bee populations are of great concern and have prompted critical evaluations of the role of pathogen introductions and host resistance in bee health. One factor that may influence host resilience when facing infection is the gut microbiota. Previous experiments with Bombus terrestris, a European bumble bee, showed that the gut microbiota can protect against Crithidia bombi, a widespread trypanosomatid parasite of bumble bees. However, the particular characteristics of the microbiome responsible for this protective effect have thus far eluded identification. Using wild and commercially sourced Bombus impatiens, an important North American pollinator, we conducted cross-wise microbiota transplants to naive hosts of both backgrounds and challenged them with a Crithidia parasite. As with B. terrestris, we find that microbiota-dependent protection against Crithidia operates in B. impatiens Lower Crithidia infection loads were experimentally associated with high microbiome diversity, large gut bacterial populations, and the presence of Apibacter, Lactobacillus Firm-5, and Gilliamella spp. in the gut community. These results indicate that even subtle differences between gut community structures can have a significant impact on a microbiome's ability to defend against parasite infections.IMPORTANCE Many wild bumble bee populations are under threat due to human activity, including through the introduction of pathogens via commercially raised bees. Recently, it was found that the bumble bee gut microbiota can help defend against a common parasite, Crithidia bombi, but the particular factors contributing to this protection are unknown. Using both wild and commercially raised bees, we conducted microbiota transplants to show that microbiome diversity, total gut bacterial load, and the presence of certain core members of the microbiota may all impact bee susceptibility to Crithidia infection. Bee origin (genetic background) was also a factor. Finally, by examining this phenomenon in a previously uninvestigated bee species, our study demonstrates that microbiome-mediated resistance to Crithidia is conserved across multiple bumble bee species. These findings highlight how intricate interactions between hosts, microbiomes, and parasites can have wide-ranging consequences for the health of ecologically important species.

Pesticide reduces bumblebee colony initiation and increases probability of population extinction.

Pollinators are in global decline and agricultural pesticides are a potential driver of this. Recent studies have suggested that pesticides may significantly impact bumblebee colonies-an important and declining group of pollinators. Here, we show that colony-founding queens, a critical yet vulnerable stage of the bumblebee lifecycle, are less likely to initiate a colony after exposure to thiamethoxam, a neonicotinoid insecticide. Bombus terrestris queens were exposed to field-relevant levels of thiamethoxam and two natural stressors: the parasite Crithidia bombi and varying hibernation durations. Exposure to thiamethoxam caused a 26% reduction in the proportion of queens that laid eggs, and advanced the timing of colony initiation, although we did not detect impacts of any experimental treatment on the ability of queens to produce adult offspring during the 14-week experimental period. As expected from previous studies, the hibernation duration also had an impact on egg laying, but there was no significant interaction with insecticide treatment. Modelling the impacts of a 26% reduction in colony founding on population dynamics dramatically increased the likelihood of population extinction. This shows that neonicotinoids can affect this critical stage in the bumblebee lifecycle and may have significant impacts on population dynamics.