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

Plant-pollinator network architecture does not impact intraspecific microbiome variability

Variation in how individuals interact with food resources can directly impact, and be affected by, their microbial interactions due to the potential for transmission. The degree to which this transmission occurs, however, may depend on the structure of forager networks, which determine the community-scale transmission opportunities. In particular, how the community-scale opportunity for transfer balances individual-scale barriers to transmission is unclear. Examining the bee-flower and bee-microbial interactions of over 1000 individual bees, we tested (1) the degree to which individual floral visits predicted microbiome composition and (2) whether plant-bee networks with increased opportunity for microbial transmission homogenized the microbiomes of bees within that network. The pollen community composition carried by bees was associated with microbiome composition at some sites, suggesting that microbial transmission at flowers occurred. Contrary to our predictions, however, microbiome variability did not differ based on transfer opportunity: bee microbiomes in asymmetric networks with high opportunity for microbial transfer were similarly variable compared to microbiomes in networks with more evenly distributed links. These findings suggest that microbial transmission at flowers is frequent enough to be observed at the community level, but that community network structure did not substantially change the dynamics of this transmission, perhaps due to filtering processes in host guts.

Bumble bee microbiota shows temporal succession and increase of lactic acid bacteria when exposed to outdoor environments

Question

The large earth bumble bee (Bombus terrestris) maintains a social core gut-microbiota, similar as known from the honey bee, which plays an important role for host health and resistance. Experiments under laboratory conditions with commercial hives are limited to vertically transmitted microbes and neglect influences of environmental factors or external acquisition of microbes. Various environmental and landscape-level factors may have an impact on the gut-microbiota of pollinating insects, with consequences for pollinator health and fitness in agroecosystems. Still, it is not fully clear whether access to different flower diversities will have a significant influence on the bumble bee microbiota. Here, we tested in a semi-field experiment if the bumble bee microbiota changes over time when exposed to different flower diversities within outdoor flight cages. We used commercial hives to distinguish between vertically and horizontally transmitted bacteria, respectively from the nest environment or the exposed outside environment.

Result

The sequential sampling of foraging workers over a period of 35 days indicated a temporal progression of the bumble bee microbiota when placed outside. The microbiota increased in diversity and changed in composition and variability over time. We observed a major increase in relative abundance of the families Lactobacillaceae, Bifidobacteriaceae and Weeksellaceae. In contrast, major core-taxa like Snodgrassella and Gilliamella declined in their relative abundance over time. The genus Lactobacillus showed a high diversity and strain specific turnover, so that only specific ASVs showed an increase over time, while others had a more erratic occurrence pattern. Exposure to different flower diversities had no significant influence on the progression of the bumble bee microbiota.

Conclusion

The bumble bee microbiota showed a dynamic temporal succession with distinct compositional changes and diversification over time when placed outdoor. The exposure of bumble bees to environmental conditions, or environmental microbes, increases dissimilarity and changes the gut-community composition. This shows the importance of environmental influences on the temporal dynamic and progression of the bumble bee microbiota.

Ecological and social factors influence interspecific pathogens occurrence among bees

The interspecific transmission of pathogens can occur frequently in the environment. Among wild bees, the main spillover cases are caused by pathogens associated with Apis mellifera, whose colonies can act as reservoirs. Due to the limited availability of data in Italy, it is challenging to accurately assess the impact and implications of this phenomenon on the wild bee populations. In this study, a total of 3372 bees were sampled from 11 Italian regions within the BeeNet project, evaluating the prevalence and the abundance of the major honey bee pathogens (DWV, BQCV, ABPV, CBPV, KBV, Nosema ceranae, Ascosphaera apis, Crithidia mellificae, Lotmaria passim, Crithidia bombi). The 68.4% of samples were positive for at least one pathogen. DWV, BQCV, N. ceranae and CBPV showed the highest prevalence and abundance values, confirming them as the most prevalent pathogens spread in the environment. For these pathogens, Andrena, Bombus, Eucera and Seladonia showed the highest mean prevalence and abundance values. Generally, time trends showed a prevalence and abundance decrease from April to July. In order to predict the risk of infection among wild bees, statistical models were developed. A low influence of apiary density on pathogen occurrence was observed, while meteorological conditions and agricultural management showed a greater impact on pathogen persistence in the environment. Social and biological traits of wild bees also contributed to defining a higher risk of infection for bivoltine, communal, mining and oligolectic bees. Out of all the samples tested, 40.5% were co-infected with two or more pathogens. In some cases, individuals were simultaneously infected with up to five different pathogens. It is essential to increase knowledge about the transmission of pathogens among wild bees to understand dynamics, impact and effects on pollinator populations. Implementing concrete plans for the conservation of wild bee species is important to ensure the health of wild and human-managed bees within a One-Health perspective.

Distribution of infectious and parasitic agents among three sentinel bee species across European agricultural landscapes

Infectious and parasitic agents (IPAs) and their associated diseases are major environmental stressors that jeopardize bee health, both alone and in interaction with other stressors. Their impact on pollinator communities can be assessed by studying multiple sentinel bee species. Here, we analysed the field exposure of three sentinel managed bee species (Apis mellifera, Bombus terrestris and Osmia bicornis) to 11 IPAs (six RNA viruses, two bacteria, three microsporidia). The sentinel bees were deployed at 128 sites in eight European countries adjacent to either oilseed rape fields or apple orchards during crop bloom. Adult bees of each species were sampled before their placement and after crop bloom. The IPAs were detected and quantified using a harmonised, high-throughput and semi-automatized qPCR workflow. We describe differences among bee species in IPA profiles (richness, diversity, detection frequencies, loads and their change upon field exposure, and exposure risk), with no clear patterns related to the country or focal crop. Our results suggest that the most frequent IPAs in adult bees are more appropriate for assessing the bees' IPA exposure risk. We also report positive correlations of IPA loads supporting the potential IPA transmission among sentinels, suggesting careful consideration should be taken when introducing managed pollinators in ecologically sensitive environments.

Body mass, temperature, and pathogen intensity differentially affect critical thermal maxima and their population-level variation in a solitary bee

Climate change presents a major threat to species distribution and persistence. Understanding what abiotic or biotic factors influence the thermal tolerances of natural populations is critical to assessing their vulnerability under rapidly changing thermal regimes. This study evaluates how body mass, local climate, and pathogen intensity influence heat tolerance and its population-level variation (SD) among individuals of the solitary bee Xenoglossa pruinosa. We assess the sex-specific relationships between these factors and heat tolerance given the differences in size between sexes and the ground-nesting behavior of the females. We collected X. pruinosa individuals from 14 sites across Pennsylvania, USA, that varied in mean temperature, precipitation, and soil texture. We measured the critical thermal maxima (CTmax) of X. pruinosa individuals as our proxy for heat tolerance and used quantitative PCR to determine relative intensities of three parasite groups-trypanosomes, Spiroplasma apis (mollicute bacteria), and Vairimorpha apis (microsporidian). While there was no difference in CTmax between the sexes, we found that CTmax increased significantly with body mass and that this relationship was stronger for males than for females. Air temperature, precipitation, and soil texture did not predict mean CTmax for either sex. However, population-level variation in CTmax was strongly and negatively correlated with air temperature, which suggests that temperature is acting as an environmental filter. Of the parasites screened, only trypanosome intensity correlated with heat tolerance. Specifically, trypanosome intensity negatively correlated with the CTmax of female X. pruinosa but not males. Our results highlight the importance of considering size, sex, and infection status when evaluating thermal tolerance traits. Importantly, this study reveals the need to evaluate trends in the variation of heat tolerance within and between populations and consider implications of reduced variation in heat tolerance for the persistence of ectotherms in future climate conditions.

Methods matter: the influence of method on infection estimates of the bumblebee parasite Crithidia bombi

The bumblebee gut parasite, Crithidia bombi, is widespread and prevalent in the field. Its interaction with Bombus spp. is a well-established epidemiological model. It is spread faecal-orally between colonies via the shared use of flowers when foraging. Accurately measuring the level of infection in bumblebees is important for assessing its distribution in the field, and also when conducting epidemiological experiments. Studies generally use 1 of 2 methods for measuring infection. One approach measures infection in faeces whereas the other method measures infection in guts. We tested whether the method of measuring infection affected the estimation of infection. Bumblebees were inoculated with a standardized inoculum and infection was measured 1 week later using either the faecal or gut method. We found that when the gut method was used to measure infection intensity estimates were significantly different to and approximately double those from the faecal method. These results have implications for the interpretation of previous study results and for the planning of future studies. Given the importance of bumblebees as pollinators, the impact of C. bombi on bumblebee health, and its use as an epidemiological model, we call on researchers to move towards consistent quantification of infections to enable future comparisons and meta-analyses of studies.

A Time for Every Purpose: Using Time-Dependent Sensitivity Analysis to Help Understand and Manage Dynamic Ecological Systems

AbstractSensitivity analysis is often used to help understand and manage ecological systems by assessing how a constant change in vital rates or other model parameters might affect the management outcome. This allows the manager to identify the most favorable course of action. However, realistic changes are often localized in time-for example, a short period of culling leads to a temporary increase in the mortality rate over the period. Hence, knowing when to act may be just as important as knowing what to act on. In this article, we introduce the method of time-dependent sensitivity analysis (TDSA) that simultaneously addresses both questions. We illustrate TDSA using three case studies: transient dynamics in static disease transmission networks, disease dynamics in a reservoir species with seasonal life history events, and endogenously driven population cycles in herbivorous invertebrate forest pests. We demonstrate how TDSA often provides useful biological insights, which are understandable on hindsight but would not have been easily discovered without the help of TDSA. However, as a caution, we also show how TDSA can produce results that mainly reflect uncertain modeling choices and are therefore potentially misleading. We provide guidelines to help users maximize the utility of TDSA while avoiding pitfalls.

Honeybee visitation to shared flowers increases Vairimorpha ceranae prevalence in bumblebees

Vairimorpha (=Nosema) ceranae is a widespread pollinator parasite that commonly infects honeybees and wild pollinators, including bumblebees. Honeybees are highly competent V. ceranae hosts and previous work in experimental flight cages suggests V. ceranae can be transmitted during visitation to shared flowers. However, the relationship between floral visitation in the natural environment and the prevalence of V. ceranae among multiple bee species has not been explored. Here, we analyzed the number and duration of pollinator visits to particular components of squash flowers-including the petals, stamen, and nectary-at six farms in southeastern Michigan, USA. We also determined the prevalence of V. ceranae in honeybees and bumblebees at each site. Our results showed that more honeybee flower contacts and longer duration of contacts with pollen and nectar were linked with greater V. ceranae prevalence in bumblebees. Honeybee visitation patterns appear to have a disproportionately large impact on V. ceranae prevalence in bumblebees even though honeybees are not the most frequent flower visitors. Floral visitation by squash bees or other pollinators was not linked with V. ceranae prevalence in bumblebees. Further, V. ceranae prevalence in honeybees was unaffected by floral visitation behaviors by any pollinator species. These results suggest that honeybee visitation behaviors on shared floral resources may be an important contributor to increased V. ceranae spillover to bumblebees in the field. Understanding how V. ceranae prevalence is influenced by pollinator behavior in the shared floral landscape is critical for reducing parasite spillover into declining wild bee populations.

Widespread distribution of honey bee-associated pathogens in native bees and wasps: Trends in pathogen prevalence and co-occurrence

Pollinators have experienced significant declines in the past decade, in part due to emerging infectious diseases. Historically, studies have primarily focused on pathogens in the Western honey bee, Apis mellifera. However, recent work has demonstrated that these pathogens are shared by other pollinators and can negatively affect their health. Here, we surveyed honey bees and 15 native bee and wasp species for 13 pathogens traditionally associated with honey bees. The native bee and wasp species included 11 species not previously screened for pathogens. We found at least one honey bee-associated pathogen in 53% of native bee and wasp samples. The most widely distributed and commonly detected pathogens were the microsporidian Nosema ceranae, the bacterium Melissococcus plutonius, and the viruses deformed wing virus and black queen cell virus. The prevalence of viruses was generally higher in honey bees than in native bees and wasps. However, the prevalence of M. plutonius and the brood fungus Ascosphaera apis was significantly higher in some native bee species than in honey bees. The data also reveal novel trends in the association between co-occurring pathogens in honey bees and native bees and wasps at the pathogen community level. These results can inform the assessment of risks that native pollinator species face from pathogen stress, and indicate that many non-viral pathogens, notably M. plutonius and N. ceranae, are far more widely distributed and commonly found in native bees and wasps than previously thought.

Can the examination of different types of hive samples be a non-invasive method for detection and quantification of viruses in honey bee (Apis mellifera L.) colonies?

Introduction

Honey bee viruses have been shown to negatively affect the vigour and longevity of European honey bees (Apis mellifera L). In the present work, beehive materials were tested for their potential to serve as non-invasive samples for honey bee virus detection.

Material and Methods

Honey, pollen, hive debris, hive grid smears and forager honey bees were collected from 24 hives at four locations in the Czech Republic. Deformed wing virus (DWV), acute bee paralysis virus (ABPV), sacbrood virus (SBV) and black queen cell virus (BQCV) were detected using a reverse transcription PCR (RT-PCR) and real-time quantitative RT-PCR and the results for bees and alternative materials compared.

Results

All forager bee samples contained DWV, BQCV and SBV and 54.2% had ABPV. When comparing beehive materials to bees, the most promising results were obtained from honey and pollen samples, with BQCV and SBV detected in all honey samples and ABPV in 12.5%. Detection of SBV was achieved in 91.6% of pollen samples, detection of BQCV in 87.5% and detection of DWW in 75%. The results for debris and smears were less consistent with the viral profile of the forager samples.

Conclusion

The best candidate materials for honey bee virus detection in a non-invasive technique are honey and pollen.

Deformed wing virus of honey bees is inactivated by cold plasma ionized hydrogen peroxide

Deformed wing virus (DWV) is a widespread pathogen of Apis mellifera honey bees, and is considered a major causative factor for the collapse of infected honey bee colonies. DWV can be horizontally transmitted among bees through various oral routes, including via food sharing and by interactions of bees with viral-contaminated solid hive substrates. Cold plasma ionized hydrogen peroxide (iHP) is used extensively by the food production, processing and medical industries to clean surfaces of microbial contaminants. In this study, we investigated the use of iHP to inactivate DWV particles in situ on a solid substrate. iHP-treated DWV sources were ~105-fold less infectious when injected into naïve honey bee pupae compared to DWV receiving no iHP treatment, matching injected controls containing no DWV. iHP treatment also greatly reduced the incidence of overt DWV infections (i.e., pupae having >109 copies of DWV). The level of DWV inactivation achieved with iHP treatment was higher than other means of viral inactivation such as gamma irradiation, and iHP treatment is likely simpler and safer. Treatment of DWV contaminated hive substrates with iHP, even with honey bees present, may be an effective way to decrease the impacts of DWV infection on honey bees.

Integrative population genetics and metagenomics reveals urbanization increases pathogen loads and decreases connectivity in a wild bee

As urbanization continues to increase, it is expected that two-thirds of the human population will reside in cities by 2050. Urbanization fragments and degrades natural landscapes, threatening wildlife including economically important species such as bees. In this study, we employ whole genome sequencing to characterize the population genetics, metagenome and microbiome, and environmental stressors of a common wild bee, Ceratina calcarata. Population genomic analyses revealed the presence of low genetic diversity and elevated levels of inbreeding. Through analyses of isolation by distance, resistance, and environment across urban landscapes, we found that green spaces including shrubs and scrub were the most optimal pathways for bee dispersal, and conservation efforts should focus on preserving these land traits to maintain high connectivity across sites for wild bees. Metagenomic analyses revealed landscape sites exhibiting urban heat island effects, such as high temperatures and development but low precipitation and green space, had the highest taxa alpha diversity across all domains even when isolating for potential pathogens. Notably, the integration of population and metagenomic data showed that reduced connectivity in urban areas is not only correlated with lower relatedness among individuals but is also associated with increased pathogen diversity, exposing vulnerable urban bees to more pathogens. Overall, our combined population and metagenomic approach found significant environmental variation in bee microbiomes and nutritional resources even in the absence of genetic differentiation, as well as enabled the potential early detection of stressors to bee health.

Transmission of a bumblebee parasite is robust despite parasite exposure to extreme temperatures

All organisms are exposed to fluctuating environmental conditions, such as temperature. How individuals respond to temperature affects their interactions with one another. Changes to the interaction between parasites and their hosts can have a large effect on disease dynamics. The gut parasite, Crithidia bombi, can be highly prevalent in the bumblebee, Bombus terrestris, and is an established epidemiological model. The parasite is transmitted between bumblebees via flowers, exposing it to a range of environmental temperatures prior to infection. We investigated whether incubation duration and temperature exposure, prior to infection, affects parasite infectivity. Prior to inoculation in B. terrestris, C. bombi was incubated at 10, 20, 30, 40 or 50°C for either 10 or 60 min. These times were chosen to reflect the length of time that the parasite remains infective when outside the host and the rate of floral visitation in bumblebees. Prevalence and infection intensity were measured in bees 1 week later. Incubation duration and the interaction between incubation temperature and duration affected the prevalence of C. bombi at 50°C, resulting in no infections after 60 min. Below 50°C, C. bombi prevalence was not affected by incubation temperature or duration. Extreme temperatures induced morphological changes in C. bombi cells; however, infection intensity was not affected by incubation duration or temperature. These results highlight that this parasite is robust to a wide range of temperatures. The parasite was not infective after being exposed to 50°C for 60 min, such temperatures likely exceed the flight abilities of bumblebees, and thus the potential for transmission. This study shows the importance of understanding the effects of environmental conditions on both hosts and parasites, which is needed to predict transmission under different environmental conditions.

Comparative analysis of 3 pollen sterilization methods for feeding bumble bees

Pollen is an essential component of bee diets, and rearing bumble bees (Bombus spp.) for commercial use necessitates feeding pollen in mass quantities. This pollen is collected from honey bee (Apis mellifera L.) colonies because neither an artificial diet nor an economical, large-scale pollen collection process from flowers is available. The provenance of honey bee-collected pollen is often unknown, and in some cases has crossed international borders. Both deformed wing virus (DWV) and the fungal pathogen Ascosphaera apis (Claussen) Olive & Spiltoir (cause of chalkbrood disease); occur in honey bee-collected pollen, and infections have been observed in bumble bees. We used these pathogens as general surrogates for viruses and spore-forming fungal diseases to test the efficacy of 3 sterilization methods, and assessed whether treatment altered pollen quality for the bumble bee. Using honey bee-collected pollen spiked with known doses of DWV and A. apis, we compared gamma irradiation (GI), ozone fumigation (OZ), and ethylene oxide fumigation (EO) against an untreated positive control and a negative control. Following sterilization treatments, we tested A. apis spore viability, detected viral presence with PCR, and tested palatability to the bumble bee Bombus impatiens Cresson. We also measured bacterial growth from pollens treated with EO and GI. GI and EO outperformed OZ treatment in pathogen suppression. EO had the highest sterilizing properties under commercial conditions and retained palatability and supported bee development better than other treatments. These results suggest that EO sterilization reduces pathogen risks while retaining pollen quality as a food source for rearing bumble bees.

From Gene to Transcript and Peptide: A Deep Overview on Non-Specific Lipid Transfer Proteins (nsLTPs)

Non-specific lipid transfer proteins (nsLTPs) stand out among plant-specific peptide superfamilies due to their multifaceted roles in plant molecular physiology and development, including their protective functions against pathogens. These antimicrobial agents have demonstrated remarkable efficacy against bacterial and fungal pathogens. The discovery of plant-originated, cysteine-rich antimicrobial peptides such as nsLTPs has paved the way for exploring the mentioned organisms as potential biofactories for synthesizing antimicrobial compounds. Recently, nsLTPs have been the focus of a plethora of research and reviews, providing a functional overview of their potential activity. The present work compiles relevant information on nsLTP omics and evolution, and it adds meta-analysis of nsLTPs, including: (1) genome-wide mining in 12 plant genomes not studied before; (2) latest common ancestor analysis (LCA) and expansion mechanisms; (3) structural proteomics, scrutinizing nsLTPs' three-dimensional structure/physicochemical characteristics in the context of nsLTP classification; and (4) broad nsLTP spatiotemporal transcriptional analysis using soybean as a study case. Combining a critical review with original results, we aim to integrate high-quality information in a single source to clarify unexplored aspects of this important gene/peptide family.

Local habitat type influences bumble bee pathogen loads and bee species distributions

Bumble bees (Hymenoptera: Apidae, Bombus Latreille) perform important ecological services in both managed and natural ecosystems. Anthropogenically induced change has altered floral resources, climate, and insecticide exposure, factors that impact health and disease levels in these bees. Habitat management presents a solution for improving bee health and biodiversity, but this requires better understanding of how different pathogens and bee species respond to habitat conditions. We take advantage of the washboard of repeated ridges (forested) and valleys (mostly developed) in central Pennsylvania to examine whether local variation in habitat type and other landscape factors influence bumble bee community composition and levels of 4 leading pathogens in the common eastern bumble bee, Bombus impatiens Cresson. Loads of viruses (DWV and BQCV) were found to be lowest in forest habitats, whereas loads of a gut parasite, Crithidia bombi, were highest in forests. Ridgetop forests hosted the most diverse bumble bee communities, including several habitat specialists. B. impatiens was most abundant in valleys, and showed higher incidence in areas of greater disturbance, including more developed, unforested, and lower floral resource sites, a pattern which mirrors its success in the face of anthropogenic change. Additionally, DNA barcoding revealed that B. sandersoni is much more common than is apparent from databases. Our results provide evidence that habitat type can play a large role in pathogen load dynamics, but in ways that differ by pathogen type, and point to a need for consideration of habitat at both macro-ecological and local spatial scales.

A time for every purpose: using time-dependent sensitivity analysis to help understand and manage dynamic ecological systems

Sensitivity analysis is often used to help understand and manage ecological systems, by assessing how a constant change in vital rates or other model parameters might affect the management outcome. This allows the manager to identify the most favorable course of action. However, realistic changes are often localized in time-for example, a short period of culling leads to a temporary increase in the mortality rate over the period. Hence, knowing when to act may be just as important as knowing what to act upon. In this article, we introduce the method of time-dependent sensitivity analysis (TDSA) that simultaneously addresses both questions. We illustrate TDSA using three case studies: transient dynamics in static disease transmission networks, disease dynamics in a reservoir species with seasonal life-history events, and endogenously-driven population cycles in herbivorous invertebrate forest pests. We demonstrate how TDSA often provides useful biological insights, which are understandable on hindsight but would not have been easily discovered without the help of TDSA. However, as a caution, we also show how TDSA can produce results that mainly reflect uncertain modeling choices and are therefore potentially misleading. We provide guidelines to help users maximize the utility of TDSA while avoiding pitfalls.

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.

An integrative bioinformatics pipeline shows that honeybee-associated microbiomes are driven primarily by pollen composition

The microbiomes associated with bee nests influence colony health through various mechanisms, although it is not yet clear how honeybee congeners differ in microbiome assembly processes, in particular the degrees to which floral visitations and the environment contribute to different aspects of diversity. We used DNA metabarcoding to sequence bacterial 16S rRNA from honey and stored pollen from nests of 4 honeybee species (Apis cerana, A. dorsata, A. florea, and A. laboriosa) sampled throughout Yunnan, China, a global biodiversity hotspot. We developed a computational pipeline integrating multiple databases for quantifying key facets of diversity, including compositional, taxonomic, phylogenetic, and functional ones. Further, we assessed candidate drivers of observed microbiome dissimilarity, particularly differences in floral visitations, habitat disturbance, and other key environmental variables. Analyses revealed that microbiome alpha diversity was broadly equivalent across the study sites and between bee species, apart from functional diversity which was very low in nests of the reclusive A. laboriosa. Turnover in microbiome composition across Yunnan was driven predominantly by pollen composition. Human disturbance negatively impacted both compositional and phylogenetic alpha diversity of nest microbiomes, but did not correlate with microbial turnover. We herein make progress in understanding microbiome diversity associated with key pollinators in a biodiversity hotspot, and provide a model for the use of a comprehensive informatics framework in assessing pattern and drivers of diversity, which enables the inclusion of explanatory variables both subtly and fundamentally different and enables elucidation of emergent or unexpected drivers.

Manipulation of multiple floral traits demonstrates role in pollinator disease transmission

Plants modulate multitrophic ecological interactions, and variation in plant traits can affect these interactions. Pollinators are exposed to pathogens at flowers and acquire or transmit pathogens at different rates on different plant species, but the traits mediating those interactions are almost entirely unknown. We experimentally manipulated five plant traits that span scales including flower, inflorescence, and plant, to determine their effects on pathogen transmission between foraging bees. Specifically, we manipulated two morphological traits (corolla lip length and flower orientation within an inflorescence) and three resource distribution traits (inflorescence nectar, plant patch nectar, and plant aggregation) in tents to test how plant traits affect bee pathogen transmission. We also quantified foraging behavior and fecal deposition patterns as potential mechanisms driving differences in transmission, and assessed trait manipulation consequences for bee reproduction. We found that pathogen transmission was reduced when we trimmed the corolla lip, evenly dispersed nectar distribution within an inflorescence, or aggregated plants in space. Some traits also affected bee reproduction; tents with trimmed corollas had more larval production than control tents, and tents with evenly distributed nectar across plant patches had more larval production than tents with clumped resources. Thus, some trait manipulations both reduced transmission and increased bee microcolony reproduction, although our design does not allow us to discern whether these are related or separate effects. Taken together, our results demonstrate causal effects of several floral traits on pathogen transmission and pollinator reproduction, indicating the importance of intraspecific plant trait variation for pollinator health and population dynamics.

Conservation measures or hotspots of disease transmission? Agri-environment schemes can reduce disease prevalence in pollinator communities

Insects are under pressure from agricultural intensification. To protect pollinators, conservation measures such as the EU agri-environment schemes (AES) promote planting wildflowers along fields. However, this can potentially alter disease ecology by serving as transmission hubs or by diluting infections. We tested this by measuring plant-pollinator interactions and virus infections (DWV-A, DWV-B and ABPV) across pollinator communities in agricultural landscapes over a year. AES had a direct effect on DWV-B, reducing prevalence and load in honeybees, with a tentative general dilution effect on load in early summer. DWV-A prevalence was reduced both under AES and with increasing niche overlap between competent hosts, likely via a dilution effect. By contrast, AES had no impact on ABPV, its prevalence driven by the proportion of bumblebees in the community. Epidemiological differences were also reflected in the virus phylogenies, with DWV-B showing recent rapid expansion, while DWV-A and ABPV showed slower growth rates and geographical population structure. Phylogenies indicate that all three viruses freely circulate across their host populations. Our study illustrates how complex interactions between environmental, ecological and evolutionary factors may influence wildlife disease dynamics. Supporting pollinator nutrition can mitigate the transmission of important bee diseases, providing an unexpected boost to pollinator conservation. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Meta-Analysis of the Effects of Insect Pathogens: Implications for Plant Reproduction

Despite extensive work on both insect disease and plant reproduction, there is little research on the intersection of the two. Insect-infecting pathogens could disrupt the pollination process by affecting pollinator population density or traits. Pathogens may also infect insect herbivores and change herbivory, potentially altering resource allocation to plant reproduction. We conducted a meta-analysis to (1) summarize the literature on the effects of pathogens on insect pollinators and herbivores and (2) quantify the extent to which pathogens affect insect traits, with potential repercussions for plant reproduction. We found 39 articles that fit our criteria for inclusion, extracting 218 measures of insect traits for 21 different insect species exposed to 25 different pathogens. We detected a negative effect of pathogen exposure on insect traits, which varied by host function: pathogens had a significant negative effect on insects that were herbivores or carried multiple functions but not on insects that solely functioned as pollinators. Particular pathogen types were heavily studied in certain insect orders, with 7 of 11 viral pathogen studies conducted in Lepidoptera and 5 of 9 fungal pathogen studies conducted in Hymenoptera. Our results suggest that most studies have focused on a small set of host-pathogen pairs. To understand the implications for plant reproduction, future work is needed to directly measure the effects of pathogens on pollinator effectiveness.

Sunflower-Associated Reductions in Varroa Mite Infestation of Honey Bee Colonies

Landscapes can affect parasite epidemiology in wild and agricultural animals. Honey bees are threatened by loss of floral resources and by parasites, principally the mite Varroa destructor and the viruses it vectors. Existing mite control relies heavily on chemical treatments that can adversely affect bees. Alternative, pesticide-free control methods are needed to mitigate infestation with these ectoparasites. Many flowering plants provide nectar and pollen that confer resistance to parasites. Enrichment of landscapes with antiparasitic floral resources could therefore provide a sustainable means of parasite control in pollinators. Floral rewards of Asteraceae plants can reduce parasitic infection in diverse bee species, including honey and bumble bees. Here, we tested the effects of sunflower (Helianthus annuus) cropland and pollen supplementation on honey bee resistance to macro- and microparasites. Although sunflower had nonsignificant effects on microparasites, We found that increased sunflower pollen availability correlated with reduced Varroa mite infestation in landscapes and pollen-supplemented colonies. At the landscape level, each doubling of sunflower crop area was associated with a 28% reduction in mite infestation. In field trials, late-summer supplementation of colonies with sunflower pollen reduced mite infestation by 2.75-fold relative to artificial pollen. United States sunflower crop acreage has declined by 2% per year since 1980, however, suggesting reduced availability of this floral resource. Although further research is needed to determine whether the observed effects represent direct inhibition of mite fecundity or mite-limiting reductions in honey bee brood-rearing, our findings suggest the potential for sunflower plantings or pollen supplements to counteract a major driver of honey bee losses worldwide.

Stingless Bee (Apidae: Apinae: Meliponini) Ecology

Stingless bees form perennial colonies of honey-making insects. The >600 species of stingless bees, mainly Neotropical, live throughout tropical latitudes. Foragers influence floral biology, plant reproduction, microbe dispersal, and diverse ecosystem functions. As tropical forest residents since the upper Cretaceous, they have had a long evolutionary history without competition from honey bees. Most stingless bees are smaller than any Apis species and recruit nest mates to resources, while their defense strategies exclude stinging behavior but incorporate biting. Stingless bees have diversified ecologically; excel in nesting site selection and mutualisms with plants, arthropods, and microbes; and display opportunism, including co-opting plant defenses. As their biology becomes better known, applications to human endeavors are imposing selective pressures from exploitation and approaches to conservation that entail colony extraction from wildlands. Although some meliponines can adjust to new conditions, their populations shall require tropical diversity for survival and reproduction.

Honeybees affect floral microbiome composition in a central food source for wild pollinators in boreal ecosystems

Basic knowledge on dispersal of microbes in pollinator networks is essential for plant, insect, and microbial ecology. Thorough understanding of the ecological consequences of honeybee farming on these complex plant-pollinator-microbe interactions is a prerequisite for sustainable honeybee keeping. Most research on plant-pollinator-microbe interactions have focused on temperate agricultural systems. Therefore, information on a wild plant that is a seasonal bottleneck for pollinators in cold climate such as Salix phylicifolia is of specific importance. We investigated how floral visitation by insects influences the community structure of bacteria and fungi in Salix phylicifolia inflorescences under natural conditions. Insect visitors were experimentally excluded with net bags. We analyzed the microbiome and measured pollen removal in open and bagged inflorescences in sites where honeybees were foraging and in sites without honeybees. Site and plant individual explained most of the variation in floral microbial communities. Insect visitation and honeybees had a smaller but significant effect on the community composition of microbes. Honeybees had a specific effect on the inflorescence microbiome and, e.g., increased the relative abundance of operational taxonomic units (OTUs) from the bacterial order Lactobacillales. Site had a significant effect on the amount of pollen removed from inflorescences but this was not due to honeybees. Insect visitors increased bacterial and especially fungal OTU richness in the inflorescences. Pollinator visits explained 38% variation in fungal richness, but only 10% in bacterial richness. Our work shows that honeybee farming affects the floral microbiome in a wild plant in rural boreal ecosystems.

Mutualism has its limits: consequences of asymmetric interactions between a well-defended plant and its herbivorous pollinator

Concern for pollinator health often focuses on social bees and their agricultural importance at the expense of other pollinators and their ecosystem services. When pollinating herbivores use the same plants as nectar sources and larval hosts, ecological conflicts emerge for both parties, as the pollinator's services are mitigated by herbivory and its larvae are harmed by plant defences. We tracked individual-level metrics of pollinator health-growth, survivorship, fecundity-across the life cycle of a pollinating herbivore, the common hawkmoth, Hyles lineata, interacting with a rare plant, Oenothera harringtonii, that is polymorphic for the common floral volatile (R)-(-)-linalool. Linalool had no impact on floral attraction, but its experimental addition suppressed oviposition on plants lacking linalool. Plants showed robust resistance against herbivory from leaf-disc to whole-plant scales, through poor larval growth and survivorship. Higher larval performance on other Oenothera species indicates that constitutive herbivore resistance by O. harringtonii is not a genus-wide trait. Leaf volatiles differed among populations of O. harringtonii but were not induced by larval herbivory. Similarly, elagitannins and other phenolics varied among plant tissues but were not herbivore-induced. Our findings highlight asymmetric plant-pollinator interactions and the importance of third parties, including alternative larval host plants, in maintaining pollinator health. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Potential effects of nectar microbes on pollinator health

Floral nectar is prone to colonization by nectar-adapted yeasts and bacteria via air-, rain-, and animal-mediated dispersal. Upon colonization, microbes can modify nectar chemical constituents that are plant-provisioned or impart their own through secretion of metabolic by-products or antibiotics into the nectar environment. Such modifications can have consequences for pollinator perception of nectar quality, as microbial metabolism can leave a distinct imprint on olfactory and gustatory cues that inform foraging decisions. Furthermore, direct interactions between pollinators and nectar microbes, as well as consumption of modified nectar, have the potential to affect pollinator health both positively and negatively. Here, we discuss and integrate recent findings from research on plant-microbe-pollinator interactions and their consequences for pollinator health. We then explore future avenues of research that could shed light on the myriad ways in which nectar microbes can affect pollinator health, including the taxonomic diversity of vertebrate and invertebrate pollinators that rely on this reward. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Flower sharing and pollinator health: a behavioural perspective

Disease is an integral part of any organisms' life, and bees have evolved immune responses and a suite of hygienic behaviours to keep them at bay in the nest. It is now evident that flowers are another transmission hub for pathogens and parasites, raising questions about adaptations that help pollinating insects stay healthy while visiting hundreds of plants over their lifetime. Drawing on recent advances in our understanding of how bees of varying size, dietary specialization and sociality differ in their foraging ranges, navigational strategies and floral resource preferences, we explore the behavioural mechanisms and strategies that may enable foraging bees to reduce disease exposure and transmission risks at flowers by partitioning overlapping resources in space and in time. By taking a novel behavioural perspective, we highlight the missing links between disease biology and the ecology of plant-pollinator relationships, critical for improving the understanding of disease transmission risks and the better design and management of habitat for pollinator conservation. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Complex networks of parasites and pollinators: moving towards a healthy balance

Parasites are viewed as a major threat to wild pollinator health. While this may be true for epidemics driven by parasite spillover from managed or invasive species, the picture is more complex for endemic parasites. Wild pollinator species host and share a species-rich, generalist parasite community. In contrast to the negative health impacts that these parasites impose on individual hosts, at a community level they may act to reduce competition from common and abundant pollinator species. By providing rare species with space in which to exist, this will act to support and maintain a diverse and thus healthier pollinator community. At this level, and perhaps paraxodically, parasites may be good for pollinators. This stands in clear contrast to the obvious negative impacts of epidemic and spillover parasites on wild pollinator communities. Research into floral resources that control parasites could be best employed to help design landscapes that provide pollinators with the opportunity to moderate their parasite community, rather than attempting to eliminate specific parasites from wild pollinator communities. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Floral shape predicts bee-parasite transmission potential

The spread of parasites is one of the primary drivers of population decline of both managed and wild bees. Several bee parasites are transmitted by the shared use of flowers, turning floral resources into potential disease hotspots. However, we know little about how floral morphology and floral species identity affect different steps of the transmission process. Here, we used the gut parasite Crithidia bombi and its primary host, bumble bees (Bombus spp.), to examine whether floral traits or species identity better predict three basic steps of parasite transmission on flowers: feces deposition on flowers, survival of the parasite on flowers, and acquisition by a new host. We also identified which traits and/or species were most strongly associated with each step in the transmission process. We found that both trait‐ and species‐based models fit the data on deposition of feces and survival of C. bombi on flowers, but that species‐based models provided a better fit compared with trait‐based ones. However, trait‐based models were better at predicting the acquisition of C. bombi on flowers. Although different species tended to support higher fecal deposition or parasite survival, we found that floral shape provided explanatory power for each of the transmission steps. When we assessed overall transmission potential, floral shape had the largest explanatory effect, with wider, shorter flowers promoting higher transmission. Taken together, our results highlight the importance of flower species identity and floral traits in disease transmission dynamics of bee parasites, and floral shape as an important predictor of overall transmission potential. Identifying traits associated with transmission potential may help us create seed mix that presents lower parasite transmission risk for bees for use in pollinator habitat.

Understanding effects of floral products on bee parasites: Mechanisms, synergism, and ecological complexity

Floral nectar and pollen commonly contain diverse secondary metabolites. While these compounds are classically thought to play a role in plant defense, recent research indicates that they may also reduce disease in pollinators. Given that parasites have been implicated in ongoing bee declines, this discovery has spurred interest in the potential for 'medicinal' floral products to aid in pollinator conservation efforts. We review the evidence for antiparasitic effects of floral products on bee diseases, emphasizing the importance of investigating the mechanism underlying antiparasitic effects, including direct or host-mediated effects. We discuss the high specificity of antiparasitic effects of even very similar compounds, and highlight the need to consider how nonadditive effects of multiple compounds, and the post-ingestion transformation of metabolites, mediate the disease-reducing capacity of floral products. While the bulk of research on antiparasitic effects of floral products on bee parasites has been conducted in the lab, we review evidence for the impact of such effects in the field, and highlight areas for future research at the floral product-bee disease interface. Such research has great potential both to enhance our understanding of the role of parasites in shaping plant-bee interactions, and the role of plants in determining bee-parasite dynamics. This understanding may in turn reveal new avenues for pollinator conservation.

Bowel Movement: Integrating Host Mobility and Microbial Transmission Across Host Taxa

The gut microbiota of animals displays a high degree of plasticity with respect to environmental or dietary adaptations and is shaped by factors like social interactions, diet diversity or the local environment. But the contribution of these drivers varies across host taxa and our ability to explain microbiome variability within wild populations remains limited. Terrestrial animals have divergent mobility ranges and can either crawl, walk or fly, from a couple of centimeters toward thousands of kilometers. Animal movement has been little regarded in host microbiota frameworks, though it can directly influence major drivers of the host microbiota: (1) Aggregation movement can enhance social transmissions, (2) foraging movement can extend range of diet diversity, and (3) dispersal movement determines the local environment of a host. Here, I would like to outline how movement behaviors of different host taxa matter for microbial acquisition across mammals, birds as well as insects. Host movement can have contrasting effects and either reduce or enlarge spatial scale. Increased dispersal movement could dissolve local effects of sampling location, while aggregation could enhance inter-host transmissions and uniformity among social groups. Host movement can also extend the boundaries of microbial dispersal limitations and connect habitat patches across plant-pollinator networks, while the microbiota of wild populations could converge toward a uniform pattern when mobility is interrupted in captivity or laboratory settings. Hence, the implementation of host movement would be a valuable addition to the metacommunity concept, to comprehend microbial dispersal within and across trophic levels.

Experimental cross species transmission of a major viral pathogen in bees is predominantly from honeybees to bumblebees

Cross-species transmission of a pathogen from a reservoir to a recipient host species, spillover, can have major impacts on biodiversity, domestic species and human health. Deformed wing virus (DWV) is a panzootic RNA virus in honeybees that is causal in their elevated colony losses, and several correlative field studies have suggested spillover of DWV from managed honeybees to wild bee species such as bumblebees. Yet unequivocal demonstration of DWV spillover is lacking, while spillback, the transmission of DWV from a recipient back to the reservoir host, is rarely considered. Here, we show in fully crossed laboratory experiments that the transmission of DWV (genotype A) from honeybees to bumblebees occurs readily, yet we neither detected viral transmission from bumblebees to honeybees nor onward transmission from experimentally infected to uninoculated bumblebees. Our results support the potential for viral spillover from honeybees to other bee species in the field when robbing resources from heterospecific nests or when visiting the same flowers. They also underscore the importance of studies on the virulence of DWV in wild bee species so as to evaluate viral impact on individual and population fitness as well as viral adaption to new host species.

Floral resources shape parasite and pathogen dynamics in bees facing urbanization

Urbanization is associated with increases in impervious land cover, which alters the distribution of resources available to wildlife and concentrates activity in un-built spaces such as parks and gardens. How resource shifts alter the dynamics of parasite and pathogen transmission has not been addressed for many important species in urban systems. We focus on urban gardens, resource-rich "islands" within the urban matrix, to examine how the availability of floral resources at local and landscape scales influences the prevalence of 6 RNA viruses and 3 parasites in honey bees and bumble bees. Because parasites and pathogens are transmitted at flowers between visitors, we expected that floral abundance would concentrate bees within gardens, amplifying infection rates in pollinators, unless increases in floral resources would enhance bee diversity enough to dilute transmission. We found that garden size and flowering perennial plant abundance had a positive, direct effect on parasite and pathogen richness in bumble bees, suggesting that resource provisioning amplifies transmission. We also found that parasitism rates in honey bees were positively associated with parasites and pathogens in bumble bees, suggesting spillover between species. Encouragingly, we found evidence that management may mitigate parasitism through indirect effects: garden size had a positive impact on bee diversity, which in-turn was negatively associated with parasite and pathogen richness in bumble bees. Unexpectedly, we observed that that parasite and pathogen richness in honey bees had no significant predictors, highlighting the complexity of comparing transmission dynamics between species. Although floral resources provide bees with food, we suggest more research on the tradeoffs between resource provisioning and disease transmission to implement conservation plantings in changing landscapes.

Honey bees and climate explain viral prevalence in wild bee communities on a continental scale

Viruses are omnipresent, yet the knowledge on drivers of viral prevalence in wild host populations is often limited. Biotic factors, such as sympatric managed host species, as well as abiotic factors, such as climatic variables, are likely to impact viral prevalence. Managed and wild bees, which harbor several multi-host viruses with a mostly fecal-oral between-species transmission route, provide an excellent system with which to test for the impact of biotic and abiotic factors on viral prevalence in wild host populations. Here we show on a continental scale that the prevalence of three broad host viruses: the AKI-complex (Acute bee paralysis virus, Kashmir bee virus and Israeli acute paralysis virus), Deformed wing virus, and Slow bee paralysis virus in wild bee populations (bumble bees and solitary bees) is positively related to viral prevalence of sympatric honey bees as well as being impacted by climatic variables. The former highlights the need for good beekeeping practices, including Varroa destructor management to reduce honey bee viral infection and hive placement. Furthermore, we found that viral prevalence in wild bees is at its lowest at the extreme ends of both temperature and precipitation ranges. Under predicted climate change, the frequency of extremes in precipitation and temperature will continue to increase and may hence impact viral prevalence in wild bee communities.

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.

The gut microbiota of bumblebees

Bumblebees (Bombus) are charismatic and important pollinators. They are one of the best studied insect groups, especially in terms of ecology, behavior, and social structure. As many species are declining, there is a clear need to understand more about them. Microbial symbionts, which can influence many dimensions of animal life, likely have an outsized role in bumblebee biology. Recent research has shown that a conserved set of beneficial gut bacterial symbionts is ubiquitous across bumblebees. These bacteria are related to gut symbionts of honeybees, but have not been studied as intensively. Here we synthesize studies of bumblebee gut microbiota, highlight major knowledge gaps, and suggest future directions. Several patterns emerge, such as symbiont-host specificity maintained by sociality, frequent symbiont loss from individual bees, symbiont-conferred protection from trypanosomatid parasites, and divergence between bumblebee and honeybee microbiota in several key traits. For many facets of bumblebee-microbe interactions, however, underlying mechanisms and ecological functions remain unclear. Such information is important if we are to understand how bumblebees shape, and are shaped by, their gut microbiota. Bumblebees may provide a useful system for microbiome scientists, providing insights into general principles of host-microbe interactions. We also note how microbiota could influence bumblebee traits and responses to stressors. Finally, we propose that tinkering with the microbiota could be one way to aid bumblebee resilience in the face of global change.

Honey bee viruses are highly prevalent but at low intensities in wild pollinators of cucurbit agroecosystems

Managed and wild bee populations are in decline around the globe due to several biotic and abiotic stressors. Pathogenic viruses associated with the Western honey bee (Apis mellifera) have been identified as key contributors to losses of managed honey bee colonies, and are known to be transmitted to wild bee populations through shared floral resources. However, little is known about the prevalence and intensity of these viruses in wild bee populations, or how bee visitation to flowers impacts viral transmission in agroecosystems. This study surveyed honey bee, bumble bee (Bombus impatiens) and wild squash bee (Eucera (Peponapis) pruinosa) populations in Cucurbita agroecosystems across Pennsylvania (USA) for the prevalence and intensity of five honey bee viruses: acute bee paralysis virus (ABPV), deformed wing virus (DWV), Israeli acute paralysis virus (IAPV), Kashmir bee virus (KBV), and slow bee paralysis virus (SBPV). We investigated the potential role of bee visitation rate to flowers on DWV intensity among species in the pollinator community, with the expectation that increased bee visitation to flowers would increase the opportunity for transmission events between host species. We found that honey bee viruses are highly prevalent but in lower titers in wild E. pruinosa and B. impatiens than in A. mellifera populations throughout Pennsylvania (USA). DWV was detected in 88% of B. impatiens, 48% of E. pruinosa, and 95% of A. mellifera. IAPV was detected in 5% of B. impatiens and 4% of E. pruinosa, compared to 9% in A. mellifera. KBV was detected in 1% of B. impatiens and 5% of E. pruinosa, compared to 32% in A. mellifera. Our results indicate that DWV titers are not correlated with bee visitation in Cucurbita fields. The potential fitness impacts of these low viral titers detected in E. pruinosa remain to be investigated.

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.

Big bees spread disease: body size mediates transmission of a bumble bee pathogen

Trait variation can have important consequences for the outcomes of species interactions. Even though some traits vary as much within species as across related species, models and empirical studies typically do not consider the role of intraspecific trait variation for processes such as disease transmission. For example, many pollinator species are in decline because of a variety of stressors including pathogens, but the role of intraspecific trait variation in mediating disease dynamics is rarely considered. For example, pollinator body size could affect pathogen transmission via differences in resistance, foraging behavior and physiology. We tested effects of body size on pollinator pathogen transmission using the common eastern bumble bee Bombus impatiens in field tents, introducing an infected "donor" microcolony of large or small workers with an uninfected average-sized "recipient" microcolony and allowing bees to forage for 9-16 d. Small donor bees had nearly 50% higher infection intensity (cells/0.02  μL) than large donor bees, but large donor bees were twice as likely to transmit Crithidia bombi to recipient bees. Both behavioral and physiological mechanisms may underlie this apparent paradox. Compared to small bees, large bees foraged more and produced more feces; simulations showed that foraging and defecation rates together had stronger effects on transmission than did donor infection intensity. Thus, effects of bee size on contact rates and pathogen supply may play significant roles in disease transmission, demonstrating the multifaceted impacts of traits on transmission dynamics.

Evaluating competition for forage plants between honey bees and wild bees in Denmark

A recurrent concern in nature conservation is the potential competition for forage plants between wild bees and managed honey bees. Specifically, that the highly sophisticated system of recruitment and large perennial colonies of honey bees quickly exhaust forage resources leading to the local extirpation of wild bees. However, different species of bees show different preferences for forage plants. We here summarize known forage plants for honey bees and wild bee species at national scale in Denmark. Our focus is on floral resources shared by honey bees and wild bees, with an emphasis on both threatened wild bee species and foraging specialist species. Across all 292 known bee species from Denmark, a total of 410 plant genera were recorded as forage plants. These included 294 plant genera visited by honey bees and 292 plant genera visited by different species of wild bees. Honey bees and wild bees share 176 plant genera in Denmark. Comparing the pairwise niche overlap for individual bee species, no significant relationship was found between their overlap and forage specialization or conservation status. Network analysis of the bee-plant interactions placed honey bees aside from most other bee species, specifically the module containing the honey bee had fewer links to any other modules, while the remaining modules were more highly inter-connected. Despite the lack of predictive relationship from the pairwise niche overlap, data for individual species could be summarized. Consequently, we have identified a set of operational parameters that, based on a high foraging overlap (>70%) and unfavorable conservation status (Vulnerable+Endangered+Critically Endangered), can guide both conservation actions and land management decisions in proximity to known or suspected populations of these species.

Functional traits linked to pathogen prevalence in wild bee communities

Reports of pollinator declines have prompted efforts to understand contributing factors and protect vulnerable species. While pathogens can be widespread in bee communities, less is known about factors shaping pathogen prevalence among species. Functional traits are often used to predict susceptibility to stressors, including pathogens, in other species-rich communities. Here, we evaluated the relationship between bee functional traits (body size, phenology, nesting location, sociality, and foraging choice) and prevalence of trypanosomes, neogregarines, and the microsporidian Nosema ceranae in wild bee communities. For the most abundant bee species in our system, Bombus impatiens, we also evaluated the relationship between intra-specific size variation and pathogen prevalence. A trait-based model fit the neogregarine prevalence data better than a taxa-based model, while the taxonomic model provided a better model fit for N. ceranae prevalence, and there was no marked difference between the models for trypanosome prevalence. We found that Augochlorella aurata was more likely to harbor trypanosomes than many other bee taxa. Similarly, we found that bigger bees and those with peak activity later in the season were less likely to harbor trypanosomes, though the effect of size was largely driven by A. aurata. We found no clear intra-specific size patterns for pathogen prevalence in B. impatiens. These results indicate that functional traits are not always better than taxonomic affinity in predicting pathogen prevalence, but can help to explain prevalence depending on the pathogen in species-rich bee communities.

Floral traits affecting the transmission of beneficial and pathogenic pollinator-associated microbes

Flowers provide resources for pollinators, and can also be transmission venues for beneficial or pathogenic pollinator-associated microbes. Floral traits could mediate transmission similarly for beneficial and pathogenic microbes, although some beneficial microbes can grow in flowers while pathogenic microbes may only survive until acquired by a new host. In spite of conceptual similarities, research on beneficial and pathogenic pollinator-associated microbes has progressed mostly independently. Recent advances demonstrate that floral traits are associated with transmission of beneficial and pathogenic microbes, with consequences for pollinator populations and communities. However, there is a near-absence of experimental manipulations of floral traits to determine causal effects on transmission, and a need to understand how floral, microbe and host traits interact to mediate transmission.

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 5–11 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 5–11-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.

More is less: mass-flowering fruit tree crops dilute parasite transmission between bees

Parasites influence wild bee population dynamics and are regarded as one of the main drivers of wild bee decline. Most of these parasites are mainly transmitted between bee species via the use of shared floral resources. Disturbance of the plant-pollinator network at a location can hence disturb the transmission of these parasites. Expansion and intensification of agriculture, another major driver of wild bee decline, often disturbs local plant-pollinator networks by altering the availability and diversity of floral resources. Mass-flowering crops are an extreme example as they provide an abundance of floral resources for a short period of time, substantially altering the present plant-pollinator network. This likely has repercussions on parasite transmission in the pollinator community. Using the bloom of mass-flowering crops we tested the hypothesis that an increase in floral resources can dilute parasite transmission in the pollinator community. To test this, we analysed the presence of parasites in the pollen of the brood cell provisions of Osmia spp., collected from trap nests placed in apple and sweet cherry orchards. We collected pollen at several time intervals during and after mass bloom, and found that pollen collected during mass bloom had significantly lower parasite prevalence compared with pollen collected after mass bloom. Furthermore, using pollen barcoding data we found that the presence of MFCs in pollen was a good predictor for lower parasite prevalence. Taken together, our results indicate that an increase in flower availability can reduce parasite transmission between bees.

Detection of deformed wing virus (DWV) in the Vietnamese walking stick Medauroidea extradentata (Phasmatodea)

Deformed wing virus (DWV) is a single-stranded positive sense RNA virus that mainly infects honey bees (Apis mellifera) and can have devastating impacts on the colony. Recent studies have shown the presence of this virus in several species of Apis spp. and some other Hymenoptera, but our knowledge of their host range is very limited. We screened previously sequenced RNAseq libraries from different tissues of Vietnamese Walking Stick, Medauroidea extradentata (Phasmatodea) for DWV. We only found this virus in six libraries from anterior and posterior midgut tissue. From the midgut libraries we were able to construct a complete DWV genome sequence, which consisted of 10,140 nucleotides and included one open reading frame. Pairwise genome comparison confirmed strong similarity (98.89 %) of these assembled sequences with only 113 SNPs to the original DWV genome. We hypothesize the M. extradentata acquired this virus via a foodborne transmission by consuming DWV-infected material such as pollen or leaves contaminated with virus infected bee faeces.

Network Centrality as an Indicator for Pollinator Parasite Transmission via Flowers

Parasites are important actors within ecosystems. However, a key aspect to unraveling parasite epidemiology is understanding transmission. The bee pollinator community harbors several multihost parasites, which have been shown to be able to spread between species via flowers. Hence the plant-pollinator network can provide valuable information on the transmission of these parasites between species. Although several controlled experiments have shown that flowers function as a transmission hub for parasites, the link with the plant-pollinator network has rarely been addressed in the field. Here, one can hypothesize that the most central flowers in the network are more likely to enable parasite transmission between species. In this study, we test this hypothesis in three local plant-pollinator networks and show that the centrality of a plant in a weighted plant-pollinator network is a good predictor of the presence of multihost pollinator parasites on the plant's flowers.