Dominant bee species and floral abundance drive parasite temporal dynamics in plant-pollinator communities

Landscape structure affects temporal dynamics in the bumble bee virome: Landscape heterogeneity supports colony resilience

Virus spillovers from managed honey bees, Apis mellifera, are thought to contribute to the decline of wild pollinators, including bumble bees. However, data on the impact of such viruses on wild pollinators remain scarce, and the influence of landscape structure on virus dynamics is poorly understood. In this study, we deployed bumble bee colonies in an agricultural landscape and studied changes in the bumble bee virome during field placement under varying habitat composition and configuration using a multiscale analytical framework. We estimated prevalence of viruses and viral loads (i.e. number of viral genomic equivalent copies) in bumble bees before and after placing them in the field using next generation sequencing and quantitative PCR. The results show that viral loads and number of different viruses present increased during placement in the field and that the virus composition of the colonies shifted from an initial dominance of honey bee associated viruses to a higher number (in both viral loads and number of viruses present) of bumble bee associated viruses. Especially DWV-B, typical for honey bees, drastically decreased after the time in the field. Viral loads prior to placing colonies in the field showed no effect on colony development, suggesting low impacts of these viruses in field settings. Notably, we further demonstrate that increased habitat diversity results in a lower number of different viruses present in Bombus colonies, while colonies in areas with well-connected farmland patches decreased in their total viral load after field placement. Our results emphasize the importance of landscape heterogeneity and connectivity for wild pollinator health and that these influences predominate at fine spatial scales.

Species traits, landscape quality and floral resource overlap with honeybees determine virus transmission in plant-pollinator networks

Emerging infectious diseases pose a threat to pollinators. Virus transmission among pollinators via flowers may be reinforced by anthropogenic land-use change and concomitant alteration of plant-pollinator interactions. Here, we examine how species' traits and roles in flower-visitation networks and landscape-scale factors drive key honeybee viruses-black queen cell virus (BQCV) and deformed wing virus-in 19 wild bee and hoverfly species, across 12 landscapes varying in pollinator-friendly (flower-rich) habitat. Viral loads were on average more than ten times higher in managed honeybees than in wild pollinators. Viral loads in wild pollinators were higher when floral resource use overlapped with honeybees, suggesting these as reservoir hosts, and increased with pollinator abundance and viral loads in honeybees. Viral prevalence decreased with the amount of pollinator-friendly habitat in a landscape, which was partly driven by reduced floral resource overlap with honeybees. Black queen cell virus loads decreased with a wild pollinator's centrality in the network and the proportion of visited dish-shaped flowers. Our findings highlight the complex interplay of resource overlap with honeybees, species traits and roles in flower-visitation networks and flower-rich pollinator habitat shaping virus transmission.

Prevalence and population genetic analyses of parasites in invasive Vespa velutina and native Hymenoptera

Invasive species pose a threat to the ecological balance of the ecosystems they invade by altering local host-pathogen dynamics. To investigate these relationships and their potential consequences, we examined the prevalence and genetic diversity patterns of Trypanosomatidae, Lipotrophidae, and Nosematidae in a collection of sympatric isolates of the invasive hornet Vespa velutina and local Hymenoptera from two recently colonized areas: Europe and South Korea. Data were gathered through PCR amplification and massive parallel sequencing, and analyses were conducted using population genetics tools. Parasite prevalences showed substantial variation depending on (i) the parasite family (Trypanosomatidae and Nosematidae were the most and less prevalent, respectively), (ii) location (e.g. Galicia displayed the highest pooled values), (iii) the season (highest in spring for Trypanosomatidae and Lipotrophidae), and (iv) the host. V. velutina exhibited significantly lower parasite occurrence than native Hymenoptera across all parasite families (consistent with the enemy release hypothesis), although this difference was less pronounced during the periods of heightened predatory activity, suggestive of trophic transmission. Parasite species displayed significant genetic differentiation between European and South Korean isolates, yet no differentiation was observed across hosts, suggesting that all Hymenoptera are exposed to a common local pathogen population. There was no indication that V. velutina acted as a carrier of foreign parasites to the invaded territories.

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.

Nesting biology and nest structure of the exotic bee Megachile sculpturalis

From the 1990s, the Southeast Asia native giant resin bee Megachile sculpturalis (Smith, 1853) was introduced first to North America, and then to many countries in Europe. Despite increasing studies on its invasive potential and geographical expansion, information on nesting behaviour of this species is still extremely scarce. To increase knowledge on the nesting biology of M. sculpturalis, we studied multiple aspects of nesting and pollen provisioning in three consecutive years in artificial nests in Bologna, Italy. We observed 166 bees visiting nests, and followed individual nesting behaviour and success of 41 adult females. We measured cavity diameter in 552 nests and characterised the structure in 100 of them. More than 95% of nest diameters ranged between 0.6 and 1.2 cm, overlapping with several sympatric species of cavity-nesting hymenopterans in the study area. Most nests had a first chamber from the entrance of variable length without brood, followed by an average of about two brood cells with a mean length of 2.85 ± 0.13 cm each. The pollen stored in brood cells was almost monofloral, belonging to the ornamental plant Styphnolobium japonicum (L.) Schott. We estimated that a single female should visit ≈180 flowers to collect enough pollen for a single brood cell. These results fill knowledge gaps on the nesting biology and nest structure of the exotic M. sculpturalis, and they are discussed in relation to possible competition with native bees for nesting sites and foraging resources.

Identification of fungi isolated from commercial bumblebee colonies

Fungi can have important beneficial and detrimental effects on animals, yet our understanding of the diversity and function of most bee-associated fungi is poor. Over 2 million bumblebee colonies are traded globally every year, but the presence and transport of viable fungi within them is unknown. Here, we explored whether any culturable fungi could be isolated from commercial bumblebee nests. We collected samples of various substrates from within 14 bumblebee colonies, including the honey, honey cup wall, egg cup wall, and frass then placed them on agar and recorded any growth. Fungal morphotypes were then subcultured and their ITS region sequenced for identification. Overall, we cultured 11 fungal species from the various nest substrates. These included both pathogenic and non-pathogenic fungi, such as Aspergillus sp., Penicillium sp., and Candida sp. Our results provide the first insights into the diversity of viable fungal communities in commercial bumblebee nests. Further research is needed to determine if these fungi are unique to commercial colonies or prevalent in wild bumblebee nests, and crucially to determine the ecological and evolutionary implications of these fungi in host colonies.

Using individual-based trait frequency distributions to forecast plant-pollinator network responses to environmental change

Determining how and why organisms interact is fundamental to understanding ecosystem responses to future environmental change. To assess the impact on plant-pollinator interactions, recent studies have examined how the effects of environmental change on individual interactions accumulate to generate species-level responses. Here, we review recent developments in using plant-pollinator networks of interacting individuals along with their functional traits, where individuals are nested within species nodes. We highlight how these individual-level, trait-based networks connect intraspecific trait variation (as frequency distributions of multiple traits) with dynamic responses within plant-pollinator communities. This approach can better explain interaction plasticity, and changes to interaction probabilities and network structure over spatiotemporal or other environmental gradients. We argue that only through appreciating such trait-based interaction plasticity can we accurately forecast the potential vulnerability of interactions to future environmental change. We follow this with general guidance on how future studies can collect and analyse high-resolution interaction and trait data, with the hope of improving predictions of future plant-pollinator network responses for targeted and effective conservation.

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.

Bees just wanna have fungi: a review of bee associations with nonpathogenic fungi

Bee-fungus associations are common, and while most studies focus on entomopathogens, emerging evidence suggests that bees associate with a variety of symbiotic fungi that can influence bee behavior and health. Here, we review nonpathogenic fungal taxa associated with different bee species and bee-related habitats. We synthesize results of studies examining fungal effects on bee behavior, development, survival, and fitness. We find that fungal communities differ across habitats, with some groups restricted mostly to flowers (Metschnikowia), while others are present almost exclusively in stored provisions (Zygosaccharomyces). Starmerella yeasts are found in multiple habitats in association with many bee species. Bee species differ widely in the abundance and identity of fungi hosted. Functional studies suggest that yeasts affect bee foraging, development, and pathogen interactions, though few bee and fungal taxa have been examined in this context. Rarely, fungi are obligately beneficial symbionts of bees, whereas most are facultative bee associates with unknown or ecologically contextual effects. Fungicides can reduce fungal abundance and alter fungal communities associated with bees, potentially disrupting bee-fungi associations. We recommend that future study focus on fungi associated with non-honeybee species and examine multiple bee life stages to document fungal composition, abundance, and mechanistic effects on bees.

Urban habitat fragmentation and floral resources shape the occurrence of gut parasites in two bumblebee species

Urbanization and the expansion of human activities foster radical ecosystem changes with cascading effects also involving host-pathogen interactions. Urban pollinator insects face several stressors related to landscape and local scale features such as green habitat loss, fragmentation and availability reduction of floral resources with unpredictable effects on parasite transmission. Furthermore, beekeeping may contribute to the spread of parasites to wild pollinators by increasing the number of parasite hosts. Here we used DNA-based diagnostics tools to evaluate how the occurrence of parasites, namely microsporidians (Nosema spp.), trypanosomatids (Crithidia spp.) and neogregarines (Apicystis bombi), is shaped by the above-mentioned stressors in two bumblebee species (i.e. Bombus terrestris and Bombus pascuorum). Infection rates of the two species were different and generally higher in B. terrestris. Moreover, they showed different responses towards the same ecological variables, possibly due to differences in body size and foraging habits supposed to affect their susceptibility to parasite infection. The probability of infection was found to be reduced in B. pascuorum by green habitat fragmentation, while increased along with floral resource availability. Unexpectedly, B. terrestris had a lower parasite richness nearby apiaries maybe due to the fact that parasites are prone to be transmitted among the most abundant species. Our finding supports the need to design proper conservation measures based on species-specific knowledge, as suggested by the variation in the parasite occurrence of the two species. Moreover, conservation policies aiming at safeguarding pollinators through flower planting should consider the indirect effects of these measures for parasite transmission together with pollinator biodiversity issues.

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.

Sunflower plantings reduce a common gut pathogen and increase queen production in common eastern bumblebee colonies

Community diversity can reduce the prevalence and spread of disease, but certain species may play a disproportionate role in diluting or amplifying pathogens. Flowers act as both sources of nutrition and sites of pathogen transmission, but the effects of specific plant species in shaping bee disease dynamics are not well understood. We evaluated whether plantings of sunflower (Helianthus annuus), whose pollen reduces infection by some pathogens when fed to bees in captivity, lowered pathogen levels and increased reproduction in free-foraging bumblebee colonies (Bombus impatiens). Sunflower abundance reduced the prevalence of a common gut pathogen, Crithidia bombi, and reduced infection intensity, with an order of magnitude lower infection intensity at high sunflower sites compared with sites with little to no sunflower. Sunflower abundance was also positively associated with greater queen production in colonies. Sunflower did not affect prevalence of other detected pathogens. This work demonstrates that a single plant species can drive disease dynamics in foraging B. impatiens, and that sunflower plantings can be used as a tool for mitigating a prevalent pathogen while also increasing reproduction of an agriculturally important bee species.

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.

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'.

The pollen virome: A review of pollen-associated viruses and consequences for plants and their interactions with pollinators

The movement of pollen grains from anthers to stigmas, often by insect pollinator vectors, is essential for plant reproduction. However, pollen is also a unique vehicle for viral spread. Pollen-associated plant viruses reside on the outside or inside of pollen grains, infect susceptible individuals through vertical or horizontal infection pathways, and can decrease plant fitness. These viruses are transferred with pollen between plants by pollinator vectors as they forage for floral resources; thus, pollen-associated viral spread is mediated by floral and pollen grain phenotypes and pollinator traits, much like pollination. Most of what is currently known about pollen-associated viruses was discovered through infection and transmission experiments in controlled settings, usually involving one virus and one plant species of agricultural or horticultural interest. In this review, we first provide an updated, comprehensive list of the recognized pollen-associated viruses. Then, we summarize virus, plant, pollinator vector, and landscape traits that can affect pollen-associated virus transmission, infection, and distribution. Next, we highlight the consequences of plant-pollinator-virus interactions that emerge in complex communities of co-flowering plants and pollinator vectors, such as pollen-associated virus spread between plant species and viral jumps from plant to pollinator hosts. We conclude by emphasizing the need for collaborative research that bridges pollen biology, virology, and pollination biology.

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.

Squash bees host high diversity and prevalence of parasites in the northeastern United States

The squash bee Eucera (Peponapis) pruinosa is emerging as a model species to study how stressors impact solitary wild bees in North America. Here, we describe the prevalence of trypanosomes, microsporidians and mollicute bacteria in E. pruinosa and two other species, Bombus impatiens and Apis mellifera, that together comprise over 97% of the pollinator visitors of Cucurbita agroecosystems in Pennsylvania (United States). Our results indicate that all three parasite groups are commonly detected in these bee species, but E. pruinosa often exhibit higher prevalences. We further describe novel trypanosome parasites detected in E. pruinosa, however it is unknown how these parasites impact these bees. We suggest future work investigates parasite replication and infection outcomes.

First Detection of Honeybee Pathogenic Viruses in Butterflies

Several pathogens are important causes of the observed pollinator decline, some of which could be transmitted between different pollinator species. To determine whether honeybee viruses can be transmitted to butterflies, a total of 120 butterflies were sampled at four locations in Slovenia. At each location, butterflies from three families (Pieridae, Nymphalidae, Hesperiidae/Lycenidae) and Carniolan honeybees (Apis mellifera carnica) were collected. The RNA of six honeybee viruses, i.e., acute bee paralysis virus (ABPV), black queen cell virus (BQCV), chronic bee paralysis virus (CBPV), deformed wing virus A (DWV-A), Sacbrood bee virus (SBV), and Lake Sinai virus 3 (LSV3), was detected by a specific quantitative method (RT-PCR). The presence of ABPV, BQCV, LSV3, and SBV was detected in both butterflies and honeybees. All butterfly and bee samples were negative for CBPV, while DWV-A was detected only in honeybees. The viral load in the positive butterfly samples was much lower than in the positive bee samples, which could indicate that butterflies are passive carriers of bee viruses. The percentage of positive butterfly samples was higher when the butterflies were collected at sampling sites with a higher density of apiaries. Therefore, we believe that infected bees are a necessary condition for the presence of viruses in cohabiting butterflies. This is the first study on the presence of pathogenic bee viruses in butterflies.

Increasing flower species richness in agricultural landscapes alters insect pollinator networks: Implications for bee health and competition

Ecological restoration programs are established to reverse land degradation, mitigate biodiversity loss, and reinstate ecosystem services. Following recent agricultural intensification that led to a decrease in flower diversity and density in rural areas and subsequently to the decline of many insects, conservation measures targeted at pollinators have been established, including sown wildflower strips (WFS) along field margins. Historically successful in establishing a high density of generalist bees and increasing pollinator diversity, the impact of enhanced flower provision on wider ecological interactions and the structure of pollinator networks has been rarely investigated. Here, we tested the effects of increasing flower species richness and flower density in agricultural landscapes on bee-plant interaction networks. We measured plant species richness and flower density and surveyed honeybee and bumblebee visits on flowers across a range of field margins on 10 UK farms that applied different pollinator conservation measures. We found that both flower species richness and flower density significantly increased bee abundance, in early and late summer, respectively. At the network level, we found that higher flower species richness did not significantly alter bee species' generality indices, but significantly reduced network connectance and marginally reduced niche overlap across honeybees and bumblebee species, a proxy for insect competition. While higher connectance and niche overlap is believed to strengthen network robustness and often is the aim for the restoration of pollinator networks, we argue that carefully designed WFS may benefit bees by partitioning their foraging niche, limiting competition for resources and the potential for disease transmission via shared floral use. We also discuss the need to extend WFS and their positive effects into spring when wild bee populations are established.

"Migratory beekeeping and its influence on the prevalence and dispersal of pathogens to managed and wild bees"

Demand for food is growing along with the human population, leading to an increase in plant production. Many crops are pollinated by insects, so the global demand for managed pollinators is also increasing. The honey bee has traditionally been considered the main provider of crop pollination services. For providing it beekeepers seasonally transport hives to different locations after the flowering of different crops. These movements could be detrimental to pollinators by: i) stressing honey bees, making them more susceptible to pathogens and parasites; ii) spreading bee parasites and pathogens across locations; iii) increasing the transmission of parasites and pathogens between managed and wild pollinators and vice versa (spillover and spillback, respectively). To understand the impact of migratory beekeeping on bee health, we conducted a systematic review to identify the main trends and provide a complete picture of existing knowledge on the subject. We found 52 studies analysing pathogen-related impacts of migratory beekeeping on honey bees. However, only 16 investigations tested the effect of migratory practices on the prevalence and spread of pathogens and parasites. We found no studies that assessed the impact of migratory beekeeping on the occurrence and spread of pests and diseases in wild bees. In general, migratory beekeeping tends to increase the prevalence of pathogens and parasites in honey bee colonies. However, the results were very heterogeneous, probably due to several uncontrolled underlying factors such as management, biological and geographical factors, and the interactions between them. In conclusion, there is an urgent need for studies to assess the impact of migratory beekeeping on bee health, given the current global bee decline and the expected increase in migratory beekeeping due to climate change and crop pollination demand.

Do pesticide and pathogen interactions drive wild bee declines?

There is clear evidence for wild insect declines globally. Habitat loss, climate change, pests, pathogens and environmental pollution have all been shown to cause detrimental effects on insects. However, interactive effects between these stressors may be the key to understanding reported declines. Here, we review the literature on pesticide and pathogen interactions for wild bees, identify knowledge gaps, and suggest avenues for future research fostering mitigation of the observed declines. The limited studies available suggest that effects of pesticides most likely override effects of pathogens. Bees feeding on flowers and building sheltered nests, are likely less adapted to toxins compared to other insects, which potential susceptibility is enhanced by the reduced number of genes encoding detoxifying enzymes compared with other insect species. However, to date all 10 studies using a fully-crossed design have been conducted in the laboratory on social bees using Crithidia spp. or Nosema spp., identifying an urgent need to test solitary bees and other pathogens. Similarly, since laboratory studies do not necessarily reflect field conditions, semi-field and field studies are essential if we are to understand these interactions and their potential effects in the real-world. In conclusion, there is a clear need for empirical (semi-)field studies on a range of pesticides, pathogens, and insect species to better understand the pathways and mechanisms underlying their potential interactions, in particular their relevance for insect fitness and population dynamics. Such data are indispensable to drive forward robust modelling of interactive effects in different environmental settings and foster predictive science. This will enable pesticide and pathogen interactions to be put into the context of other stressors more broadly, evaluating their relative importance in driving the observed declines of wild bees and other insects. Ultimately, this will enable the development of more effective mitigation measures to protect bees and the ecosystem services they supply.

The impact of mass-flowering crops on bee pathogen dynamics

Nearly two fifths of the Earth's land area is currently used for agriculture, substantially impacting the environment and ecosystems. Besides the direct impact through land use change, intensive agriculture can also have an indirect impact, for example by changing wildlife epidemiology. We review here the potential effects of mass-flowering crops (MFCs), which are rapidly expanding in global cropping area, on the epidemiology of known pathogens in bee pollinators. We bring together the fifty MFCs with largest global area harvested and give an overview of their pollination dependency as well as their impact on bee pollinators. When in bloom these crops provide an abundance of flowers, which can provide nutrition for bees and increase bee reproduction. After their short bloom peak, however, the fields turn into green deserts. These big changes in floral availability strongly affect the plant-pollinator network, which in turn affects the pathogen transmission network, mediated by shared flowers. We address this dual role of flowers provided by MFCs, serving as nutritional resources as well as pathogen transmission spots, and bring together the current knowledge to assess how MFCs could affect pathogen prevalence in bee pollinator communities.

Detection of Microsporidia in Pollinator Communities of a Mediterranean Biodiversity Hotspot for Wild Bees

Insect pollination is crucial for the maintenance of natural and managed ecosystems but the functioning of this ecosystem service is threatened by a worldwide decline of pollinators. Key factors in this situation include the spread and interspecific transmission of pathogens worldwide through the movement of managed pollinators. Research on this field has been mainly conducted in some particular species, while studies assessing the interspecific transmission of pathogens at a community level are scarce. However, this information is pivotal to design strategies to protect pollinators. Herein, we analysed the prevalence of two common microsporidia pathogens of managed honey bees (Nosema ceranae and N. apis) in bee communities of semiarid Mediterranean areas from the Southeast of the Iberian Peninsula. Our results confirm the ability of N. ceranae to disperse across wild bee communities in semiarid Mediterranean ecosystems since it was detected in 36 Apoidea species (39% of the sampling; for the first time in nine genera). The prevalence of the pathogen did not show any phylogenetic signal which suggests a superfamily host range of the pathogen or that wild bees may be acting only as vectors of N. ceranae. In addition, N. apis was detected in an Eucera species, which is the second time it has been detected by molecular techniques in a host other than the honey bee. Our study represents the primary assessment of the prevalence of microsporidia at community level in Mediterranean areas and provides outstanding results on the ability of Nosema pathogens to spread across the landscape.

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'.

Effects of planted pollinator habitat on pathogen prevalence and interspecific detection between bee species

Shared resources can instigate pathogen spread due to large congregations of individuals in both natural and human modified resources. Of current concern is the addition of pollinator habitat in conservation efforts as it attracts bees of various species, potentially instigating interspecific sharing of pathogens. Common pathogens have been documented across a wide variety of pollinators with shared floral resources instigating their spread in some, but not all, cases. To evaluate the impact of augmented pollinator habitat on pathogen prevalence, we extracted RNA from samples of eight bee species across three families and screened these samples for nine pathogens using RT-qPCR. We found that some habitat characteristics influenced pathogen detection; however, we found no evidence that pathogen detection in one bee species was correlated with pathogen detection in another. In fact, pathogen detection was rare in wild bees. While gut parasites were detected in 6 out of the 8 species included in this study, viruses were only detected in honey bees. Further, virus detection in honey bees was low with a maximum 21% of samples testing positive for BQCV, for example. These findings suggest factors other than the habitat itself may be more critical in the dissemination of pathogens among bee species. However, we found high relative prevalence and copy number of gut parasites in some bee species which may be of concern, such as Bombus pensylvanicus. Long-term monitoring of pathogens in different bee species at augmented pollinator habitat is needed to evaluate if these patterns will change over time.

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.

The threat of pesticide and disease co-exposure to managed and wild bee larvae

Brood diseases and pesticides can reduce the survival of bee larvae, reduce bee populations, and negatively influence ecosystem biodiversity. However, major gaps persist in our knowledge regarding the routes and implications of co-exposure to these stressors in managed and wild bee brood. In this review, we evaluate the likelihood for co-exposure to brood pathogen and pesticide stressors by examining the routes of potential co-exposure and the possibility for pollen and nectar contaminated with pathogens and pesticides to become integrated into brood food. Furthermore, we highlight ways in which pesticides may increase brood disease morbidity directly, through manipulating host immunity, and indirectly through disrupting microbial communities in the guts of larvae, or compromising brood care provided by adult bees. Lastly, we quantify the brood research bias towards Apis species and discuss the implications the bias has on brood disease and pesticide risk assessment in wild bee communities. We advise that future studies should place a higher emphasis on evaluating bee brood afflictions and their interactions with commonly encountered stressors, especially in wild bee species.

•

Brood exposure to pathogens and pesticides may occur frequently and in combination during the consumption of pollen and nectar.

•

Brood pathogen virulence can be directly increased due to pesticide-mediated manipulation of larvae immune responses.

•

Pesticides may indirectly increase brood disease morbidity by affecting larval gut microbial compositionand adult bee health.

•

Research bias towards Apis species skews our understanding and management of brood disease and pesticide risks in wild bees.

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.

Effects of an alternative host on the prevalence and intensity of infection of a bumble bee parasite

Several bee parasites are transmitted through flowers, and some of them can infect multiple host species. Given the shared use of flowers by bee species, parasites can potentially encounter multiple host species, which could affect the evolution of parasite virulence. We used the trypanosomatid parasite Crithidia bombi and its host, the common eastern bumble bee (Bombus impatiens), to explore the effect of infecting an alternative host, the alfalfa leaf-cutter bee (Megachile rotundata), on parasite infectivity and ability to replicate. We conducted a serial passage experiment on primary and alternative hosts, assessing infectivity and intensity of infection during five passes. Parasite cells from each pass through the alternative host were also used to infect a group of primary hosts. We found that serial passes through the alternative host increased infectivity, but there was no effect on intensity of infection. Interestingly, both the probability and intensity of infection on the primary host increased after serial passage through the alternative host. This increase in intensity of infection could be due to maladaptation after selection of new C. bombi strains has occurred in the alternative host. This study suggests that host switching has the potential to affect the adaptation of bee parasites to their hosts.

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.

Critical View on the Importance of Host Defense Strategies on Virus Distribution of Bee Viruses: What Can We Learn from SARS-CoV-2 Variants?

Bees, both wild and domesticated ones, are hosts to a plethora of viruses, with most of them infecting a wide range of bee species and genera. Although viral discovery and research on bee viruses date back over 50 years, the last decade is marked by a surge of new studies, new virus discoveries, and reports on viral transmission in and between bee species. This steep increase in research on bee viruses was mainly initiated by the global reports on honeybee colony losses and the worldwide wild bee decline, where viruses are regarded as one of the main drivers. While the knowledge gained on bee viruses has significantly progressed in a short amount of time, we believe that integration of host defense strategies and their effect on viral dynamics in the multi-host viral landscape are important aspects that are currently still missing. With the large epidemiological dataset generated over the last two years on the SARS-CoV-2 pandemic, the role of these defense mechanisms in shaping viral dynamics has become eminent. Integration of these dynamics in a multi-host system would not only greatly aid the understanding of viral dynamics as a driver of wild bee decline, but we believe bee pollinators and their viruses provide an ideal system to study the multi-host viruses and their epidemiology.

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.

Bumble Bee (Bombus vosnesenskii) Queen Nest Searching Occurs Independent of Ovary Developmental Status

Studies on the physiological states of wild-caught organisms are essential to uncovering the links between ecological and physiological processes. Bumble bee queens emerge from overwintering in the spring. At this time, queens develop their ovaries and search for a nest site in which to start a colony. Whether these two processes, ovary development and nest-searching, interact with or influence one another remains an unresolved question in behavioral physiology. We explored the hypothesis that ovary development and nest-searching might be mechanistically connected, by testing whether (1) ovary development precedes nest-searching behavior; (2) nest occupation precedes ovary development; or (3) ovary development and nest-searching occur independently, in bumble bee (Bombus vosnesenskii) queens. We collected queens either nest-searching (and thus prior to occupying a nest) or pollen-collecting (and thus provisioning an occupied nest) and measured their degree of ovary activation. We further screened these queens for parasites or other symbionts, to identify additional factors that may impact their reproductive success at this time. We found that queens searched for and occupied nests at all stages of ovary development, indicating that these processes occur independently in this system. Nest-searching queens were more likely to have substantial mite loads than pollen-collecting queens, who had already located and occupied a nest. However, mite loads did not significantly predict ovary developmental status. Collectively, our work shows that nesting status and symbionts alone are insufficient to explain the variation in spring bumble bee queen ovary development. We propose that ovary development and nest-searching occur opportunistically, which may enable queens to begin laying eggs earlier in the season than if these processes occurred in discrete succession.

Pathogen transport amplifies or dilutes disease transmission depending on the host dose-response relationship

Pathogen transport by biotic or abiotic processes (e.g. mechanical vectors, wind, rain) can increase disease transmission by creating more opportunities for host exposure. But transport without replication has an inherent trade-off, that creating new venues for exposure decreases the average pathogen abundance at each venue. The host dose-response relationship is therefore required to correctly assess infection risk. We model and analyse two examples-biotic mechanical vectors in plant-pollinator networks, and abiotic-facilitated long-distance pathogen dispersal-to illustrate how oversimplifying the dose-response relationship can lead to incorrect epidemiological predictions. When the minimum infective dose is high, mechanical vectors amplify disease transmission less than suggested by simple compartment models, and may even dilute transmission. When long-distance dispersal leads to infrequent large exposures, models that assume a linear force of infection can substantially under-predict the speed of epidemic spread. Our work highlights an important general interplay between dose-response relationships and pathogen transport.

Screening of Honey Bee Pathogens in the Czech Republic and Their Prevalence in Various Habitats

Western honey bee (Apis mellifera) is one of the most important pollinators in the world. Thus, a recent honey bee health decline and frequent honey bee mass losses have drawn attention and concern. Honey bee fitness is primarily reduced by pathogens, parasites, and viral load, exposure to pesticides and their residues, and inadequate nutrition from both the quality and amount of food resources. This study evaluated the prevalence of the most common honey bee pathogens and viruses in different habitats across the Czech Republic. The agroecosystems, urban ecosystems, and national park were chosen for sampling from 250 colonies in 50 apiaries. Surprisingly, the most prevalent honey bee pathogens belong to the family Trypanosomatidae including Lotmaria passim and Crithidia mellificae. As expected, the most prevalent viruses were DWV, followed by ABPV. Additionally, the occurrence of DWV-B and DWV-C were correlated with honey bee colony mortality. From the habitat point of view, most pathogens occurred in the town habitat, less in the agroecosystem and least in the national park. The opposite trend was observed in the occurrence of viruses. However, the prevalence of viruses was not affected by habitat.

Mass-flowering monoculture attracts bees, amplifying parasite prevalence

As the global agricultural footprint expands, it is increasingly important to address the link between the resource pulses characteristic of monoculture farming and wildlife epidemiology. To understand how mass-flowering crops impact host communities and subsequently amplify or dilute parasitism, we surveyed wild and managed bees in a monoculture landscape with varying degrees of floral diversification. We screened 1509 bees from 16 genera in sunflower fields and in non-crop flowering habitat across 200 km² of the California Central Valley. We found that mass-flowering crops increase bee abundance. Wild bee abundance was subsequently associated with higher parasite presence, but only in sites with a low abundance of non-crop flowers. Bee traits related to higher dispersal ability (body size) and diet breadth (pollen lecty) were also positively related to parasite presence. Our results highlight the importance of non-crop flowering habitat for supporting bee communities. We suggest monoculture alone cannot support healthy 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.

The managed-to-invasive species continuum in social and solitary bees and impacts on native bee conservation

Invasive bee species have negative impacts on native bee species and are a source of conservation concern. The invasion of bee species is mediated by the abiotic environment, biotic communities, and propagule pressure of the invader. Each of these factors is further affected by management, which can amplify the magnitude of the impact on native bee species. The ecological traits and behavior of invasive bees also play a role in whether and to what degree they compete with or otherwise negatively affect native bee species. The magnitude of impact of an invasive bee species relates both to its population size in the introduced habitat and the degree of overlap between its resources and the resources native bees require.

The Pathogens Spillover and Incidence Correlation in Bumblebees and Honeybees in Slovenia

Slovenia has a long tradition of beekeeping and a high density of honeybee colonies, but less is known about bumblebees and their pathogens. Therefore, a study was conducted to define the incidence and prevalence of pathogens in bumblebees and to determine whether there are links between infections in bumblebees and honeybees. In 2017 and 2018, clinically healthy workers of bumblebees (Bombus spp.) and honeybees (Apis mellifera) were collected on flowers at four different locations in Slovenia. In addition, bumblebee queens were also collected in 2018. Several pathogens were detected in the bumblebee workers using PCR and RT-PCR methods: 8.8% on acute bee paralysis virus (ABPV), 58.5% on black queen cell virus (BQCV), 6.8% on deformed wing virus (DWV), 24.5% on sacbrood bee virus (SBV), 15.6% on Lake Sinai virus (LSV), 16.3% on Nosema bombi, 8.2% on Nosema ceranae, 15.0% on Apicystis bombi and 17.0% on Crithidia bombi. In bumblebee queens, only the presence of BQCV, A. bombi and C. bombi was detected with 73.3, 26.3 and 33.3% positive samples, respectively. This study confirmed that several pathogens are regularly detected in both bumblebees and honeybees. Further studies on the pathogen transmission routes are required.

Identification of pathogens in the invasive hornet Vespa velutina and in native Hymenoptera (Apidae, Vespidae) from SW-Europe

Invasive species contribute to deteriorate the health of ecosystems due to their direct effects on native fauna and the local parasite-host dynamics. We studied the potential impact of the invasive hornet Vespa velutina on the European parasite-host system by comparing the patterns of diversity and abundance of pathogens (i.e. Microsporidia: Nosematidae; Euglenozoa: Trypanosomatidae and Apicomplexa: Lipotrophidae) in European V. velutina specimens with those in the native European hornet Vespa crabro, as well as other common Hymenoptera (genera Vespula, Polistes and Bombus). We show that (i) V. velutina harbours most common hymenopteran enteropathogens as well as several new parasitic taxa. (ii) Parasite diversity in V. velutina is most similar to that of V. crabro. (iii) No unambiguous evidence of pathogen release by V. velutina was detected. This evidence together with the extraordinary population densities that V. velutina reaches in Europe (around of 100,000 individuals per km² per year), mean that this invasive species could severely alter the native pathogen-host dynamics either by actively contributing to the dispersal of the parasites and/or by directly interacting with them, which could have unexpected long-term harmful consequences on the native entomofauna.

Do Viruses From Managed Honey Bees (Hymenoptera: Apidae) Endanger Wild Bees in Native Prairies?

Populations of wild and managed pollinators are declining in North America, and causes include increases in disease pressure and decreases in flowering resources. Tallgrass prairies can provide floral resources for managed honey bees (Hymenoptera: Apidae, Apis mellifera Linnaeus) and wild bees. Honey bees kept near prairies may compete with wild bees for floral resources, and potentially transfer viral pathogens to wild bees. Measurements of these potential interactions are lacking, especially in the context of native habitat conservation. To address this, we assessed abundance and richness of wild bees in prairies with and without honey bee hives present, and the potential spillover of several honey bee viruses to bumble bees (Hymenoptera: Apidae, Bombus Latrielle). We found no indication that the presence of honey bee hives over 2 yr had a negative effect on population size of wild bee taxa, though a potential longer-term effect remains unknown. All levels of viruses quantified in bumble bees were lower than those observed in honey bees. Higher levels of deformed wing virus and Israeli acute paralysis virus were found in Bombus griseocollis DeGeer (Hymenoptera: Apidae) collected at sites with hives than those without hives. These data suggest that the presence of honey bees in tallgrass prairie could increase wild bee exposure to viruses. Additional studies on cross-species transmission of viruses are needed to inform decisions regarding the cohabitation of managed bees within habitat utilized by wild bees.

Virus Prevalence and Genetic Diversity Across a Wild Bumblebee Community

Viruses are key population regulators, but we have limited knowledge of the diversity and ecology of viruses. This is even the case in wild host populations that provide ecosystem services, where small fitness effects may have major ecological impacts in aggregate. One such group of hosts are the bumblebees, which have a major role in the pollination of food crops and have suffered population declines and range contractions in recent decades. In this study, we investigate the diversity of four recently discovered bumblebee viruses (Mayfield virus 1, Mayfield virus 2, River Liunaeg virus, and Loch Morlich virus), and two previously known viruses that infect both wild bumblebees and managed honeybees (Acute bee paralysis virus and Slow bee paralysis virus) from isolates in Scotland. We investigate the ecological and environmental factors that determine viral presence and absence. We show that the recently discovered bumblebee viruses were more genetically diverse than the viruses shared with honeybees. Coinfection is potentially important in shaping prevalence: we found a strong positive association between River Liunaeg virus and Loch Morlich virus presence after controlling for host species, location and other relevant ecological variables. We tested for a relationship between environmental variables (temperature, UV radiation, wind speed, and prevalence), but as we had few sampling sites, and thus low power for site-level analyses, we could not conclude anything regarding these variables. We also describe the relationship between the bumblebee communities at our sampling sites. This study represents a first step in the description of predictors of bumblebee infection in the wild.

Pathways for Novel Epidemiology: Plant-Pollinator-Pathogen Networks and Global Change

Multiple global change pressures, and their interplay, cause plant-pollinator extinctions and modify species assemblages and interactions. This may alter the risks of pathogen host shifts, intra- or interspecific pathogen spread, and emergence of novel population or community epidemics. Flowers are hubs for pathogen transmission. Consequently, the structure of plant-pollinator interaction networks may be pivotal in pathogen host shifts and modulating disease dynamics. Traits of plants, pollinators, and pathogens may also govern the interspecific spread of pathogens. Pathogen spillover-spillback between managed and wild pollinators risks driving the evolution of virulence and community epidemics. Understanding this interplay between host-pathogen dynamics and global change will be crucial to predicting impacts on pollinators and pollination underpinning ecosystems and human wellbeing.

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.