Emerging dangers: deadly effects of an emergent parasite in a new pollinator host

Pathogen occurrence in different developmental stages of the invasive Vespa velutina nigrithorax (Buysson, 1905)

BACKGROUND

The yellow-legged hornet (Vespa velutina nigrithorax) is a predatory species native to South-East Asia. The hornet is invasive in Europe, spreading to several countries and becoming a pest for Apis mellifera due to its behaviour of preying in front of apiaries. The aim of this study was (i) to investigate the presence of honey bee pathogens within the developmental stages of V. velutina after neutralizing a nest in Bologna province (Emilia-Romagna, Italy) and (ii) to analyze the mitochondrial DNA to determine if the population derived from the population initially introduced in Europe.

RESULTS

The results indicated that deformed wing virus (82.76%) and Nosema ceranae (67.28%) were the most prevalent pathogens. Deformed wing virus, N. ceranae and sacbrood virus were found in all investigated stages, while chronic bee paralysis virus and Kashmir bee virus were exclusively found in foraging adults. All detected viruses were found to be replicative, highlighting active infection in the hosts. The mtDNA analysis demonstrated that the origin derived from the invasive population arrived in France.

CONCLUSION

This study underscores the importance of further research to understand the effect of interspecific transmission, especially concerning the potential role of these pathogens as a biocontrol for the invasive V. velutina nigrithorax. © 2024 Society of Chemical Industry.

Differentiating spillover: an examination of cross-habitat movement in ecology spillover in ecology

Organisms that immigrate into a recipient habitat generate a movement pattern that affects local population dynamics and the environment. Spillover is the pattern of unidirectional movement from a donor habitat to a different, adjacent recipient habitat. However, ecological definitions are often generalized to include any cross-habitat movement, which limits within- and cross-discipline collaboration. To assess spillover nomenclature, we reviewed 337 studies within the agriculture, disease, fisheries and habitat fragmentation disciplines. Each study's definition of spillover and the methods used were analysed. We identified four descriptors (movement, habitat type and arrangement, and effect) used that differentiate spillover from other cross-habitat movement patterns (dispersal, foray loops and edge movement). Studies often define spillover as movement (45%) but rarely measure it as such (4%), particularly in disease and habitat fragmentation disciplines. Consequently, 98% of studies could not distinguish linear from returning movement out of a donor habitat, which can overestimate movement distance. Overall, few studies (12%) included methods that matched their own definition, revealing a distinct mismatch. Because theory shows that long-term impacts of the different movement patterns can vary, differentiating spillover from other movement patterns is necessary for effective long-term and inter-disciplinary management of organisms that use heterogeneous landscapes.

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.

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.

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.

Sulfoxaflor effects depend on the interaction with other pesticides and Nosema ceranae infection in the honey bee (Apis mellifera)

Honey bees health is compromised by many factors such as the use of agrochemicals in agriculture and the various diseases that can affect them. Multiple studies have shown that these factors can interact, producing a synergistic effect that can compromise the viability of honey bees. This study analyses the interactions between different pesticides and the microsporidium Nosema ceranae and their effect on immune and detoxification gene expression, sugar consumption and mortality in the Iberian western honey bee (Apis mellifera iberiensis). For this purpose, workers were infected with N. ceranae and subjected to a sugar-water diet with field concentrations of the pesticides sulfoxaflor, azoxystrobin and glyphosate. Increased sugar intake and altered immune and cytochrome P450 gene expression were observed in workers exposed to sulfoxaflor and infected with N. ceranae. None of the pesticides affected Nosema spore production in honey bee gut. Of the three pesticides tested (alone or in combination) only sulfoxaflor increased mortality in honey bees. Taken together, our results suggest that the effects of sulfoxaflor were attenuated in contact with other pesticides, and that Nosema infection leads to increase sugar intake in sulfoxaflor-exposed bees. Overall, this underlines the importance of studying the interaction between different stressors to understand their overall impact not only on honey bee but also on wild bees health.

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.

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.

Different effects of pesticides on transcripts of the endocrine regulation and energy metabolism in honeybee foragers from different colonies

Honeybees are important pollinators of many crops and contribute to biological biodiversity. For years, a decline in bee populations has been observed in certain areas. This decline in honeybees is accompanied by a decrease in pollinator services. One factor contributing to the decline of bee colonies is the exposure to pesticides. Pesticide exposure of bees, among other effects, can negatively affect orientation, memory, immune system function and gene expression. Among the altered expressed genes are transcripts of endocrine regulation and oxidative phosphorylation. Endocrine regulation plays an important role in the development of nurse bees into foragers and oxidative phosphorylation is involved in energy metabolism. Most of these transcriptional changes were investigated using mixed aged honeybees derived from the same colony. Experiments using nurse bees or foragers of the same age but from different colonies are rare. In the present study, effects of the two pesticides chlorpyrifos and pyraclostrobin on the expression of transcripts linked to endocrine regulation and oxidative phosphorylation in foragers of the same age from three different colonies are investigated to fill this gap. These two pesticides were selected because negative effects at sublethal concentrations on bees are known and because they are found in pollen and nectar of crops and wild plants. For this purpose, 20-22 days old foragers of three different colonies were exposed to different sublethal concentrations of the selected fungicides for 24 h, followed by analysis of the expression of buffy, vitellogenin, hbg-3, ilp-1, mrjp1, 2 and 3, cox5a, cox5b and cox17. Some significant changes in gene expression of both endocrine regulation transcripts and oxidative phosphorylation were shown. Furthermore, it became clear that forager bees from different colonies react differently. This is especially important in relation to the risk analysis of pesticides. In addition, it could be shown that the expression of hbg-3 in the brain of bees is a robust marker to distinguish nurse bees from foragers at the molecular biological level. In summary, this study clearly shows that pesticides, which are often detected in pollen and nectar, display negative effects at sublethal concentrations on bees and that it is important to use bees from different colonies for risk assessment of pesticides.

Occurrence of Honey Bee (Apis mellifera L.) Pathogens in Wild Pollinators in Northern Italy

Diseases contribute to the decline of pollinator populations, which may be aggravated by the interspecific transmission of honey bee pests and pathogens. Flowers increase the risk of transmission, as they expose the pollinators to infections during the foraging activity. In this study, both the prevalence and abundance of 21 honey bee pathogens (11 viruses, 4 bacteria, 3 fungi, and 3 trypanosomatids) were assessed in the flower-visiting entomofauna sampled from March to September 2021 in seven sites in the two North-Italian regions, Emilia-Romagna and Piedmont. A total of 1,028 specimens were collected, identified, and analysed. Of the twenty-one pathogens that were searched for, only thirteen were detected. Altogether, the prevalence of the positive individuals reached 63.9%, with Nosema ceranae, deformed wing virus (DWV), and chronic bee paralysis virus (CBPV) as the most prevalent pathogens. In general, the pathogen abundance averaged 5.15 * 10⁶ copies, with CBPV, N. ceranae, and black queen cell virus (BQCV) as the most abundant pathogens, with 8.63, 1.58, and 0.48 * 10⁷ copies, respectively. All the detected viruses were found to be replicative. The sequence analysis indicated that the same genetic variant was circulating in a specific site or region, suggesting that interspecific transmission events among honey bees and wild pollinators are possible. Frequently, N. ceranae and DWV were found to co-infect the same individual. The circulation of honey bee pathogens in wild pollinators was never investigated before in Italy. Our study resulted in the unprecedented detection of 72 wild pollinator species as potential hosts of honey bee pathogens. Those results encourage the implementation of monitoring actions aiming to improve our understanding of the environmental implications of such interspecific transmission events, which is pivotal to embracing a One Health approach to pollinators’ welfare.

The gut parasite Nosema ceranae impairs olfactory learning in bumblebees

Pollinators are exposed to numerous parasites and pathogens when foraging on flowers. These biological stressors may affect critical cognitive abilities required for foraging. Here, we tested whether exposure to Nosema ceranae, one of the most widespread parasites of honey bees also found in wild pollinators, impacts cognition in bumblebees. We investigated different forms of olfactory learning and memory using conditioning of the proboscis extension reflex. Seven days after feeding parasite spores, bumblebees showed lower performances in absolute, differential, and reversal learning than controls. The consistent observations across different types of olfactory learning indicates a general negative effect of N. ceranae exposure that did not specifically target particular brain areas or neural processes. We discuss the potential mechanisms by which N. ceranae impairs bumblebee cognition and the broader consequences for populations of pollinators.

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

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

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 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.

Polymorphism of 16s rRNA Gene: Any Effect on the Biomolecular Quantitation of the Honey Bee (Apis mellifera L., 1758) Pathogen Nosema ceranae?

The microsporidian Nosema ceranae is a severe threat to the western honey bee Apis mellifera, as it is responsible for nosemosis type C, which leads the colonies to dwindle and collapse. Infection quantification is essential to clinical and research aims. Assessment is made often with molecular assays based on rRNA genes, which are present in the N. ceranae genome as multiple and polymorphic copies. This study aims to compare two different methods of Real-Time PCR (qPCR), respectively relying on the 16S rRNA and Hsp70 genes, the first of which is described as a multiple and polymorphic gene. Young worker bees, hatched in the laboratory and artificially inoculated with N. ceranae spores, were incubated at 33 °C and subject to different treatment regimens. Samples were taken post-infection and analyzed with both qPCR methods. Compared to Hsp70, the 16S rRNA method systematically detected higher abundance. Straightforward conversion between the two methods is made impossible by erratic 16s rRNA/Hsp70 ratios. The 16s rRNA polymorphism showed an increase around the inoculated dose, where a higher prevalence of ungerminated spores was expected due to the treatment effects. The possible genetic background of that irregular distribution is discussed in detail. The polymorphic nature of 16S rRNA showed to be a limit in the infection quantification. More reliably, the N. ceranae abundance can be assessed in honey bee samples with methods based on the single-copy gene Hsp70.

Pathogens Spillover from Honey Bees to Other Arthropods

Honey bees, and pollinators in general, play a major role in the health of ecosystems. There is a consensus about the steady decrease in pollinator populations, which raises global ecological concern. Several drivers are implicated in this threat. Among them, honey bee pathogens are transmitted to other arthropods populations, including wild and managed pollinators. The western honey bee, Apis mellifera, is quasi-globally spread. This successful species acted as and, in some cases, became a maintenance host for pathogens. This systematic review collects and summarizes spillover cases having in common Apis mellifera as the mainteinance host and some of its pathogens. The reports are grouped by final host species and condition, year, and geographic area of detection and the co-occurrence in the same host. A total of eighty-one articles in the time frame 1960-2021 were included. The reported spillover cases cover a wide range of hymenopteran host species, generally living in close contact with or sharing the same environmental resources as the honey bees. They also involve non-hymenopteran arthropods, like spiders and roaches, which are either likely or unlikely to live in close proximity to honey bees. Specific studies should consider host-dependent pathogen modifications and effects on involved host species. Both the plasticity of bee pathogens and the ecological consequences of spillover suggest a holistic approach to bee health and the implementation of a One Health approach.

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.

Viral load, not food availability or temperature, predicts colony longevity in an invasive eusocial wasp with plastic life history

Social insect colonies exhibit a variety of life history strategies, from the annual, semelparous colonies of temperate bees and wasps to the long-lived colonies of many ants and honeybees. Species introduced to novel habitats may exhibit plasticity in life history strategies as a result of the introduction, but the factors governing these changes often remain obscure. Vespula pensylvanica, a yellowjacket wasp, exhibits such plasticity in colony longevity. Multi-year (perennial) colonies are relatively common in introduced populations in Hawaii, while source populations in the western United States are typically on an annual cycle. Here, we use experiments and observational data to examine how diet, disease, nest thermal environment, and nest location influence colony longevity in a population with both annual and perennial colonies. Counter to our predictions, experimental feeding and warming did not increase colony survival in the winter in the introduced range. However, Moku Virus load and wasp colony density predicted colony survival in one year, suggesting a potential role for disease in modulating colony phenology. We also found that local V. pensylvanica colony density was positively correlated with Moku Virus loads, and that Arsenophonus sp. bacterial loads in V. pensylvanica colonies were positively associated with proximity to feral honeybee (Apis mellifera) hives, suggesting potential transmission routes for these poorly understood symbionts. The factors influencing colony longevity in this population are likely multiple and interactive. More important than food availability, we propose winter precipitation as a critical factor that may explain temporal and spatial variation in colony longevity in these invasive wasps.

Parasites and RNA viruses in wild and laboratory reared bumble bees Bombus pauloensis (Hymenoptera: Apidae) from Uruguay

Bumble bees (Bombus spp.) are important pollinators insects involved in the maintenance of natural ecosystems and food production. Bombus pauloensis is a widely distributed species in South America, that recently began to be managed and commercialized in this region. The movement of colonies within or between countries may favor the dissemination of parasites and pathogens, putting into risk while populations of B. pauloensis and other native species. In this study, wild B. pauloensis queens and workers, and laboratory reared workers were screened for the presence of phoretic mites, internal parasites (microsporidia, protists, nematodes and parasitoids) and RNA viruses (Black queen cell virus (BQCV), Deformed wing virus (DWV), Acute paralysis virus (ABCV) and Sacbrood virus (SBV)). Bumble bee queens showed the highest number of mite species, and it was the only group where Conopidae and S. bombi were detected. In the case of microsporidia, a higher prevalence of N. ceranae was detected in field workers. Finally, the bumble bees presented the four RNA viruses studied for A. mellifera, in proportions similar to those previously reported in this species. Those results highlight the risks of spillover among the different species of pollinators.

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.

Differential Viral Distribution Patterns in Reproductive Tissues of Apis mellifera and Apis cerana Drones

Honeybee drones are male bees that mate with virgin queens during the mating flight, consequently transferring their genes to offspring. Therefore, the health of drones affects the overall fitness of the offspring and ultimately the survivability of the colony. Honeybee viruses are considered to be a major threat to the health of honeybees. In the present study, we demonstrated the pattern of common honeybee viruses in various tissues of drones in the western honeybee, Apis mellifera, and the eastern honeybee, Apis cerana. Drones were collected during the mating flight and analyzed using quantitative real-time (qRT-PCR) to detect the presence of seven honeybee viruses. The qRT-PCR result revealed that three honeybee viruses, namely Black Queen Cell Virus (BQCV), Deformed Wing Virus (DWV), and Chinese Sacbrood Virus (CSBV), were detected in the reproductive tissues of A. mellifera and A. cerana drones. The results from qRT-PCR showed that the Israeli Acute Paralysis Virus (IAPV) was only detected in A. mellifera drone body tissues. Moreover, the prevalence of DWV and BQCV in the drones collected from A. mellifera colonies was significantly higher than that of A. cerana. In addition, virus multiple infections were higher in A. mellifera drones compared to those in A. cerana. CSBV was found predominantly in the reproductive tissues of A. cerana drones. This study is the first report describing the presence of the CSBV in reproductive tissues of A. mellifera drones. Our results may reflect the preference of honeybee viruses in honeybee species and may provide a piece of interesting evidence for understanding the virus transmission in A. cerana.

Artificial Diets Modulate Infection Rates by Nosema ceranae in Bumblebees

Parasites alter the physiology and behaviour of their hosts. In domestic honey bees, the microsporidia Nosema ceranae induces energetic stress that impairs the behaviour of foragers, potentially leading to colony collapse. Whether this parasite similarly affects wild pollinators is little understood because of the low success rates of experimental infection protocols. Here, we present a new approach for infecting bumblebees (Bombus terrestris) with controlled amounts of N. ceranae by briefly exposing individual bumblebees to parasite spores before feeding them with artificial diets. We validated our protocol by testing the effect of two spore dosages and two diets varying in their protein to carbohydrate ratio on the prevalence of the parasite (proportion of PCR-positive bumblebees), the intensity of parasites (spore count in the gut and the faeces), and the survival of bumblebees. Overall, insects fed a low-protein, high-carbohydrate diet showed the highest parasite prevalence (up to 70%) but lived the longest, suggesting that immunity and survival are maximised at different protein to carbohydrate ratios. Spore dosage did not affect parasite infection rate and host survival. The identification of experimental conditions for successfully infecting bumblebees with N. ceranae in the lab will facilitate future investigations of the sub-lethal effects of this parasite on the behaviour and cognition of wild pollinators.

Multiple stressors interact to impair the performance of bumblebee Bombus terrestris colonies

Bumblebees are constantly exposed to a wide range of biotic and abiotic stresses which they must defend themselves against to survive. Pathogens and pesticides represent important stressors that influence bumblebee health, both when acting alone or in combination. To better understand bumblebee health, we need to investigate how these factors interact, yet experimental studies to date generally focus on only one or two stressors. The aim of this study is to evaluate how combined effects of four important stressors (the gut parasite Nosema ceranae, the neonicotinoid insecticide thiamethoxam, the pyrethroid insecticide cypermethrin and the EBI fungicide tebuconazole) interact to affect bumblebees at the individual and colony levels. We established seven treatment groups of colonies that we pulse exposed to different combinations of these stressors for 2 weeks under laboratory conditions. Colonies were subsequently placed in the field for 7 weeks to evaluate the effect of treatments on the prevalence of N. ceranae in inoculated bumblebees, expression levels of immunity and detoxification-related genes, food collection, weight gain, worker and male numbers, and production of worker brood and reproductives. Exposure to pesticide mixtures reduced food collection by bumblebees. All immunity-related genes were upregulated in the bumblebees inoculated with N. ceranae when they had not been exposed to pesticide mixtures, and bumblebees exposed to the fungicide and the pyrethroid were less likely to have N. ceranae. Combined exposure to the three-pesticide mixture and N. ceranae reduced bumblebee colony growth, and all treatments had detrimental effects on brood production. The groups exposed to the neonicotinoid insecticide produced 40%-76% fewer queens than control colonies. Our findings show that exposure to combinations of stressors that bumblebees frequently come into contact with have detrimental effects on colony health and performance and could therefore have an impact at the population level. These results also have significant implications for current practices and policies for pesticide risk assessment and use as the combinations tested here are frequently applied simultaneously in the field. Understanding the interactions between different stressors will be crucial for improving our ability to manage bee populations and for ensuring pollination services into the future.

Nosema ceranae causes cellular immunosuppression and interacts with thiamethoxam to increase mortality in the stingless bee Melipona colimana

The microsporidian parasite Nosema ceranae and neonicotinoid insecticides affect the health of honey bees (Apis mellifera). However, there is limited information about the effect of these stressors on other pollinators such as stingless bees (Hymenoptera: Meliponini). We examined the separate and combined effects of N. ceranae and the neonicotinoid thiamethoxam at field-exposure levels on the survivorship and cellular immunity (hemocyte concentration) of the stingless bee Melipona colimana. Newly-emerged bees were subjected to four treatments provided in sucrose syrup: N. ceranae spores, thiamethoxam, thiamethoxam and N. ceranae, and control (bees receiving only syrup). N. ceranae developed infections of > 467,000 spores/bee in the group treated with spores only. However, in the bees subjected to both stressors, infections were < 143,000 spores/bee, likely due to an inhibitory effect of thiamethoxam on the microsporidium. N. ceranae infections did not affect bee survivorship, but thiamethoxam plus N. ceranae significantly increased mortality. Hemocyte counts were significantly lower in N. ceranae infected-bees than in the other treatments. These results suggest that N. ceranae may infect, proliferate and cause cellular immunosuppression in stingless bees, that exposure to sublethal thiamethoxam concentrations is toxic to M. colimana when infected with N. ceranae, and that thiamethoxam restrains N. ceranae proliferation. These findings have implications on pollinators' conservation.

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

Pollinator reductions can leave communities less diverse and potentially at increased risk of infectious diseases. Species-rich plant and bee communities have high species turnover, making the study of disease dynamics challenging. To address how temporal dynamics shape parasite prevalence in plant and bee communities, we screened >5,000 bees and flowers over an entire growing season for five common bee microparasites (Nosema ceranae, Nosema bombi, Crithidia bombi, Crithidia expoeki and neogregarines). Over 110 bee species and 89 flower species were screened, revealing that 42% of bee species (12.2% individual bees) and 70% of flower species (8.7% individual flowers) had at least one parasite in or on them, respectively. Some common flowers (for example, Lychnis flos-cuculi) harboured multiple parasite species whilst others (for example, Lythrum salicaria) had few. Significant temporal variation of parasite prevalence in bees was linked to bee diversity, bee and flower abundance and community composition. Specifically, we found that bee communities had the highest prevalence late in the season, when social bees (Bombus spp. and Apis mellifera) were dominant and bee diversity was lowest. Conversely, prevalence on flowers was lowest late in the season when floral abundance was highest. Thus turnover in the bee community impacted community-wide prevalence, and turnover in the plant community impacted when parasite transmission was likely to occur at flowers. These results imply that efforts to improve bee health will benefit from the promotion of high floral numbers to reduce transmission risk, maintaining bee diversity to dilute parasites and monitoring the abundance of dominant competent hosts.

Cross-infectivity of honey and bumble bee-associated parasites across three bee families

Recent declines of wild pollinators and infections in honey, bumble and other bee species have raised concerns about pathogen spillover from managed honey and bumble bees to other pollinators. Parasites of honey and bumble bees include trypanosomatids and microsporidia that often exhibit low host specificity, suggesting potential for spillover to co-occurring bees via shared floral resources. However, experimental tests of trypanosomatid and microsporidial cross-infectivity outside of managed honey and bumble bees are scarce. To characterize potential cross-infectivity of honey and bumble bee-associated parasites, we inoculated three trypanosomatids and one microsporidian into five potential hosts – including four managed species – from the apid, halictid and megachilid bee families. We found evidence of cross-infection by the trypanosomatids Crithidia bombi and C. mellificae, with evidence for replication in 3/5 and 3/4 host species, respectively. These include the first reports of experimental C. bombi infection in Megachile rotundata and Osmia lignaria, and C. mellificae infection in O. lignaria and Halictus ligatus. Although inability to control amounts inoculated in O. lignaria and H. ligatus hindered estimates of parasite replication, our findings suggest a broad host range in these trypanosomatids, and underscore the need to quantify disease-mediated threats of managed social bees to sympatric pollinators.

Rapid Gastrointestinal Passage May Protect Bombus terrestris from Becoming a True Host for Nosema ceranae

Pollination provided by managed honey bees as well as by all the wild bee species is a crucial ecosystem service contributing to the conservation of biodiversity and human food security. Therefore, it is not only the health status of honey bees but also the health status of wild bees that concerns us all. In this context, recent field studies suggesting interspecies transmission of the microsporidium parasite Nosema ceranae from honey bees (Apis mellifera) to bumblebees (Bombus spp.) were alarming. On the basis of these studies, N. ceranae was identified as an emerging infectious agent (EIA) of bumblebees, although knowledge of its impact on its new host was still elusive. In order to investigate the infectivity, virulence, and pathogenesis of N. ceranae infections in bumblebees, we performed controlled laboratory exposure bioassays with Bombus terrestris by orally inoculating the bees with infectious N. ceranae spores. We comprehensively analyzed the infection status of the bees via microscopic analysis of squash preparations, PCR-based detection of N. ceranae DNA, histology of Giemsa-stained tissue sections, and species-specific fluorescence in situ hybridization. We did not find any evidence for a true infection of bumblebees by N. ceranae Through a series of experiments, we ruled out the possibility that spore infectivity, spore dosage, incubation time, or age and source of the bumblebees caused these negative results. Instead, our results clearly demonstrate that no infection and production of new spores took place in bumblebees after they ingested N. ceranae spores in our experiments. Thus, our results question the classification of N. ceranae as an emerging infectious agent for bumblebees.IMPORTANCE Emerging infectious diseases (EIDs) pose a major health threat to both humans and animals. EIDs include, for instance, those that have spread into hitherto naive populations. Recently, the honey bee-specific microsporidium Nosema ceranae has been detected by molecular methods in field samples of bumblebees. This detection of N. ceranae DNA in bumblebees led to the assumption that N. ceranae infections represent an EID of bumblebees and resulted in speculations on the role of this pathogen in driving bumblebee declines. In order to address the issue of whether N. ceranae is an emerging infectious agent for bumblebees, we experimentally analyzed host susceptibility and pathogen reproduction in this new host-pathogen interaction. Surprisingly, we did not find any evidence for a true infection of Bombus terrestris by N. ceranae, questioning the classification of N. ceranae infections as EIDs of bumblebees and demonstrating that detection of microsporidian DNA does not equal detection of microsporidian infection.

Microbial Diversity Associated with the Pollen Stores of Captive-Bred Bumble Bee Colonies

The pollen stores of bumble bees host diverse microbiota that influence overall colony fitness. Yet, the taxonomic identity of these symbiotic microbes is relatively unknown. In this descriptive study, we characterized the microbial community of pollen provisions within captive-bred bumble bee hives obtained from two commercial suppliers located in North America. Findings from 16S rRNA and ITS gene-based analyses revealed that pollen provisions from the captive-bred hives shared several microbial taxa that have been previously detected among wild populations. While diverse microbes across phyla Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, and Ascomycota were detected in all commercial hives, significant differences were detected at finer-scale taxonomic resolution based on the supplier source. The causative agent of chalkbrood disease in honey bees, Ascosphaera apis, was detected in all hives obtained from one supplier source, although none of the hives showed symptoms of infection. The shared core microbiota across both commercial supplier sources consisted of two ubiquitous bee-associated groups, Lactobacillus and Wickerhamiella/Starmerella clade yeasts that potentially contribute to the beneficial function of the microbiome of bumble bee pollen provisions.

Global Trends in Bumble Bee Health

Bumble bees (Bombus) are unusually important pollinators, with approximately 260 wild species native to all biogeographic regions except sub-Saharan Africa, Australia, and New Zealand. As they are vitally important in natural ecosystems and to agricultural food production globally, the increase in reports of declining distribution and abundance over the past decade has led to an explosion of interest in bumble bee population decline. We summarize data on the threat status of wild bumble bee species across biogeographic regions, underscoring regions lacking assessment data. Focusing on data-rich studies, we also synthesize recent research on potential causes of population declines. There is evidence that habitat loss, changing climate, pathogen transmission, invasion of nonnative species, and pesticides, operating individually and in combination, negatively impact bumble bee health, and that effects may depend on species and locality. We distinguish between correlational and causal results, underscoring the importance of expanding experimental research beyond the study of two commercially available species to identify causal factors affecting the diversity of wild species.

Exposure of Larvae of the Solitary Bee Osmia bicornis to the Honey Bee Pathogen Nosema ceranae Affects Life History

Wild bees are important pollinators of wild plants and agricultural crops and they are threatened by several environmental stressors including emerging pathogens. Honey bees have been suggested as a potential source of pathogen spillover. One prevalent pathogen that has recently emerged as a honey bee disease is the microsporidian Nosema ceranae. While the impacts of N. ceranae in honey bees are well documented, virtually nothing is known about its effects in solitary wild bees. The solitary mason bee Osmia bicornis is a common pollinator in orchards and amenable to commercial management. Here, we experimentally exposed larvae of O. bicornis to food contaminated with N. ceranae and document spore presence during larval development. We measured mortality, growth parameters, and timing of pupation in a semi-field experiment. Hatched individuals were assessed for physiological state including fat body mass, wing muscle mass, and body size. We recorded higher mortality in the viable-spore-exposed group but could only detect a low number of spores among the individuals of this treatment. Viable-spore-treated individuals with higher head capsule width had a delayed pupation start. No impact on the physiological status could be detected in hatched imagines. Although we did not find overt evidence of O. bicornis infection, our findings indicate that exposure of larvae to viable N. ceranae spores could affect bee development.

Microsporidia Nosema spp. - obligate bee parasites are transmitted by air

Microsporidia Nosema are transferred among bees via the faecal-oral route. Nosema spp. spores have been detected on flowers and transferred to hives along with the bee pollen. The aim of the present study was to determine whether Nosema microsporidia are transferred by air in an apiary, in a control area (without the presence of bee colonies), and/or in a laboratory during cage experiments with artificially infected bees. The novel way of transmission by air was investigated by the volumetric method using a Hirst-type aerobiological sampler located on the ground in the apiary, in the Botanical Garden and on the laboratory floor. Concurrently, the mean rate of Nosema infections in the foragers in the apiary was estimated with the Bürker haemocytometer method. Spore-trapping tapes were imaged by means of light microscopy, Nomarski interference contrast microscopy and scanning electron microscopy. The highest concentration of Nosema spores per 1m3 of air (4.65) was recorded in August, while the lowest concentration (2.89) was noted in July. This was confirmed by a Real-Time PCR analysis. The presence of N. apis as well as N. ceranae was detected in each of the tested tapes from the apiary. The average copy number of N. apis was estimated at 14.4 × 104 copies per 1 cm2 of the tape; whereas the number of N. ceranae was 2.24 × 104 copies per tape per 1 cm2. The results indicate that Nosema microsporidia were transferred by the wind in the apiary, but not in the Botanical Garden and laboratory by air. This was confirmed by genetic analyses. DNA from immobilised biological material was isolated and subjected to a PCR to detect the Nosema species. A fragment of the 16S rRNA gene, characteristic of Nosema apis and N. ceranae, was detected. Our research adds knowledge about the transfer of Nosema spp. microsporidia in the natural environment and indicates the season associated with the greatest risk of a bee colony infection with Nosema spp.

Pathogen spillover from Apis mellifera to a stingless bee

Pathogen spillover from managed bees is increasingly considered as a possible cause of pollinator decline. Though spillover has been frequently documented, evidence of the pathogen's virulence in the new host or mechanism of transmission is rare. Stingless bees (Apocrita: Meliponini) are crucial pollinators pan-tropically and overlap with managed honeybees (Apis mellifera) in much of their range. Nosema ceranae is the most prevalent disease of adult A. mellifera. We used laboratory experiments and field surveys to investigate the susceptibility of stingless bees (Tetragonula hockingsi) to N. ceranae, infection prevalence and transmissibility via flowers. We found that 67% of T. hockingsi fed sucrose with N. ceranae had detectable spores in their ventriculus, and they died at 2.96 times the rate of sucrose-only fed bees. Five of six field hives harboured bees with N. ceranae present at least once during our five-month survey, with prevalence up to 20%. In our floral transmission experiment, 67% of inflorescences exposed to infected A. mellifera yielded N. ceranae spores, and all resulted in T. hockingsi with N. ceranae spores in their guts. We conclude that N. ceranae is virulent in T. hockingsi under laboratory conditions, is common in the local T. hockingsi population and is transmissible via flowers.

Impact of Biotic and Abiotic Stressors on Managed and Feral Bees

Large-scale declines in bee abundance and species richness over the last decade have sounded an alarm, given the crucial pollination services that bees provide. Population dips have specifically been noted for both managed and feral bee species. The simultaneous increased cultivation of bee-dependent agricultural crops has given rise to additional concern. As a result, there has been a surge in scientific research investigating the potential stressors impacting bees. A group of environmental and anthropogenic stressors negatively impacting bees has been isolated. Habitat destruction has diminished the availability of bee floral resources and nest habitats, while massive monoculture plantings have limited bee access to a variety of pollens and nectars. The rapid spread and increased resistance buildup of various bee parasites, pathogens, and pests to current control methods are implicated in deteriorating bee health. Similarly, many pesticides that are widely applied on agricultural crops and within beehives are toxic to bees. The global distribution of honey bee colonies (including queens with attendant bees) and bumble bee colonies from crop to crop for pollination events has been linked with increased pathogen stress and increased competition with native bee species for limited resources. Climatic alterations have disrupted synchronous bee emergence with flower blooming and reduced the availability of diverse floral resources, leading to bee physiological adaptations. Interactions amongst multiple stressors have created colossal maladies hitting bees at one time, and in some cases delivering additive impacts. Initiatives including the development of wild flower plantings and assessment of pesticide toxicity to bees have been undertaken in efforts to ameliorate current bee declines. In this review, recent findings regarding the impact of these stressors on bees and strategies for mitigating them are discussed.

RNA virus spillover from managed honeybees (Apis mellifera) to wild bumblebees (Bombus spp.)

The decline of many bumblebee species (Bombus spp.) has been linked to an increased prevalence of pathogens caused by spillover from managed bees. Although poorly understood, RNA viruses are suspected of moving from managed honeybees (Apis mellifera) into wild bumblebees through shared floral resources. We examined if RNA viruses spillover from managed honeybees, the extent to which viruses are replicating within bumblebees, and the role of flowers in transmission. Prevalence and active infections of deformed wing virus (DWV) were higher in bumblebees collected near apiaries and when neighboring honeybees had high infection levels. We found no DWV in bumblebees where honeybee foragers and honeybee apiaries were absent. The prevalence of black queen cell virus (BQCV) was also higher in bumblebees collected near apiaries. Furthermore, we detected viruses on 19% of flowers, all of which were collected within apiaries. Our results corroborate the hypothesis that viruses are spilling over from managed honeybees to wild bumblebees and that flowers may be an important route for transmission.

Trait-Based Modeling of Multihost Pathogen Transmission: Plant-Pollinator Networks

Epidemiological models for multihost pathogen systems often classify individuals taxonomically and use species-specific parameter values, but in species-rich communities that approach may require intractably many parameters. Trait-based epidemiological models offer a potential solution but have not accounted for within-species trait variation or between-species trait overlap. Here we propose and study trait-based models with host and vector communities represented as trait distributions without regard to species identity. To illustrate this approach, we develop susceptible-infectious-susceptible models for disease spread in plant-pollinator networks with continuous trait distributions. We model trait-dependent contact rates in two common scenarios: nested networks and specialized plant-pollinator interactions based on trait matching. We find that disease spread in plant-pollinator networks is impacted the most by selective pollinators, universally attractive flowers, and cospecialized plant-pollinator pairs. When extreme pollinator traits are rare, pollinators with common traits are most important for disease spread, whereas when extreme flower traits are rare, flowers with uncommon traits impact disease spread the most. Greater nestedness and specialization both typically promote disease persistence. Given recent pollinator declines caused in part by pathogens, we discuss how trait-based models could inform conservation strategies for wild and managed pollinators. Furthermore, while we have applied our model to pollinators and pathogens, its framework is general and can be transferred to any kind of species interactions in any community.

Honey bee-collected pollen is a potential source of Ascosphaera apis infection in managed bumble bees

The trade of bumble bees started in the early nineties for pollinator-dependent greenhouse plants. Nowadays, its rearing and transport have received public attention, since managed bees can transfer pathogens to wild bee populations. Therefore, guaranteeing pathogen-free bumble bees is fundamental. The major protein source used in rearing facilities is honey bee-collected pollen. This can carry pathogens, however to date, solid data on the risk of this food source to the health of bumble bees is lacking. Here we performed a large pathogen screening of non-irradiated honey bee-collected pollen to discover particles infective to Bombus terrestris. We identified seven parasites (Apicystis bombi, Ascosphaera apis, Crithidia mellificae, Nosema ceranae, Paenibacillus larvae and two parasites resembling Nosema thomsoni and Microsporidium sp. Oise) and four viruses (CBPV, DWV, IAPV and SBV) in 17 pollen batches from two major European pollen source regions (Spain and Romania). Ascosphaera apis was capable of infecting bumble bees; the larvae showed similar symptoms to chalkbrood disease reported in honey bees. Bumble bee breeding facilities need to be cautious about the potential presence of this disease, which was originally reported in honey bees. Thorough diagnostic and control methods are needed, as risk of spillover to wild bee species is possible.

Prevalence of infection by the microsporidian Nosema spp. in native bumblebees (Bombus spp.) in northern Thailand

Bumblebees (tribe Bombini, genus Bombus Latreille) play a pivotal role as pollinators in mountain regions for both native plants and for agricultural systems. In our survey of northern Thailand, four species of bumblebees (Bombus (Megabombus) montivagus Smith, B. (Alpigenobombus) breviceps Smith, B. (Orientalibombus) haemorrhoidalis Smith and B. (Melanobombus) eximius Smith), were present in 11 localities in 4 provinces (Chiang Mai, Mae Hong Son, Chiang Rai and Nan). We collected and screened 280 foraging worker bumblebees for microsporidia (Nosema spp.) and trypanosomes (Crithidia spp.). Our study is the first to demonstrate the parasite infection in bumblebees in northern Thailand. We found N. ceranae in B. montivagus (5.35%), B. haemorrhoidalis (4.76%), and B. breviceps (14.28%) and N. bombi in B. montivagus (14.28%), B. haemorrhoidalis (11.64%), and B. breviceps (28.257%).

Pathogen shifts in a honeybee predator following the arrival of the Varroa mite

Emerging infectious diseases (EIDs) are a global threat to honeybees, and spillover from managed bees threaten wider insect populations. Deformed wing virus (DWV), a widespread virus that has become emergent in conjunction with the spread of the mite Varroa destructor, is thought to be partly responsible for global colony losses. The arrival of Varroa in honeybee populations causes a dramatic loss of viral genotypic diversity, favouring a few virulent strains. Here, we investigate DWV spillover in an invasive Hawaiian population of the wasp, Vespula pensylvanica, a honeybee predator and honey-raider. We show that Vespula underwent a parallel loss in DWV variant diversity upon the arrival of Varroa, despite the mite being a honeybee specialist. The observed shift in Vespula DWV and the variant-sharing between Vespula and Apis suggest that these wasps can acquire DWV directly or indirectly from honeybees. Apis prey items collected from Vespula foragers were positive for DWV, indicating predation is a possible route of transmission. We also sought cascading effects of DWV shifts in a broader Vespula pathogen community. We identified concurrent changes in a suite of additional pathogens, as well as shifts in the associations between these pathogens in Vespula. These findings reveal how hidden effects of the Varroa mite can, via spillover, transform the composition of pathogens in interacting species, with potential knock-on effects for entire pathogen communities.

Home sick: impacts of migratory beekeeping on honey bee (Apis mellifera) pests, pathogens, and colony size

Honey bees are important pollinators of agricultural crops and the dramatic losses of honey bee colonies have risen to a level of international concern. Potential contributors to such losses include pesticide exposure, lack of floral resources and parasites and pathogens. The damaging effects of all of these may be exacerbated by apicultural practices. To meet the pollination demand of US crops, bees are transported to areas of high pollination demand throughout the year. Compared to stationary colonies, risk of parasitism and infectious disease may be greater for migratory bees than those that remain in a single location, although this has not been experimentally established. Here, we conducted a manipulative experiment to test whether viral pathogen and parasite loads increase as a result of colonies being transported for pollination of a major US crop, California almonds. We also tested if they subsequently transmit those diseases to stationary colonies upon return to their home apiaries. Colonies started with equivalent numbers of bees, however migratory colonies returned with fewer bees compared to stationary colonies and this difference remained one month later. Migratory colonies returned with higher black queen cell virus loads than stationary colonies, but loads were similar between groups one month later. Colonies exposed to migratory bees experienced a greater increase of deformed wing virus prevalence and load compared to the isolated group. The three groups had similar infestations of Varroa mites upon return of the migratory colonies. However, one month later, mite loads in migratory colonies were significantly lower compared to the other groups, possibly because of lower number of host bees. Our study demonstrates that migratory pollination practices has varying health effects for honey bee colonies. Further research is necessary to clarify how migratory pollination practices influence the disease dynamics of honey bee diseases we describe here.

Climate change-driven range losses among bumblebee species are poised to accelerate

Climate change has shaped bee distributions over the past century. Here, we conducted the first species-specific assessment of future climate change impacts on North American bumblebee distributions, using the most recent global change scenarios developed in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). We assessed potential shifts in bumblebee species distributions with models generated using Maxent. We tested different assumptions about bumblebee species’ dispersal capacities, drawing on observed patterns of range shifts to date, dispersal rates observed for bumblebee queens, and, lastly, assuming unlimited dispersal. Models show significant contractions of current ranges even under scenarios in which dispersal rates were high. Results suggest that dispersal rates may not suffice for bumblebees to track climate change as rapidly as required under any IPCC scenario for future climate change. Areas where species losses are projected overlap for many species and climate scenarios, and are concentrated in eastern parts of the continent. Models also show overlap for range expansions across many species, suggesting the presence of “hotspots” where management activities could benefit many species, across all climate scenarios. Broad-scale strategies are likely to be necessary to improve bumblebee conservation prospects under climate change.

Floral Resource Competition Between Honey Bees and Wild Bees: Is There Clear Evidence and Can We Guide Management and Conservation?

Supporting managed honey bees by pasturing in natural landscapes has come under review due to concerns that honey bees could negatively impact the survival of wild bees through competition for floral resources. Critique and assessment of the existing body of published literature against our criteria focussing on studies that can support best management resulted in 19 experimental papers. Indirect measures of competition examining foraging patterns and behavior yielded equivocal results. Direct measures of reproduction and growth were investigated in only seven studies, with six indicating negative impacts to wild bees from the presence of managed honey bees. Three of these studies examined fitness impacts to BombusLatreille and all three indicated reduced growth or reduced reproductive output. Because there is a severe lack of literature, yet potential that honey bee presence could negatively impact wild bees, exemplified with bumble bee studies, we advocate for further research into the fitness impacts of competition between managed and wild pollinators. Conservative approaches should be taken with respect to pasturing honey bees on natural lands with sensitive bumble bee populations. Correspondingly, forage opportunities for honey bees in managed, agricultural landscapes, should be increased in an effort to reduce potential pressure and infringement on wild bee populations in natural areas.

Pollinator decline - an ecological calamity in the making?

Since pollination by insects is vitally important for much of global crop production, and to provide pollination services more widely throughout the planetary ecosystems, the prospect of an imminent 'pollination crisis', due to a die-off of flying insects, is most disquieting, to say the least. Indeed, the term 'ecological Armageddon' has been used in the media. However, to know whether or not a wholesale decline in flying pollinators (including non-bee species) is occurring across the world is very difficult, due to an insufficiency of geographically widespread and long-term data. Bees, as the best documented species, can be seen to be suffering from chronic exposure to a range of stressors, which include: a loss of abundance and diversity of flowers, and a decline in suitable habitat for them to build nests; long-term exposure to agrochemicals, including pesticides such as neonicotinoids; and infection by parasites and pathogens, many inadvertently spread by the actions of humans. It is likely that climate change may impact further on particular pollinators, for example bumble bees, which are cool-climate specialists. Moreover, the co-operative element of various different stress factors should be noted; thus, for example, exposure to pesticides is known to diminish detoxification mechanisms and also immune responses, hence lowering the resistance of bees to parasitic infections. It is further conspicuous that for those wild non-bee insects - principally moths and butterflies - where data are available, the picture is also one of significant population losses. Alarmingly, a recent study in Germany indicated that a decline in the biomass of flying insects had occurred by 76% in less than three decades, as sampled in nature reserves across the country. Accordingly, to fully answer the question posed in the title of this article 'pollinator decline - an ecological calamity in the making?' will require many more detailed, more geographically encompassing, more species-inclusive, and longer-term studies, but the available evidence points to a clear 'probably', and the precautionary principle would suggest this is not a prospect we can afford to ignore.