Varroa destructor parasitism and Deformed wing virus infection in honey bees are linked to peroxisome-induced pathways

The ectoparasitic mite Varroa destructor transmits and triggers viral infections that have deleterious effects on honey bee colonies worldwide. We performed a manipulative experiment in which worker bees collected at emergence were exposed to Varroa for 72 h, and their proteomes were compared with those of untreated control bees. Label-free quantitative proteomics identified 77 differentially expressed A. mellifera proteins (DEPs). In addition, viral proteins were identified by orthogonal analysis, and most importantly, Deformed wing virus (DWV) was found at high levels/intensity in Varroa-exposed bees. Pathway enrichment analysis suggested that the main pathways affected included peroxisomal metabolism, cyto-/exoskeleton reorganization, and cuticular proteins. Detailed examination of individual DEPs revealed that additional changes in DEPs were associated with peroxisomal function. In addition, the proteome data support the importance of TGF-β signaling in Varroa-DWV interaction and the involvement of the mTORC1 and Hippo pathways. These results suggest that the effect of DWV on bees associated with Varroa feeding results in aberrant autophagy. In particular, autophagy is selectively modulated by peroxisomes, to which the observed proteome changes strongly corresponded. This study complements previous research with different study designs and suggests the importance of the peroxisome, which plays a key role in viral infections.

Virus replication in the honey bee parasite, Varroa destructor

IMPORTANCE

The parasitic mite Varroa destructor is a significant driver of worldwide colony losses of our most important commercial pollinator, the Western honey bee Apis mellifera. Declines in honey bee health are frequently attributed to the viruses that mites vector to honey bees, yet whether mites passively transmit viruses as a mechanical vector or actively participate in viral amplification and facilitate replication of honey bee viruses is debated. Our work investigating the antiviral RNA interference response in V. destructor demonstrates that key viruses associated with honey bee declines actively replicate in mites, indicating that they are biological vectors, and the host range of bee-associated viruses extends to their parasites, which could impact virus evolution, pathogenicity, and spread.

Adapted tolerance to virus infections in four geographically distinct Varroa destructor-resistant honeybee populations

The ectoparasitic mite, Varroa destructor, is unarguably the leading cause of honeybee (Apis mellifera) mortality worldwide through its role as a vector for lethal viruses, in particular, strains of the Deformed wing virus (DWV) and Acute bee paralysis virus (ABPV) complexes. Several honeybee populations across Europe have well-documented adaptations of mite-resistant traits but little is known about host adaptations towards the virus infections vectored by the mite. The aim of this study was to assess and compare the possible contribution of adapted virus tolerance and/or resistance to the enhanced survival of four well-documented mite-resistant honeybee populations from Norway, Sweden, The Netherlands and France, in relation to unselected mite-susceptible honeybees. Caged adult bees and laboratory reared larvae, from colonies of these four populations, were inoculated with DWV and ABPV in a series of feeding infection experiments, while control groups received virus-free food. Virus infections were monitored using RT-qPCR assays in individuals sampled over a time course. In both adults and larvae the DWV and ABPV infection dynamics were nearly identical in all groups, but all mite-resistant honeybee populations had significantly higher survival rates compared to the mite-susceptible honeybees. These results suggest that adapted virus tolerance is an important component of survival mechanisms.

A New Strain of Virus Discovered in China Specific to the Parasitic Mite Varroa destructor Poses a Potential Threat to Honey Bees

The ectoparasitic mite, Varroa destructor, feeds directly on honey bees and serves as a vector for transmitting viruses among them. The Varroa mite causes relatively little damage to its natural host, the Eastern honey bee (Apis cerana) but it is the most devastating pest for the Western honey bee (Apis mellifera). Using Illumina HiSeq sequencing technology, we conducted a metatranscriptome analysis of the microbial community associated with Varroa mites. This study led to the identification of a new Chinese strain of Varroa destructor virus-2 (VDV-2), which is a member of the Iflaviridae family and was previously reported to be specific to Varroa mites. A subsequent epidemiological investigation of Chinese strain of VDV-2 (VDV-2-China) showed that the virus was highly prevalent among Varroa populations and was not identified in any of the adult workers from both A. mellifera and A.cerana colonies distributed in six provinces in China, clearly indicating that VDV-2-China is predominantly a Varroa-adapted virus. While A. mellifera worker pupae exposed to less than two Varroa mites tested negative for VDV-2-China, VDV-2-China was detected in 12.5% of the A. mellifera worker pupae that were parasitized by more than 10 Varroa mites, bringing into play the possibility of a new scenario where VDV-2 could be transmitted to the honey bees during heavy Varroa infestations. Bioassay for the VDV-2-China infectivity showed that A. cerana was not a permissive host for VDV-2-China, yet A. mellifera could be a biological host that supports VDV-2-China's replication. The different replication dynamics of the virus between the two host species reflect their variation in terms of susceptibility to the virus infection, posing a potential threat to the health of the Western honey bee. The information gained from this study contributes to the knowledge concerning genetic variabilities and evolutionary dynamics of Varroa-borne viruses, thereby enhancing our understanding of underlying molecular mechanisms governing honey bee Varroosis.

The Bee Hemolymph Metabolome: A Window into the Impact of Viruses on Bumble Bees

State-of-the-art virus detection technology has advanced a lot, yet technology to evaluate the impacts of viruses on bee physiology and health is basically lacking. However, such technology is sorely needed to understand how multi-host viruses can impact the composition of the bee community. Here, we evaluated the potential of hemolymph metabolites as biomarkers to identify the viral infection status in bees. A metabolomics strategy based on ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry was implemented. First, we constructed a predictive model for standardized bumble bees, in which non-infected bees were metabolically differentiated from an overt Israeli acute paralysis virus (IAPV) infection (R²Y = 0.993; Q² = 0.906), as well as a covert slow bee paralysis virus (SBPV) infection (R²Y = 0.999; Q² = 0.875). Second, two sets of potential biomarkers were identified, being descriptors for the metabolomic changes in the bee's hemolymph following viral infection. Third, the biomarker sets were evaluated in a new dataset only containing wild bees and successfully discriminated virus infection versus non-virus infection with an AUC of 0.985. We concluded that screening hemolymph metabolite markers can underpin physiological changes linked to virus infection dynamics, opening promising avenues to identify, monitor, and predict the effects of virus infection in a bee community within a specific environment.

RNAseq of Deformed Wing Virus and Other Honey Bee-Associated Viruses in Eight Insect Taxa with or without Varroa Infestation

The global spread of a parasitic mite (Varroa destructor) has resulted in Deformed wing virus (DWV), a previously rare pathogen, now dominating the viromes in honey bees and contributing to large-scale honey bee colony losses. DWV can be found in diverse insect taxa and has been implicated in spilling over from honey bees into associated ("apiary") and other ("non-apiary") insects. Here we generated next generation sequence data from 127 insect samples belonging to diverse taxa collected from Hawaiian islands with and without Varroa to identify whether the mite has indirectly affected the viral landscapes of key insect taxa across bees, wasps, flies and ants. Our data showed that, while Varroa was associated with a dramatic increase in abundance of (predominantly recombinant) DWV in honey bees (and no other honey bee-associated RNA virus), this change was not seen in any other taxa sampled. Honey bees share their environment with other insect populations and exist as a homogenous group, frequently sharing common viruses, albeit at low levels. Our data suggest that the threat of Varroa to increase viral load in an apiary does not automatically translate to an increase in virus load in other insects living in the wider community.

Pesticide-Virus Interactions in Honey Bees: Challenges and Opportunities for Understanding Drivers of Bee Declines

Honey bees are key agricultural pollinators, but beekeepers continually suffer high annual colony losses owing to a number of environmental stressors, including inadequate nutrition, pressures from parasites and pathogens, and exposure to a wide variety of pesticides. In this review, we examine how two such stressors, pesticides and viruses, may interact in additive or synergistic ways to affect honey bee health. Despite what appears to be a straightforward comparison, there is a dearth of studies examining this issue likely owing to the complexity of such interactions. Such complexities include the wide array of pesticide chemical classes with different modes of actions, the coupling of many bee viruses with ectoparasitic Varroa mites, and the intricate social structure of honey bee colonies. Together, these issues pose a challenge to researchers examining the effects pesticide-virus interactions at both the individual and colony level.

Green Bees: Reverse Genetic Analysis of Deformed Wing Virus Transmission, Replication, and Tropism

Environmental and agricultural pollination services by honey bees, Apis mellifera, and honey production are compromised by high levels of annual colony losses globally. The majority are associated with disease caused by deformed wing virus (DWV), a positive-strand RNA virus, exacerbated by the ectoparasitic mite Varroa destructor. To improve honey bee health, a better understanding of virus transmission and pathogenesis is needed which requires the development of tools to study virus replication, transmission, and localisation. We report the use of reverse genetic (RG) systems for the predominant genetically distinct variants of DWV to address these questions. All RG-recovered viruses replicate within 24 h post-inoculation of pupae and could recapitulate the characteristic symptoms of DWV disease upon eclosion. Larvae were significantly less susceptible but could be infected orally and subsequently developed disease. Using genetically tagged RG DWV and an in vitro Varroa feeding system, we demonstrate virus replication in the mite by accumulation of tagged negative-strand viral replication intermediates. We additionally apply a modified DWV genome expressing a fluorescent reporter protein for direct in vivo observation of virus distribution in injected pupae or fed larvae. Using this, we demonstrate extensive sites of virus replication in a range of pupal tissues and organs and in the nascent wing buds in larvae fed high levels of virus, indicative of a direct association between virus replication and pathogenesis. These studies provide insights into virus replication kinetics, tropism, transmission, and pathogenesis, and produce new tools to help develop the understanding needed to control DWV-mediated colony losses.

The honeybee (Apis mellifera) developmental state shapes the genetic composition of the deformed wing virus-A quasispecies during serial transmission

The main biological threat to the western honeybee (Apis mellifera) is the parasitic mite Varroa destructor, largely because it vectors lethal epidemics of honeybee viruses that, in the absence of this mite, are relatively innocuous. The severe pathology is a direct consequence of excessive virus titres caused by this novel transmission route. However, little is known about how the virus adapts genetically during transmission and whether this influences the pathology. Here, we show that upon injection into honeybee pupae, the deformed wing virus type-A (DWV-A) quasispecies undergoes a rapid, extensive expansion of its sequence space, followed by strong negative selection towards a uniform, common shape by the time the pupae have completed their development, with no difference between symptomatic and asymptomatic adults in either DWV titre or genetic composition. This suggests that the physiological and molecular environment during pupal development has a strong, conservative influence on shaping the DWV-A quasispecies in emerging adults. There was furthermore no evidence of any progressive adaptation of the DWV-A quasispecies to serial intra-abdominal injection, simulating mite transmission, despite the generation of ample variation immediately following each transmission, suggesting that the virus either had already adapted to transmission by injection, or was unaffected by it.

Meta-analysis of honey bee neurogenomic response links Deformed wing virus type A to precocious behavioral maturation

Crop pollination by the western honey bee Apis mellifera is vital to agriculture but threatened by alarmingly high levels of colony mortality, especially in Europe and North America. Colony loss is due, in part, to the high viral loads of Deformed wing virus (DWV), transmitted by the ectoparasitic mite Varroa destructor, especially throughout the overwintering period of a honey bee colony. Covert DWV infection is commonplace and has been causally linked to precocious foraging, which itself has been linked to colony loss. Taking advantage of four brain transcriptome studies that unexpectedly revealed evidence of covert DWV-A infection, we set out to explore whether this effect is due to DWV-A mimicking naturally occurring changes in brain gene expression that are associated with behavioral maturation. Consistent with this hypothesis, we found that brain gene expression profiles of DWV-A infected bees resembled those of foragers, even in individuals that were much younger than typical foragers. In addition, brain transcriptional regulatory network analysis revealed a positive association between DWV-A infection and transcription factors previously associated with honey bee foraging behavior. Surprisingly, single-cell RNA-Sequencing implicated glia, not neurons, in this effect; there are relatively few glial cells in the insect brain and they are rarely associated with behavioral plasticity. Covert DWV-A infection also has been linked to impaired learning, which together with precocious foraging can lead to increased occurrence of infected bees from one colony mistakenly entering another colony, especially under crowded modern apiary conditions. These findings provide new insights into the mechanisms by which DWV-A affects honey bee health and colony survival.

Cell Lines for Honey Bee Virus Research

With ongoing colony losses driven in part by the Varroa mite and the associated exacerbation of the virus load, there is an urgent need to protect honey bees (Apis mellifera) from fatal levels of virus infection and from the non-target effects of insecticides used in agricultural settings. A continuously replicating cell line derived from the honey bee would provide a valuable tool for the study of molecular mechanisms of virus-host interaction, for the screening of antiviral agents for potential use within the hive, and for the assessment of the risk of current and candidate insecticides to the honey bee. However, the establishment of a continuously replicating honey bee cell line has proved challenging. Here, we provide an overview of attempts to establish primary and continuously replicating hymenopteran cell lines, methods (including recent results) of establishing honey bee cell lines, challenges associated with the presence of latent viruses (especially Deformed wing virus) in established cell lines and methods to establish virus-free cell lines. We also describe the potential use of honey bee cell lines in conjunction with infectious clones of honey bee viruses for examination of fundamental virology.

Dynamic evolution in the key honey bee pathogen deformed wing virus: Novel insights into virulence and competition using reverse genetics

The impacts of invertebrate RNA virus population dynamics on virulence and infection outcomes are poorly understood. Deformed wing virus (DWV), the main viral pathogen of honey bees, negatively impacts bee health, which can lead to colony death. Despite previous reports on the reduction of DWV diversity following the arrival of the parasitic mite Varroa destructor, the key DWV vector, we found high genetic diversity of DWV in infested United States honey bee colonies. Phylogenetic analysis showed that divergent US DWV genotypes are of monophyletic origin and were likely generated as a result of diversification after a genetic bottleneck. To investigate the population dynamics of this divergent DWV, we designed a series of novel infectious cDNA clones corresponding to coexisting DWV genotypes, thereby devising a reverse-genetics system for an invertebrate RNA virus quasispecies. Equal replication rates were observed for all clone-derived DWV variants in single infections. Surprisingly, individual clones replicated to the same high levels as their mixtures and even the parental highly diverse natural DWV population, suggesting that complementation between genotypes was not required to replicate to high levels. Mixed clone–derived infections showed a lack of strong competitive exclusion, suggesting that the DWV genotypes were adapted to coexist. Mutational and recombination events were observed across clone progeny, providing new insights into the forces that drive and constrain virus diversification. Accordingly, our results suggest that Varroa influences DWV dynamics by causing an initial selective sweep, which is followed by virus diversification fueled by negative frequency-dependent selection for new genotypes. We suggest that this selection might reflect the ability of rare lineages to evade host defenses, specifically antiviral RNA interference (RNAi). In support of this hypothesis, we show that RNAi induced against one DWV strain is less effective against an alternate strain from the same population.

Deformed wing virus, a key pathogen of honey bees, shows rapid diversification after genetic bottlenecks; a novel reverse-genetic system provides insights into the forces that shape virus diversity, suggesting that virus quasi-species diversification may be driven by selection of genotypes capable of evading host RNAi defences.

Identification of new viral variants specific to the honey bee mite Varroa destructor

Large-scale colony losses among managed Western honey bees have become a serious threat to the beekeeping industry in the last decade. Multiple factors contribute to these losses, but the impact of Varroa destructor parasitism is by far the most important, along with the contribution of some pathogenic viruses vectored by the mite. So far, more than 20 viruses have been identified infecting the honey bee, most of them RNA viruses. They may be maintained either as covert infections or causing severe symptomatic infections, compromising the viability of the colony. In silico analysis of available transcriptomic data obtained from mites collected in the USA and Europe, as well as additional investigation with new samples collected locally, allowed the description of three RNA viruses, two of them variants of the previously described VDV-2 and VDV-3 and the other a new species reported here for the first time. Our results showed that these viruses were widespread among samples and that they were present in the mites as well as in the bees but with differences in the relative abundance and prevalence. However, we have obtained strong evidence showing that these three viruses were able to replicate in the mite, but not in the bee, suggesting that they are selectively infecting the mite. This opens the door to future applications that may help controlling the mite through biological control approaches.

Does viral load alter behavior of the bee parasite Varroa destructor?

The invasive mite Varroa destructor has negatively impacted global apiculture, by being a vector for many viruses of the honey bee (Apis mellifera). Until now, most studies have been limited to varroa-honey bee or virus-honey bee interactions. The aim of this study is to bridge the important research gap of varroa-virus interactions by correlating varroa behavior with viral load. Ten-minute video recordings of 200 varroa mites were analyzed, and average speeds of the mites were compared to individual qPCR viral loads for deformed wing virus (DWV) and sacbrood virus (SBV). Statistically significant models reveal that colony, DWV, and SBV all might play a role in mite behavior, suggesting that the varroa-virus interaction needs to be an integral part of future studies on honey bee pathogens.

Disentangling host-parasite-pathogen interactions in a varroa-resistant honeybee population reveals virus tolerance as an independent, naturally adapted survival mechanism

The ectoparasitic mite, Varroa destructor, is unarguably the leading cause of honeybee (Apis mellifera) mortality worldwide through its role as a vector for lethal viruses, in particular, strains of the Deformed wing virus (DWV) and Acute bee paralysis virus (ABPV) complexes. This multi-level system of host-parasite-pathogen interactions makes it difficult to investigate effects of either the mite or the virus on natural host survival. The aim of this study was to remove confounding effects of varroa to examine the role of virus susceptibility in the enhanced survival of a naturally adapted Swedish mite-resistant (MR) honeybee population, relative to mite-susceptible (MS) honeybees. Caged adult bees and laboratory reared larvae, from varroa-free colonies, were inoculated with DWV and ABPV in a series of feeding infection experiments, while control groups received virus-free food. Virus infections were monitored using RT-qPCR assays in individuals sampled over a time course. In both adults and larvae the DWV and ABPV infection dynamics were nearly identical between MR and MS groups, but MS adults suffered significantly higher mortality than MR adults. Results suggest virus tolerance, rather than reduced susceptibility or virus resistance, is an important component of the natural survival of this honeybee population.

Genetic Diversity of Deformed Wing Virus From Apis mellifera carnica (Hymenoptera: Apidae) and Varroa Mite (Mesostigmata: Varroidae)

Deformed wing virus (DWV) is one of the most widespread viruses that infect honey bee colonies. The route of infection is directly through contaminated food, feces, and air, or indirectly through the varroa mite, which acts as a vector. Positive DWV samples were obtained from Carniolan honey bee (Apis mellifera carnica) colonies and of varroa mites from the whole territory of Slovenia during a survey between 2007 and 2014. Nucleotide sequences of 471 nucleotides for the L protein gene and 573 nucleotides for the helicase gene were compared. High genetic diversity was observed among these Slovenian Carniolan honey bee DWV field samples, as well as with almost all the strains previously found in other countries in Europe. Phylogenetic analyses in two regions of the viral genome show that several of the DWV strains obtained from honey bees and varroa are genetically very closely related, confirming the important role of varroa in the transmission of DWV. Identification of closely related sequences also confirmed that the same strains of DWV have been successfully transmitted between various honey bee colonies and apiaries. It has also been established that simultaneous infection, in one apiary, of honey bees with two or more different strains of DWV is quite frequent. This is phylogenetic study that compares honey bee and varroa DWV strains from Carniolan honeybees.

Extracts of Polypore Mushroom Mycelia Reduce Viruses in Honey Bees

Waves of highly infectious viruses sweeping through global honey bee populations have contributed to recent declines in honey bee health. Bees have been observed foraging on mushroom mycelium, suggesting that they may be deriving medicinal or nutritional value from fungi. Fungi are known to produce a wide array of chemicals with antimicrobial activity, including compounds active against bacteria, other fungi, or viruses. We tested extracts from the mycelium of multiple polypore fungal species known to have antiviral properties. Extracts from amadou (Fomes) and reishi (Ganoderma) fungi reduced the levels of honey bee deformed wing virus (DWV) and Lake Sinai virus (LSV) in a dose-dependent manner. In field trials, colonies fed Ganoderma resinaceum extract exhibited a 79-fold reduction in DWV and a 45,000-fold reduction in LSV compared to control colonies. These findings indicate honey bees may gain health benefits from fungi and their antimicrobial compounds.

A Diverse Range of Novel RNA Viruses in Geographically Distinct Honey Bee Populations

Understanding the diversity and consequences of viruses present in honey bees is critical for maintaining pollinator health and managing the spread of disease. The viral landscape of honey bees (Apis mellifera) has changed dramatically since the emergence of the parasitic mite Varroa destructor, which increased the spread of virulent variants of viruses such as deformed wing virus. Previous genomic studies have focused on colonies suffering from infections by Varroa and virulent viruses, which could mask other viral species present in honey bees, resulting in a distorted view of viral diversity. To capture the viral diversity within colonies that are exposed to mites but do not suffer the ultimate consequences of the infestation, we examined populations of honey bees that have evolved naturally or have been selected for resistance to Varroa This analysis revealed seven novel viruses isolated from honey bees sampled globally, including the first identification of negative-sense RNA viruses in honey bees. Notably, two rhabdoviruses were present in three geographically diverse locations and were also present in Varroa mites parasitizing the bees. To characterize the antiviral response, we performed deep sequencing of small RNA populations in honey bees and mites. This provided evidence of a Dicer-mediated immune response in honey bees, while the viral small RNA profile in Varroa mites was novel and distinct from the response observed in bees. Overall, we show that viral diversity in honey bee colonies is greater than previously thought, which encourages additional studies of the bee virome on a global scale and which may ultimately improve disease management.IMPORTANCE Honey bee populations have become increasingly susceptible to colony losses due to pathogenic viruses spread by parasitic Varroa mites. To date, 24 viruses have been described in honey bees, with most belonging to the order Picornavirales Collapsing Varroa-infected colonies are often overwhelmed with high levels of picornaviruses. To examine the underlying viral diversity in honey bees, we employed viral metatranscriptomics analyses on three geographically diverse Varroa-resistant populations from Europe, Africa, and the Pacific. We describe seven novel viruses from a range of diverse viral families, including two viruses that are present in all three locations. In honey bees, small RNA sequences indicate that these viruses are processed by Dicer and the RNA interference pathway, whereas Varroa mites produce strikingly novel small RNA patterns. This work increases the number and diversity of known honey bee viruses and will ultimately contribute to improved disease management in our most important agricultural pollinator.

Are Dispersal Mechanisms Changing the Host-Parasite Relationship and Increasing the Virulence of Varroa destructor (Mesostigmata: Varroidae) in Managed Honey Bee (Hymenoptera: Apidae) Colonies?

Varroa (Varroa destructor Anderson and Trueman) are a serious pest of European honey bees (Apis mellifera L.), and difficult to control in managed colonies. In our 11-mo longitudinal study, we applied multiple miticide treatments, yet mite numbers remained high and colony losses exceeded 55%. High mortality from varroa in managed apiaries is a departure from the effects of the mite in feral colonies where bees and varroa can coexist. Differences in mite survival strategies and dispersal mechanisms may be contributing factors. In feral colonies, mites can disperse through swarming. In managed apiaries, where swarming is reduced, mites disperse on foragers robbing or drifting from infested hives. Using a honey bee-varroa population model, we show that yearly swarming curtails varroa population growth, enabling colony survival for >5 yr. Without swarming, colonies collapsed by the third year. To disperse, varroa must attach to foragers that then enter other hives. We hypothesize that stress from parasitism and virus infection combined with effects that viruses have on cognitive function may contribute to forager drift and mite and virus dispersal. We also hypothesize that drifting foragers with mites can measurably increase mite populations. Simulations initialized with field data indicate that low levels of drifting foragers with mites can create sharp increases in mite populations in the fall and heavily infested colonies in the spring. We suggest new research directions to investigate factors leading to mite dispersal on foragers, and mite management strategies with consideration of varroa as a migratory pest.

Relative abundance of deformed wing virus, Varroa destructor virus 1, and their recombinants in honey bees (Apis mellifera) assessed by kmer analysis of public RNA-Seq data

Deformed wing virus (DWV) is a major pathogen of concern to apiculture, and recent reports have indicated the local predominance and potential virulence of recombinants between DWV and a related virus, Varroa destructor virus 1 (VDV). However, little is known about the frequency and titer of VDV and recombinants relative to DWV generally. In this study, I assessed the relative occurrence and titer of DWV and VDV in public RNA-seq accessions of honey bee using a rapid, kmer-based approach. Three recombinant types were detectable graphically and corroborated by de novo assembly. Recombination breakpoints did not disrupt the capsid-encoding region, consistent with previous reports, and both VDV- and DWV-derived capsids were observed in recombinant backgrounds. High abundance of VDV kmers was largely restricted to recombinant forms. Non-metric multidimensional scaling identified genotypic clusters among DWV isolates, which was corroborated by read mapping and consensus generation. The recently described DWV-C lineage was not detected in the searched accessions. The data further highlight the utility of high-throughput sequencing to monitor viral polymorphisms and statistically test biological predictors of titer, and point to the need for consistent methodologies and sampling schemes.

Persistence of subclinical deformed wing virus infections in honeybees following Varroa mite removal and a bee population turnover

Deformed wing virus (DWV) is a lethal virus of honeybees (Apis mellifera) implicated in elevated colony mortality rates worldwide and facilitated through vector transmission by the ectoparasitic mite Varroa destructor. Clinical, symptomatic DWV infections are almost exclusively associated with high virus titres during pupal development, usually acquired through feeding by Varroa mites when reproducing on bee pupae. Control of the mite population, generally through acaricide treatment, is essential for breaking the DWV epidemic and minimizing colony losses. In this study, we evaluated the effectiveness of remedial mite control on clearing DWV from a colony. DWV titres in adult bees and pupae were monitored at 2 week intervals through summer and autumn in acaricide-treated and untreated colonies. The DWV titres in Apistan treated colonies was reduced 1000-fold relative to untreated colonies, which coincided with both the removal of mites and also a turnover of the bee population in the colony. This adult bee population turnover is probably more critical than previously realized for effective clearing of DWV infections. After this initial reduction, subclinical DWV titres persisted and even increased again gradually during autumn, demonstrating that alternative non-Varroa transmission routes can maintain the DWV titres at significant subclinical levels even after mite removal. The implications of these results for practical recommendations to mitigate deleterious subclinical DWV infections and improving honeybee health management are discussed.

A Comparison of Wolbachia Infection Frequencies in Varroa With Prevalence of Deformed Wing Virus

Wolbachia are widely distributed bacterial endosymbionts of arthropods and filarial nematodes. These bacteria can affect host fitness in a variety of ways, such as protecting hosts against viruses and other pathogens. Here, we investigate the possible role of Wolbachia in the prevalence of the deformed wing virus (DWV), a highly virulent pathogen of honey bees (Apis mellifera) that is transmitted by parasitic Varroa mites (Varroa destructor). About 180 Varroa mites from 18 beehives were tested for infection with Wolbachia and DWV. We first screened for Wolbachia using two standard primers (wsp and 16S rDNA), and found 26% of the mites to be positive for Wolbachia using the wsp primer and 64% of the mites to be positive using the 16S rDNA primer. Using these intermediate Wolbachia frequencies, we then tested for statistical correlations with virus infection frequencies. The analysis revealed a significant positive correlation between DWV and Wolbachia using the wsp primer, but no significant association between DWV and Wolbachia using the 16S rDNA primer. In conclusion, there is no evidence for an anti-pathogenic effect of Wolbachia in V. destructor, but weak evidence for a pro-pathogenic effect. These results encourage further examination of Wolbachia-virus interactions in Varroa mites since an increased vector competence of the mites may significantly impact disease outbreaks in honey bees.

Moku virus; a new Iflavirus found in wasps, honey bees and Varroa

There is an increasing global trend of emerging infectious diseases (EIDs) affecting a wide range of species, including honey bees. The global epidemic of the single stranded RNA Deformed wing virus (DWV), driven by the spread of Varroa destructor has been well documented. However, DWV is just one of many insect RNA viruses which infect a wide range of hosts. Here we report the full genome sequence of a novel Iflavirus named Moku virus (MV), discovered in the social wasp Vespula pensylvanica collected in Hawaii. The novel genome is 10,056 nucleotides long and encodes a polyprotein of 3050 amino acids. Phylogenetic analysis showed that MV is most closely related to Slow bee paralysis virus (SBPV), which is highly virulent in honey bees but rarely detected. Worryingly, MV sequences were also detected in honey bees and Varroa from the same location, suggesting that MV can also infect other hymenopteran and Acari hosts.

Optimal search patterns in honeybee orientation flights are robust against emerging infectious diseases

Lévy flights are scale-free (fractal) search patterns found in a wide range of animals. They can be an advantageous strategy promoting high encounter rates with rare cues that may indicate prey items, mating partners or navigational landmarks. The robustness of this behavioural strategy to ubiquitous threats to animal performance, such as pathogens, remains poorly understood. Using honeybees radar-tracked during their orientation flights in a novel landscape, we assess for the first time how two emerging infectious diseases (Nosema sp. and the Varroa-associated Deformed wing virus (DWV)) affect bees' behavioural performance and search strategy. Nosema infection, unlike DWV, affected the spatial scale of orientation flights, causing significantly shorter and more compact flights. However, in stark contrast to disease-dependent temporal fractals, we find the same prevalence of optimal Lévy flight characteristics (μ ≈ 2) in both healthy and infected bees. We discuss the ecological and evolutionary implications of these surprising insights, arguing that Lévy search patterns are an emergent property of fundamental characteristics of neuronal and sensory components of the decision-making process, making them robust against diverse physiological effects of pathogen infection and possibly other stressors.

Varroa destructor (Mesostigmata: Varroidae) Parasitism and Climate Differentially Influence the Prevalence, Levels, and Overt Infections of Deformed Wing Virus in Honey Bees (Hymenoptera: Apidae)

The prevalence and loads of deformed wing virus (DWV) between honey bee (Apis mellifera L.) colonies from a tropical and a temperate environment were compared. The interaction between these environments and the mite Varroa destructor in relation to DWV prevalence, levels, and overt infections, was also analyzed. V. destructor rates were determined, and samples of mites, adult bees, brood parasitized with varroa mites and brood not infested by mites were analyzed. DWV was detected in 100% of the mites and its prevalence and loads in honey bees were significantly higher in colonies from the temperate climate than in colonies from the tropical climate. Significant interactions were found between climate and type of sample, with the highest levels of DWV found in varroa-parasitized brood from temperate climate colonies. Additionally, overt infections were observed only in the temperate climate. Varroa parasitism and DWV loads in bees from colonies with overt infections were significantly higher than in bees from colonies with covert infections. These results suggest that interactions between climate, V. destructor, and possibly other factors, may play a significant role in the prevalence and levels of DWV in honey bee colonies, as well as in the development of overt infections. Several hypotheses are discussed to explain these results.

Specific Cues Associated With Honey Bee Social Defence against Varroa destructor Infested Brood

Social immunity forms an essential part of the defence repertoire of social insects. In response to infestation by the parasitic mite Varroa destructor and its associated viruses, honey bees (Apis mellifera L.) have developed a specific behaviour (varroa-sensitive hygiene, or VSH) that helps protect the colony from this parasite. Brood cells heavily infested with mites are uncapped, the brood killed, and the cell contents removed. For this extreme sacrifice to be beneficial to the colony, the targeting of parasitized brood for removal must be accurate and selective. Here we show that varroa-infested brood produce uniquely identifiable cues that could be used by VSH-performing bees to identify with high specificity which brood cells to sacrifice. This selective elimination of mite-infested brood is a disease resistance strategy analogous to programmed cell death, where young bees likely to be highly dysfunctional as adults are sacrificed for the greater good of the colony.

Diversity in a honey bee pathogen: first report of a third master variant of the Deformed Wing Virus quasispecies

Treatment of emerging RNA viruses is hampered by the high mutation and replication rates that enable these viruses to operate as a quasispecies. Declining honey bee populations have been attributed to the ectoparasitic mite Varroa destructor and its affiliation with Deformed Wing Virus (DWV). In the current study we use next-generation sequencing to investigate the DWV quasispecies in an apiary known to suffer from overwintering colony losses. We show that the DWV species complex is made up of three master variants. Our results indicate that a new DWV Type C variant is distinct from the previously described types A and B, but together they form a distinct clade compared with other members of the Iflaviridae. The molecular clock estimation predicts that Type C diverged from the other variants ∼319 years ago. The discovery of a new master variant of DWV has important implications for the positive identification of the true pathogen within global honey bee populations.

Occurrence of Deformed wing virus, Chronic bee paralysis virus and mtDNA variants in haplotype K of Varroa destructor mites in Syrian apiaries

A small-scale survey was conducted on 64 beehives located in four governorates of Syria in order to assess for the first time the presence of honeybee-infecting viruses and of Varroa destructor mites in the country. RT-PCR assays conducted on 192 honeybees (Apis mellifera L.) using virus-specific primers showed that Deformed wing virus (DWV) was present in 49 (25.5%) of the tested samples and Chronic bee paralysis virus (CBPV) in 2 (1.04%), whereas Acute bee paralysis virus, Sacbrood virus, Black queen cell virus and Kashmir bee virus were absent. Nucleotide sequences of PCR amplicons obtained from DWV and CBPV genomes shared 95-97 and 100% identity with isolates reported in the GenBank, respectively. The phylogenetic tree grouped the Syrian DWV isolates in one cluster, distinct from all those of different origins reported in the database. Furthermore, 19 adult V. destructor females were genetically analyzed by amplifying and sequencing four fragments in cytochrome oxidase subunit 1 (cox1), ATP synthase 6 (atp6), cox3 and cytochrome b (cytb) mitochondrial DNA (mtDNA) genes. Sequences of concatenated V. destructor mtDNA genes (2696 bp) from Syria were similar to the Korean (K) haplotype and were found recurrently in all governorates. In addition, two genetic lineages of haplotype K with slight variations (0.2-0.3%) were present only in Tartous and Al-Qunaitra governorates.

In-depth proteomic analysis of Varroa destructor: Detection of DWV-complex, ABPV, VdMLV and honeybee proteins in the mite

We investigated pathogens in the parasitic honeybee mite Varroa destructor using nanoLC-MS/MS (TripleTOF) and 2D-E-MS/MS proteomics approaches supplemented with affinity-chromatography to concentrate trace target proteins. Peptides were detected from the currently uncharacterized Varroa destructor Macula-like virus (VdMLV), the deformed wing virus (DWV)-complex and the acute bee paralysis virus (ABPV). Peptide alignments revealed detection of complete structural DWV-complex block VP2-VP1-VP3, VDV-1 helicase and single-amino-acid substitution A/K/Q in VP1, the ABPV structural block VP1-VP4-VP2-VP3 including uncleaved VP4/VP2, and VdMLV coat protein. Isoforms of viral structural proteins of highest abundance were localized via 2D-E. The presence of all types of capsid/coat proteins of a particular virus suggested the presence of virions in Varroa. Also, matches between the MWs of viral structural proteins on 2D-E and their theoretical MWs indicated that viruses were not digested. The absence/scarce detection of non-structural proteins compared with high-abundance structural proteins suggest that the viruses did not replicate in the mite; hence, virions accumulate in the Varroa gut via hemolymph feeding. Hemolymph feeding also resulted in the detection of a variety of honeybee proteins. The advantages of MS-based proteomics for pathogen detection, false-positive pathogen detection, virus replication, posttranslational modifications, and the presence of honeybee proteins in Varroa are discussed.

Genome Characterization, Prevalence and Distribution of a Macula-Like Virus from Apis mellifera and Varroa destructor

Around 14 distinct virus species-complexes have been detected in honeybees, each with one or more strains or sub-species. Here we present the initial characterization of an entirely new virus species-complex discovered in honeybee (Apis mellifera L.) and varroa mite (Varroa destructor) samples from Europe and the USA. The virus has a naturally poly-adenylated RNA genome of about 6500 nucleotides with a genome organization and sequence similar to the Tymoviridae (Tymovirales; Tymoviridae), a predominantly plant-infecting virus family. Literature and laboratory analyses indicated that the virus had not previously been described. The virus is very common in French apiaries, mirroring the results from an extensive Belgian survey, but could not be detected in equally-extensive Swedish and Norwegian bee disease surveys. The virus appears to be closely linked to varroa, with the highest prevalence found in varroa samples and a clear seasonal distribution peaking in autumn, coinciding with the natural varroa population development. Sub-genomic RNA analyses show that bees are definite hosts, while varroa is a possible host and likely vector. The tentative name of Bee Macula-like virus (BeeMLV) is therefore proposed. A second, distantly related Tymoviridae-like virus was also discovered in varroa transcriptomes, tentatively named Varroa Tymo-like virus (VTLV).

Honey Bee Infecting Lake Sinai Viruses

Honey bees are critical pollinators of important agricultural crops. Recently, high annual losses of honey bee colonies have prompted further investigation of honey bee infecting viruses. To better characterize the recently discovered and very prevalent Lake Sinai virus (LSV) group, we sequenced currently circulating LSVs, performed phylogenetic analysis, and obtained images of LSV2. Sequence analysis resulted in extension of the LSV1 and LSV2 genomes, the first detection of LSV4 in the US, and the discovery of LSV6 and LSV7. We detected LSV1 and LSV2 in the Varroa destructor mite, and determined that a large proportion of LSV2 is found in the honey bee gut, suggesting that vector-mediated, food-associated, and/or fecal-oral routes may be important for LSV dissemination. Pathogen-specific quantitative PCR data, obtained from samples collected during a small-scale monitoring project, revealed that LSV2, LSV1, Black queen cell virus (BQCV), and Nosema ceranae were more abundant in weak colonies than strong colonies within this sample cohort. Together, these results enhance our current understanding of LSVs and illustrate the importance of future studies aimed at investigating the role of LSVs and other pathogens on honey bee health at both the individual and colony levels.

Differential viral levels and immune gene expression in three stocks of Apis mellifera induced by different numbers of Varroa destructor

The viral levels and immune responses of Italian honey bees (IHB), Russian honey bees (RHB) and an outcross of Varroa Sensitive Hygienic bees (POL) deliberately infested with one or two foundress Varroa were compared. We found that the Deformed wing virus (DWV) level in IHB inoculated with one or two foundress Varroa increased to about 10(3) or 10(5) fold the levels of their uninfested brood. In contrast, POL (10(2) or 10(4) fold) and RHB (10(2) or l0(4) fold) supported a lower increase in DWV levels. The feeding of different stages of Varroa nymphs did not increase DWV levels of their pupal hosts. Analyses of their corresponding Varroa mites showed the same trends: two foundress Varroa yielded higher DWV levels than one foundress, and the addition of nymphs did not increase viral levels. Using the same pupae examined for the presence of viruses, 16 out of 24 genes evaluated showed significant differential mRNA expression levels among the three honey bee stocks. However, only four genes (Defensin, Dscam, PPOact and spaetzle), which were expressed at similar levels in uninfested pupae, were altered by the number of feeding foundress Varroa and levels of DWV regardless of stocks. This research provides the first evidence that immune response profiles of different honey bee stocks are induced by Varroa parasitism.

A virulent strain of deformed wing virus (DWV) of honeybees (Apis mellifera) prevails after Varroa destructor-mediated, or in vitro, transmission

The globally distributed ectoparasite Varroa destructor is a vector for viral pathogens of the Western honeybee (Apis mellifera), in particular the Iflavirus Deformed Wing Virus (DWV). In the absence of Varroa low levels DWV occur, generally causing asymptomatic infections. Conversely, Varroa-infested colonies show markedly elevated virus levels, increased overwintering colony losses, with impairment of pupal development and symptomatic workers. To determine whether changes in the virus population were due Varroa amplifying and introducing virulent virus strains and/or suppressing the host immune responses, we exposed Varroa-naïve larvae to oral and Varroa-transmitted DWV. We monitored virus levels and diversity in developing pupae and associated Varroa, the resulting RNAi response and transcriptome changes in the host. Exposed pupae were stratified by Varroa association (presence/absence) and virus levels (low/high) into three groups. Varroa-free pupae all exhibited low levels of a highly diverse DWV population, with those exposed per os (group NV) exhibiting changes in the population composition. Varroa-associated pupae exhibited either low levels of a diverse DWV population (group VL) or high levels of a near-clonal virulent variant of DWV (group VH). These groups and unexposed controls (C) could be also discriminated by principal component analysis of the transcriptome changes observed, which included several genes involved in development and the immune response. All Varroa tested contained a diverse replicating DWV population implying the virulent variant present in group VH, and predominating in RNA-seq analysis of temporally and geographically separate Varroa-infested colonies, was selected upon transmission from Varroa, a conclusion supported by direct injection of pupae in vitro with mixed virus populations. Identification of a virulent variant of DWV, the role of Varroa in its transmission and the resulting host transcriptome changes furthers our understanding of this important viral pathogen of honeybees.

Honeybees are the most important managed pollinating insect, contributing billions of dollars to annual global agricultural production. Over the last century a parasitic mite, Varroa, has spread worldwide, with significant impacts on honeybee colony health as a consequence of its transmission of a cocktail of viruses while feeding on honeybee ‘blood’. The most important virus for colony health is deformed wing virus (DWV), high levels of which cause developmental deformities and premature ageing resulting in high overwintering colony losses. In experiments on individual Varroa-exposed pupae we demonstrate that a single type of virulent DWV is amplified 1,000–10,000 times in the recipient pupae, despite the mite containing a high diversity of replicating DWV strains. We could recapitulate this by direct injection of pupae with mixed virus populations, showing the virulent strain is advantaged by the route of transmission. In parallel, we detected changes in the immune response and developmental gene expression of the honeybee and propose that these contribute to the characteristic pathogenesis of DWV. Identification of a virulent strain of DWV has implications for therapeutic or prophylactic interventions to improve honeybee colony health, as well as contributing to our understanding of the biology of this important honeybee viral pathogen.

Sequence recombination and conservation of Varroa destructor virus-1 and deformed wing virus in field collected honey bees (Apis mellifera)

We sequenced small (s) RNAs from field collected honeybees (Apis mellifera) and bumblebees (Bombuspascuorum) using the Illumina technology. The sRNA reads were assembled and resulting contigs were used to search for virus homologues in GenBank. Matches with Varroadestructor virus-1 (VDV1) and Deformed wing virus (DWV) genomic sequences were obtained for A. mellifera but not B. pascuorum. Further analyses suggested that the prevalent virus population was composed of VDV-1 and a chimera of 5’-DWV-VDV1-DWV-3’. The recombination junctions in the chimera genomes were confirmed by using RT-PCR, cDNA cloning and Sanger sequencing. We then focused on conserved short fragments (CSF, size > 25 nt) in the virus genomes by using GenBank sequences and the deep sequencing data obtained in this study. The majority of CSF sites confirmed conservation at both between-species (GenBank sequences) and within-population (dataset of this study) levels. However, conserved nucleotide positions in the GenBank sequences might be variable at the within-population level. High mutation rates (Pi>10%) were observed at a number of sites using the deep sequencing data, suggesting that sequence conservation might not always be maintained at the population level. Virus-host interactions and strategies for developing RNAi treatments against VDV1/DWV infections are discussed.

Dynamics of persistent and acute deformed wing virus infections in honey bees, Apis mellifera

The dynamics of viruses are critical to our understanding of disease pathogenesis. Using honey bee Deformed wing virus (DWV) as a model, we conducted field and laboratory studies to investigate the roles of abiotic and biotic stress factors as well as host health conditions in dynamics of virus replication in honey bees. The results showed that temperature decline could lead to not only significant decrease in the rate for pupae to emerge as adult bees, but also an increased severity of the virus infection in emerged bees, partly explaining the high levels of winter losses of managed honey bees, Apis mellifera, around the world. By experimentally exposing adult bees with variable levels of parasitic mite Varroa destructor, we showed that the severity of DWV infection was positively correlated with the density and time period of Varroa mite infestation, confirming the role of Varroa mites in virus transmission and activation in honey bees. Further, we showed that host conditions have a significant impact on the outcome of DWV infection as bees that originate from strong colonies resist DWV infection and replication significantly better than bee originating from weak colonies. The information obtained from this study has important implications for enhancing our understanding of host‑pathogen interactions and can be used to develop effective disease control strategies for honey bees.