Tolerance of Honey Bees to Varroa Mite in the Absence of Deformed Wing Virus.

The global spread of the parasitic mite Varroa destructor has emphasized the significance of viruses as pathogens of honey bee (Apis mellifera) populations. In particular, the association of deformed wing virus (DWV) with V. destructor and its devastating effect on honey bee colonies has led to that virus now becoming one of the most well-studied insect viruses. However, there has been no opportunity to examine the effects of Varroa mites without the influence of DWV. In Papua New Guinea (PNG), the sister species, V. jacobsoni, has emerged through a host-shift to reproduce on the local A. mellifera population. After initial colony losses, beekeepers have maintained colonies without chemicals for more than a decade, suggesting that this bee population has an unknown mite tolerance mechanism. Using high throughput sequencing (HTS) and target PCR detection, we investigated whether the viral landscape of the PNG honey bee population is the underlying factor responsible for mite tolerance. We found A. mellifera and A. cerana from PNG and nearby Solomon Islands were predominantly infected by sacbrood virus (SBV), black queen cell virus (BQCV) and Lake Sinai viruses (LSV), with no evidence for any DWV strains. V. jacobsoni was infected by several viral homologs to recently discovered V. destructor viruses, but Varroa jacobsoni rhabdovirus-1 (ARV-1 homolog) was the only virus detected in both mites and honey bees. We conclude from these findings that A. mellifera in PNG may tolerate V. jacobsoni because the damage from parasitism is significantly reduced without DWV. This study also provides further evidence that DWV does not exist as a covert infection in all honey bee populations, and remaining free of this serious viral pathogen can have important implications for bee health outcomes in the face of Varroa.

Metagenomic analysis of Varroa-free Australian honey bees (Apis mellifera) shows a diverse Picornavirales virome.

The viral landscape of the honey bee (Apismellifera) has changed as a consequence of the global spread of the parasitic mite Varroa destructor and accompanying virulent strains of the iflavirus deformed wing virus (DWV), which the mite vectors. The presence of DWV in honey bee populations is known to influence the occurrence of other viruses, suggesting that the current known virome of A. mellifera may be undercharacterized. Here we tested this hypothesis by examining the honey bee virome in Australia, which is uniquely free of parasitic mites or DWV. Using a high-throughput sequencing (HTS) approach, we examined the RNA virome from nine pools of A. mellifera across Australia. In addition to previously reported honey bee viruses, several other insect viruses were detected, including strains related to aphid lethal paralysis virus (ALPV) and Rhopalosiphum padi virus (RhPV), which have recently been identified as infecting honey bees in the USA, as well as several other viruses recently found in Drosophila spp. A further 42 putative novel insect virus genomes spanning the order Picornavirales were assembled, which significantly increases the known viral diversity in A. mellifera. Among these novel genomes, we identified several that were similar (but different) to key A. mellifera viruses, such as DWV, that warrant further investigation. We propose that A. mellifera may be preferentially infected with viruses of the order Picornavirales and that a diverse population of these viruses may be representative of a Varroa-free landscape.

Absence of deformed wing virus and Varroa destructor in Australia provides unique perspectives on honeybee viral landscapes and colony losses.

Honeybee (Apis mellifera) health is threatened globally by the complex interaction of multiple stressors, including the parasitic mite Varroa destructor and a number of pathogenic viruses. Australia provides a unique opportunity to study this pathogenic viral landscape in the absence of V. destructor. We analysed 1,240A. mellifera colonies across Australia by reverse transcription-polymerase chain reaction (RT-PCR) and next-generation sequencing (NGS). Five viruses were prevalent: black queen cell virus (BQCV), sacbrood virus (SBV), Israeli acute paralysis virus (IAPV) and the Lake Sinai viruses (LSV1 and LSV2), of which the latter three were detected for the first time in Australia. We also showed several viruses were absent in our sampling, including deformed wing virus (DWV) and slow bee paralysis virus (SBPV). Our findings highlight that viruses can be highly prevalent in A. mellifera populations independently of V. destructor. Placing these results in an international context, our results support the hypothesis that the co-pathogenic interaction of V. destructor and DWV is a key driver of increased colony losses, but additional stressors such as pesticides, poor nutrition, etc. may enable more severe and frequent colony losses to occur.

Differential gene expression in Varroa jacobsoni mites following a host shift to European honey bees (Apis mellifera).

BACKGROUND: Varroa mites are widely considered the biggest honey bee health problem worldwide. Until recently, Varroa jacobsoni has been found to live and reproduce only in Asian honey bee (Apis cerana) colonies, while V. destructor successfully reproduces in both A. cerana and A. mellifera colonies. However, we have identified an island population of V. jacobsoni that is highly destructive to A. mellifera, the primary species used for pollination and honey production. The ability of these populations of mites to cross the host species boundary potentially represents an enormous threat to apiculture, and is presumably due to genetic variation that exists among populations of V. jacobsoni that influences gene expression and reproductive status. In this work, we investigate differences in gene expression between populations of V. jacobsoni reproducing on A. cerana and those either reproducing or not capable of reproducing on A. mellifera, in order to gain insight into differences that allow V. jacobsoni to overcome its normal species tropism. RESULTS: We sequenced and assembled a de novo transcriptome of V. jacobsoni. We also performed a differential gene expression analysis contrasting biological replicates of V. jacobsoni populations that differ in their ability to reproduce on A. mellifera. Using the edgeR, EBSeq and DESeq R packages for differential gene expression analysis, we found 287 differentially expressed genes (FDR ≤ 0.05), of which 91% were up regulated in mites reproducing on A. mellifera. In addition, mites found reproducing on A. mellifera showed substantially more variation in expression among replicates. We searched for orthologous genes in public databases and were able to associate 100 of these 287 differentially expressed genes with a functional description. CONCLUSIONS: There is differential gene expression between the two mite groups, with more variation in gene expression among mites that were able to reproduce on A. mellifera. A small set of genes showed reduced expression in mites on the A. mellifera host, including putative transcription factors and digestive tract developmental genes. The vast majority of differentially expressed genes were up-regulated in this host. This gene set showed enrichment for genes associated with mitochondrial respiratory function and apoptosis, suggesting that mites on this host may be experiencing higher stress, and may be less optimally adapted to parasitize it. Some genes involved in reproduction and oogenesis were also overexpressed, which should be further studied in regards to this host shift.

Further evidence of an oriental origin for Nosema ceranae (Microsporidia: Nosematidae).

Although Nosema ceranae was first isolated from the Asian honeybee (Apis cerana) in Asia and then subsequently recognized as a widespread gut parasite of the Western honeybee (Apis mellifera), its origins and primary host are yet to be accurately established. In this study we examined the possibility of an Asian origin for the parasite by looking for evidence of its ongoing spread out of Asia. To do this, we surveyed for the presence of N. ceranae in A. cerana and A. mellifera on isolated islands of the Solomon Islands (Pacific region), most of which were inhabited with A. mellifera that had been introduced from Australia and New Zealand at a time when N. ceranae was not present in either country, but on which some had also recently become inhabited with invasive A. cerana that originated from Asia with no prior history of contact with A. mellifera infected with N. ceranae. We also sought to verify previous findings that N. ceranae was widespread in Asian honeybees by surveying for its presence in isolated populations of the Asian honeybees, A. cerana, A. koschevnikovi, A. nigrocincta and A. florea. We obtained evidence that A. cerana introduced N. ceranae to A. mellifera in the Solomon Islands and also confirmed the widespread occurrence of the parasite in Asian honeybees, even reporting it for the first time in A. koschevnikovi from Borneo. Our findings provide further support for the hypothesis that N. ceranae has only recently emerged from Asia to become a parasite of A. mellifera.

Using simulation to evaluate time to detect incursions in honeybee biosecurity in Australia.

A key determinant of the efficiency of a surveillance system for exotic mites is whether an incursion might be detected sufficiently quickly to allow successful management actions to occur. To assess this possibility we have developed a spatial modeling system and synthesized knowledge of honeybee and mite behavior to explore the potential spread of exotic mites and the likelihood of their detection in sentinel hives. We find that increasing the number of hives and the efficiency of the detection method are the most effective means of improving the time to detection.

Genomic survey of the ectoparasitic mite Varroa destructor, a major pest of the honey bee Apis mellifera.

BACKGROUND: The ectoparasitic mite Varroa destructor has emerged as the primary pest of domestic honey bees (Apis mellifera). Here we present an initial survey of the V. destructor genome carried out to advance our understanding of Varroa biology and to identify new avenues for mite control. This sequence survey provides immediate resources for molecular and population-genetic analyses of Varroa-Apis interactions and defines the challenges ahead for a comprehensive Varroa genome project. RESULTS: The genome size was estimated by flow cytometry to be 565 Mbp, larger than most sequenced insects but modest relative to some other Acari. Genomic DNA pooled from ~1,000 mites was sequenced to 4.3× coverage with 454 pyrosequencing. The 2.4 Gbp of sequencing reads were assembled into 184,094 contigs with an N50 of 2,262 bp, totaling 294 Mbp of sequence after filtering. Genic sequences with homology to other eukaryotic genomes were identified on 13,031 of these contigs, totaling 31.3 Mbp. Alignment of protein sequence blocks conserved among V. destructor and four other arthropod genomes indicated a higher level of sequence divergence within this mite lineage relative to the tick Ixodes scapularis. A number of microbes potentially associated with V. destructor were identified in the sequence survey, including ~300 Kbp of sequence deriving from one or more bacterial species of the Actinomycetales. The presence of this bacterium was confirmed in individual mites by PCR assay, but varied significantly by age and sex of mites. Fragments of a novel virus related to the Baculoviridae were also identified in the survey. The rate of single nucleotide polymorphisms (SNPs) in the pooled mites was estimated to be 6.2 × 10-5 per bp, a low rate consistent with the historical demography and life history of the species. CONCLUSIONS: This survey has provided general tools for the research community and novel directions for investigating the biology and control of Varroa mites. Ongoing development of Varroa genomic resources will be a boon for comparative genomics of under-represented arthropods, and will further enhance the honey bee and its associated pathogens as a model system for studying host-pathogen interactions.

Predicting the economic impact of an invasive species on an ecosystem service.

Quantifying the impact of alien invasive species on ecosystem services is an essential step in developing effective practices and policy for invasive species management. Here we develop a stochastic bioeconomic model that enables the economic impact of an invasive pest to be estimated before its arrival, based on relatively poorly specified ecological and economic parameters. We developed the model by using a hypothetical invasion of the varroa bee mite (Varroa destructor) into Australia and the negative flow-on effects that it would have on pollination by reducing honey bee populations, giving rise to a loss of pollination services, reduced crop yields, and additional production costs. If the mite were to continue to be prevented from entering the country over the next 30 years, we estimate that the economic costs avoided would be U.S. $16.4-38.8 million (Aus $21.3-50.5 million) per year. We suggest that current invasion response funding arrangements in Australia, which do not acknowledge these avoided damages, require amendment.

Genetic and morphological variation of bee-parasitic Tropilaelaps mites (Acari: Laelapidae): new and re-defined species.

Mites in the genus Tropilaelaps are parasites of social honeybees. Two species, Tropilaelaps clareae and T. koenigerum, have been recorded and their primary hosts are presumed to be the giant honeybees of Asia, Apis dorsata and A. laboriosa. The most common species, T. clareae, is also an economically important pest of the introduced Western honeybee (A. mellifera) throughout Asia and is considered an emerging threat to world apiculture. In the studies reported here, genetic (mtDNA CO-I and nuclear ITS1-5.8S-ITS2 gene sequence) and morphological variation and host associations were examined among Tropilaelaps isolates collected from A. dorsata, A. laboriosa and A. mellifera throughout Asia and neighbouring regions. The results clearly indicate that the genus contains at least four species. Tropilaelaps clareae, previously assumed to be ubiquitous in Asia, was found to be two species, and it is here redefined as encompassing haplotypes (mites with distinct mtDNA gene sequences) that parasitise native A. dorsata breviligula and introduced A. mellifera in the Philippines and also native A. d. binghami on Sulawesi Island in Indonesia. Tropilaelaps mercedesae n. sp., which until now has been mistaken for T. clareae, encompasses haplotypes that, together with haplotypes of T. koenigerum, parasitise native A. d. dorsata in mainland Asia and Indonesia (except Sulawesi Island). It also parasitises introduced A. mellifera in these and surrounding regions and, with another new species, T. thaii n. sp., also parasitises A. laboriosa in mountainous Himalayan regions. Methods are described for identifying each species. These studies help to clarify the emerging threat of Tropilaelaps to world apiculture and will necessitate a revision of quarantine protocols for countries that import and export honeybees.

The invasive Korea and Japan types of Varroa destructor, ectoparasitic mites of the Western honeybee (Apis mellifera), are two partly isolated clones.

Varroa destructor, now a major pest of the Western honeybee, Apis mellifera, switched from its original host, the Eastern honeybee, A. cerana, ca. 50 years ago. So far, only two out of several known mitochondrial haplotypes of V. destructor have been found to be capable of reproducing on A. mellifera (Korea and Japan). These haplotypes are associated in almost complete cytonuclear disequilibrium to diagnostic alleles at 11 microsatellite loci. By contrast, microsatellite polymorphism within each type is virtually absent, because of a severe bottleneck at the time of host change. Accordingly, 12 mitochondrial sequences of 5185 nucleotides displayed 0.40% of nucleotide divergence between haplotypes and no intra haplotype variation. Hence, each type has a quasi-clonal structure. The nascent intratype variability is subsequent to the clone formation 50 years ago: in both types the variant alleles differ from the most common by one (in 10 cases), two (five cases) or three (one case) repeated motifs. In addition to individuals of the two 'pure' types, five F1 hybrids and 19 recombinant individuals (Japan alleles introgressed into the Korea genetic background) were detected. The existence of F1 and recombinant individuals in admixed populations requires that double infestations of honeybee cells occur in a high proportion but the persistence of pure types suggests a post-zygotic isolation between the two clones.