Oldest Varroa tolerant honey bee population provides insight into the origins of the global decline of honey bees

The ecto-parasitic mite Varroa destructor has transformed the previously inconsequential Deformed Wing Virus (DWV) into the most important honey bee viral pathogen responsible for the death of millions of colonies worldwide. Naturally, DWV persists as a low level covert infection transmitted between nest-mates. It has long been speculated that Varroa via immunosuppression of the bees, activate a covert infection into an overt one. Here we show that despite Varroa feeding on a population of 20-40 colonies for over 30 years on the remote island of Fernando de Noronha, Brazil no such activation has occurred and DWV loads have remained at borderline levels of detection. This supports the alternative theory that for a new vector borne viral transmission cycle to start, an outbreak of an overt infection must first occur within the host. Therefore, we predict that this honey bee population is a ticking time-bomb, protected by its isolated position and small population size. This unique association between mite and bee persists due to the evolution of low Varroa reproduction rates. So the population is not adapted to tolerate Varroa and DWV, rather the viral quasi-species has simply not yet evolved the necessary mutations to produce a virulent variant

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

Varroa destructor reproduction and cell re-capping in mite-resistant Apis mellifera populations

Globalization has facilitated the spread of emerging pests such as the Varroa destructor mite, resulting in the near global distribution of the pest. In South African and Brazilian honey bees, mite-resistant colonies appeared within a decade; in Europe, mite-resistant colonies are rare, but several of these exhibited high levels of “re-capping” behavior. We studied re-capping in Varroa-naïve (UK/Australia) and Varroa-resistant (South Africa and Brazil) populations and found very low and very high levels, respectively, with the resistant populations targeting mite-infested cells. Furthermore, 54% of artificially infested A. m. capensis worker cells were removed after 10 days and 83% of the remaining infested cells were re-capped. Such targeted re-capping of drone cells did not occur. We propose that cell opening is a fundamental trait in mite-resistant populations and that re-capping is an accurate proxy for this behavior

The spread and evolution of RNA viruses among honey bees and the wider insect community with particular emphasis on deformed wing virus (DWV)

The honey bee is our most significant animal pollinator and is highly valued economically and culturally throughout the world. Unfortunately, over the last 30 years beekeepers across the globe have experienced significantly increased honey bee colony losses, especially in the Northern hemisphere. The spread of RNA viruses, most notably Deformed wing virus (DWV) in association with its vector, the ectoparasitic Varroa destructor mite are now widely accepted as a major factor in these colony losses. This has had a devastating effect on beekeepers and farmers alike. It has become apparent that many of these viral pathogens are generalists, able to infect many insect species. Although much work is currently ongoing in this field, there are areas where knowledge is still lacking, which could provide clues to protecting the bees in the long term. Firstly, I investigated whether there is a unique DWV variant responsible for causing the development of deformed wings by comparing DWV in deformed and asymptomatic bees. This revealed no consistent differences, and greater variation was seen between locations rather than phenotypes, indicating that there is no unique viral variant that induces deformity. Secondly, I disproved, by studying the oldest known Varroa-tolerant honey bee population, the long held theory that Varroa feeding activity induces activation of latent DWV, since these bees have long existed with Varroa yet still harbour low level, genetically diverse DWV infections, and have had no reported colony losses. Thirdly, I discovered two new RNA viruses in ants and wasps collected from apiaries, during a honey bee collection trip to Hawaii. Milololii virus was found to infect the Ghost ant Tapinoma melanocephalum. The other, Moku virus, was sequenced in high depth from yellowjacket wasps, Vespula pensylvanica, but was also, worryingly, at low levels in both Varroa and honey bees, suggesting that it has the potential to infect diverse hosts. Finally, I detected DWV in a range of species living in Hawaiian apiaries. DWV genetic profiles grouped by species rather than location, suggesting that variants may exist which are better adapted to replicate in different host species. Together these increase our understanding of the DWV – Varroa – honey bee nexus, expand our knowledge of the circulating virosphere within the apiary, and provide new insights into how DWV spreads beyond honey bees and into the wider insect community. Further benefit could now be gained from investigating whether DWV and other viruses detected in different arthropod species are true infections by using negative strand – specific RT-PCR to detect viral replication. Furthermore, it would be of great interest to use experimental infections to discover the nature of any pathogenicity of viruses in non - Apis hosts

Novel RNA virus genome discovered in Ghost ants (Tapinoma melanocephalum) from Hawaii

Here we report the full genome sequence of Milolii virus, a novel single stranded (positive sense) RNA virus discovered from Tapinoma melanocephalum ants in Hawaii. The genome is 10,475 nucleotides long encoding a polyprotein of 3304 amino acids

Covert deformed wing virus infections have long-term deleterious effects on honeybee foraging and survival

Several studies have suggested that covert stressors can contribute to bee colony declines. Here we provide a novel case study and show using radio-frequency identification (RFID) tracking technology that covert deformed wing virus (DWV) infections in adult honeybee workers seriously impact longterm foraging and survival under natural foraging conditions. In particular, our experiments show that adult workers injected with low doses of DWV experienced increased mortality rates, that DWV caused workers to start foraging at a premature age, and that the virus reduced the workers’ total activity span as foragers. Altogether, these results demonstrate that covert deformed wing virus infections have strongly deleterious effects on honey bee foraging and survival. These results are consistent with previous studies that suggested DWV to be an important contributor to the ongoing bee declines in Europe and the US. Overall, our study underlines the strong impact that covert pathogen infections can have on individual and group-level performance in bees

Cold case : the disappearance of Egypt bee virus, a fourth distinct master strain of deformed wing virus linked to honeybee mortality in 1970’s Egypt

In 1977, a sample of diseased adult honeybees (Apis mellifera) from Egypt was found to contain large amounts of a previously unknown virus, Egypt bee virus, which was subsequently shown to be serologically related to deformed wing virus (DWV). By sequencing the original isolate, we demonstrate that Egypt bee virus is in fact a fourth unique, major variant of DWV (DWV-D): more closely related to DWV-C than to either DWV-A or DWV-B. DWV-A and DWV-B are the most common DWV variants worldwide due to their close relationship and transmission by Varroa destructor. However, we could not find any trace of DWV-D in several hundred RNA sequencing libraries from a worldwide selection of honeybee, varroa and bumblebee samples. This means that DWV-D has either become extinct, been replaced by other DWV variants better adapted to varroa-mediated transmission, or persists only in a narrow geographic or host range, isolated from common bee and beekeeping trade routes

Deformed wing virus in honeybees and other insects

Deformed wing virus (DWV) has become the most well-known, widespread, and intensively studied insect pathogen in the world. Although DWV was previously present in honeybee populations, the arrival and global spread of a new vector, the ectoparasitic mite Varroa destructor, has dramatically altered DWV epidemiology. DWV is now the most prevalent virus in honeybees, with a minimum average of 55% of colonies/apiaries infected across 32 countries. Additionally, DWV has been detected in 65 arthropod species spanning eight insect orders and three orders of Arachnida. Here, we describe the significant progress that has been made in elucidating the capsid structure of the virus, understanding its ever-expanding host range, and tracking the constantly evolving DWV genome and formation of recombinants. The construction of molecular clones, working with DWV in cell lines, and the development of immunohistochemistry methods will all help the community to move forward. Identifying the tissues in which DWV variants are replicating and understanding the impact of DWV in non-honeybee hosts are major new goals