A study to be published in Science on Friday has found that the spread of a disease wiping out millions of honeybees is manmade and driven by the European populations of the honeybee Apis mellifera.
The combination of the Deformed Wing Virus (DWV) and the Varroa mite, which feed on bee larvae, is a particularly devastating threat to global bee populations.
The team contacted beekeepers globally to collect honeybee and Varroa samples. They then sequenced DWV from these samples and combined their results with publically available information to determine the virus’ global spread. They found that the virus mainly spread from Europe to North America, Australia and New Zealand. They also found two-way movement of the disease between Europe and Asia, but none between Asia and Australasia, despite the latter being much closer to each other.
We spoke with the senior author of the study Lena Wilfert, Senior Lecturer in Molecular Evolution at the University of Exeter.
ResearchGate: What motivated you to study this?
Lena Wilfert: It has been clear for a while that the Varroa mite, which emerged as a new ectoparasite of European honeybees a century ago, has dramatically changed the host-parasite dynamics of honeybees and DWV. When Varroa is present, this otherwise harmless virus can cause honeybee hives to die over winter. We wanted to know how the virus and the mite had spread globally and which populations were the main drivers of the global spread.
Honeybees and wild pollinators are crucial for producing crops that require pollination, including much of the fruit and vegetables that make our diet both healthy and tasty, and for maintaining biodiversity of wild flowering plants. Maintaining both their numbers and their diversity is thus really important. Habitat loss and environmental stress play a major role for wild pollinators, but infectious diseases can also play an important role, especially in the domesticated honeybees. Many viral diseases are multi-host pathogens that can infect both honeybees and wild pollinators such as bumblebees or solitary bees.
RG: Can you explain how Deformed Wing Virus and the Varroa mite threaten bees? How much damage have they already done to the bee population?
LW: Varroa mites are ectoparasites that feed on the hemolymph or ‘blood’ of honeybee larvae; they can thus pick up the virus from infected larvae and transfer it by injecting it straight into the body cavity (the ‘bloodstream’) of the bees. In combination, this leads to a higher prevalence of DWV and higher levels of the virus load in individuals, causing the deformed wings that give the virus its name in infected larvae. More importantly, this has been linked to an increase in the winter mortality of colonies, which is a problem troubling beekeepers.
RG: How did you design your study?
LW: We contacted beekeepers globally to obtain honeybee and Varroa samples. We screened and sequenced DWV from these samples and complemented this with all publicly available data as well as compiling all available biogeographic data on the spread of Varroa.
RG: What were your results?
LW: In a nutshell, we found that DWV is a re-emerging disease, fueled by the emergence of Varroa. Their spread is manmade and the main drivers of the world-wide epidemic of DWV are European populations of the European honeybee Apis mellifera.
RG: What does your study mean for honey production?
LW: Actually, not much at all - honey production is a very local affair, and we are mainly worried about long-distance movement e.g. between countries and continents. It is important that beekeepers keep up the good job they are already doing – controlling Varroa in their hives and adopting best practices to prevent the spread of diseases between their hives and other apiaries, and by extension to wild pollinators.
RG: How does knowing that this is a manmade problem help us solve it? What steps can be taken?
LW: Locally, we need to keep encouraging beekeepers to control for Varroa and to use best practice to prevent disease spread between hives and other apiaries. By doing this, beekeepers are also the stewards of wild pollinators. This, for example, means not feeding bees with pollen that has not been treated against pathogens with irradiation.
At a larger level, it is important that the existing regulations on the international transport of bees are upheld - this is not a question of ‘the genie’s out of the bottle, nothing matters any longer.’
RG: Can bees recover?
LW: In the short term, we will not be able to eradicate Varroa nor to breed resistant bees. With good Varroa control, the situation can be kept under control in individual hives, and further spread can be minimized if not completely avoided.
RG: If we lost bees what kind of effect would this have?
LW: I would like to stress that this is not a scenario that we currently envisage at a large scale, although this has already been reported in some areas of China with extreme levels of environmental stress and pollution. There is resilience within the pollinator community – many flowers and crops can be pollinated by a range of pollinators including honeybees and wild pollinators. Managed honeybees have the huge advantage of providing pollination services where and when we need them, and of course providing honey and other useful materials. Much of our current crop production, notably pollination of mass flowering crops such as almonds, would have to change dramatically if honeybees were not available.
We have two issues here: maintaining the health of managed honeybees and maintaining biodiversity and population strength in wild pollinators. Together they are needed for the production of many fruits and vegetables and for maintaining biodiversity and ecosystems with wild flowering plants.
Study:
"Deformed wing virus is a recent global epidemic in honeybees driven by Varroamites", is published in Science on Friday February 5, by L.Wilfert, G Long, H.C. Leggett, P Schmid-Hempel, R. Butlin, S.J.M Martin and M Boots.
Image courtesy of Umberto Salvagnin.
