Recent spread of Varroa destructor virus-1, a honey bee pathogen, in the United States.

RNA viruses impact honey bee health and contribute to elevated colony loss rates worldwide. Deformed wing virus (DWV) and the closely related Varroa destructor virus-1 (VDV1), are the most widespread honey bee viruses. VDV1 is known to cause high rates of overwintering colony losses in Europe, however it was unknown in the United States (US). Using next generation sequencing, we identified VDV1 in honey bee pupae in the US. We tested 603 apiaries the US in 2016 and found that VDV1 was present in 66.0% of them, making it the second most prevalent virus after DWV, which was present in 89.4% of the colonies. VDV1 had the highest load in infected bees (7.45*1012  ± 1.62*1012 average copy number ± standard error) compared to other tested viruses, with DWV second (1.04*1012 ± 0.53*1012 ). Analysis of 75 colonies sourced in 2010 revealed that VDV1 was present in only 2 colonies (2.7%), suggesting its recent spread. We also detected newly emerged recombinants between the US strains of VDV1 and DWV. The presence of these recombinants poses additional risk, because similar VDV1-DWV recombinants constitute the most virulent honeybee viruses in the UK.