http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057540

Abstract

Varroa mites and viruses are the currently the high-profile suspects
in collapsing bee colonies. Therefore, seasonal variation in varroa
load and viruses (Acute-Kashmir-Israeli complex (AKI) and Deformed
Wing Virus (DWV)) were monitored in a year-long study. We investigated
the viral titres in honey bees and varroa mites from 23 colonies (15
apiaries) under three treatment conditions: Organic acids (11
colonies), pyrethroid (9 colonies) and untreated (3 colonies).
Approximately 200 bees were sampled every month from April 2011 to
October 2011, and April 2012. The 200 bees were split to 10 subsamples
of 20 bees and analysed separately, which allows us to determine the
prevalence of virus-infected bees. The treatment efficacy was often
low for both treatments. In colonies where varroa treatment reduced
the mite load, colonies overwintered successfully, allowing the mites
and viruses to be carried over with the bees into the next season. In
general, AKI and DWV titres did not show any notable response to the
treatment and steadily increased over the season from April to
October. In the untreated control group, titres increased most
dramatically. Viral copies were correlated to number of varroa mites.
Most colonies that collapsed over the winter had significantly higher
AKI and DWV titres in October compared to survivors. Only treated
colonies survived the winter. We discuss our results in relation to
the varroa-virus model developed by Stephen Martin.

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