Prevalence and persistence of deformed wing virus (DWV) in untreated or acaricide-treated Varroa destructor infested honey bee (Apis mellifera) colonies

The ectoparasitic mite Varroa destructor is a serious pest of the honey bee Apis mellifera. The naturally occurring virus known as deformed wing virus (DWV) has long been linked with the collapse of mite infested honey bee colonies. We therefore surveyed the prevalence and persistence of DWV in four heavily infested untreated colonies (Survey 1), and five heavily infested colonies that were treated with an acaricide (Survey 2). The presence of DWV in samples of adult bees, sealed brood and mites was detected using an Enzyme Linked Immunosorbent Assay (ELISA). Twenty individuals of each type were sampled monthly from each colony over the course of the study. During the summer, the proportion of adults, sealed brood and mites in which DWV was detected increased until either the colony died or was treated. When colonies were treated, thus removing mites from the colony, DWV became undetectable in the sealed bee brood at a similar rate to the loss of mites. The speed at which DWV became undetectable in adult workers depended, however, on the season, reflecting differences in life span between adult workers emerging in summer or winter. If treatment was delayed until October, DWV was still detected in adult bees during the winter even in the absence of mites. To reduce the viral load of the colony, therefore, mite treatment should be started no later than the end of August in order to remove the mites before production of the overwintering bees begins.