Date Published: June 13, 2019
Publisher: Public Library of Science
Author(s): Carl Giuffre, Sharon R. Lubkin, David R. Tarpy, Olav Rueppell.
The invasive mite Varroa destructor has negatively impacted global apiculture, by being a vector for many viruses of the honey bee (Apis mellifera). Until now, most studies have been limited to varroa-honey bee or virus-honey bee interactions. The aim of this study is to bridge the important research gap of varroa-virus interactions by correlating varroa behavior with viral load. Ten-minute video recordings of 200 varroa mites were analyzed, and average speeds of the mites were compared to individual qPCR viral loads for deformed wing virus (DWV) and sacbrood virus (SBV). Statistically significant models reveal that colony, DWV, and SBV all might play a role in mite behavior, suggesting that the varroa-virus interaction needs to be an integral part of future studies on honey bee pathogens.
The invasive mite Varroa destructor has negatively impacted apiculture worldwide . Varroa experienced an evolutionary host-shift from the Asian honey bee (Apis cerana) to the European honey bee (Apis mellifera) as early as 1960 and has been strongly implicated for playing a role in Colony Collapse Disorder and reduced health of bees in general [1,2]. These ectoparasites go through two major phases in their life cycle—the reproductive and phoretic stages. During the reproductive stage, a single varroa female infests the cell of an immature honey bee (pupa), feeding on the hemolymph of the developing bee. In doing so, the parasite can directly vector several viruses within honey bee colonies [2–4]. During the phoretic stage, varroa mites emerge with the enclosed bee and continue to feed on adult honey bee hemolymph for sustenance  and continue to spread viral pathogens horizontally among nestmates . Honey bee colonies can exceed 50,000 bees, with only one female reproductive (the queen) . When a queen is infected with viral pathogens, the health of the entire colony can be compromised as she then has the potential to vertically transmit virus to her offspring through oviposition .
The behavioral and qPCR data were statistically analyzed using JMP Pro 11.0 (SAS Institute, Cary, NC, USA). Mites were tested for a suite of seven common honey bee viruses  although the collected mites only tested positive for DWV and SBV. Of the 200 total mites, 194 remained for statistical analysis after accounting for various technical errors or small representation in categories (Table 1). Of those 194, whose tracks are displayed in Fig 4A–4C, 120 tested negative for infection (Fig 4A). Colonies 1 and 2 were primarily uninfected, with only two mites from each colony testing positive for both DWV and SBV. In contrast, colonies 3 and 4 displayed a variety of infection patterns: uninfected (Fig 4A), infected with DWV only (Fig 4B), and infected with both DWV and SBV (Fig 4C).
BIC favors selection of Model C (smallest BIC), which suggests that mite viral load seems to be unrelated to mite behavior. At the colony level, phoretic mites may modify their behavior based on environmental conditions, such as bee population, brood availability, hive temperature, or even bee grooming. Although none of these variables were measured in the current study, environmental conditions clearly play a direct role in virus’ ability to spread, by impacting vector motility. It could be argued that colonies have different viral profiles responsible for these global effects. Unfortunately, four colonies were not sufficient to determine such effects.