Diversity of Potential Resistance Mechanisms in Honey Bees (Apis mellifera) Selected for Low Population Growth of the Parasitic Mite, Varroa destructor

Honey bees (<i>Apis mellifera</i>) bred for resistance to the parasitic mite, <i>Varroa destructor</i>, were examined for potential <i>Varroa</i> resistance mechanisms following bidirectional selection for low (resistant) or high (susceptible) <i>Varroa</i> population growth (LVG and HVG, respectively) based on mite fall in colonies at two different time points. Hygienic and grooming behavior rates in LVG colonies were significantly higher than those in HVG colonies for two out of three generations of selection, indicating that behavioral resistance to the mite increased. For the third generation, grooming start time was significantly shorter, and grooming intensity more frequent in LVG bees than in HVG bees. Cellular immunity was increased as well, based on significantly higher haemocyte concentrations in non-parasitized and <i>Varroa</i>-parasitized LVG bees. Humoral immunity was increased with <i>Varroa</i>-parasitized LVG bees, which had significantly higher expression of the antimicrobial peptide gene, hymenoptaecin 2. In addition, antiviral resistance may be involved as there were significantly lower levels of deformed wing virus (DWV) in <i>Varroa</i>-parasitized LVG bees. While selection for LVG and HVG bees was solely based on <i>Varroa</i> population growth, it appears that behavioral, cellular, and humoral mechanisms were all selected along with this resistance. Thus, LVG resistance appears to be a multi-gene trait, involving multiple resistance mechanisms.