Accelerated Varroa destructor population growth in honey bee (Apis mellifera) colonies is associated with visitation from non-natal bees.

A leading cause of managed honey bee colony mortality in the US, Varroa destructor populations typically exceed damaging levels in the fall. One explanation for rapid population increases is migration of mite carrying bees between colonies. Here, the degree to which bees from high and low mite donor colonies move between apiaries, and the effect visitation has on Varroa populations was monitored. More bees from low mite colonies (n = 37) were detected in receiver apiaries than bees from high mite colonies (n = 10, p < 0.001). Receiver colony Varroa population growth was associated with visitation by non-natal bees (p = 0.03), but not high mite bees alone (p = 0.19). Finally, colonies lacking robbing screens experienced faster Varroa population growth than screened neighbors (p = 0.01). Results indicate visiting non-natal bees may vector mites to receiver colonies. These results do not support the current two leading theories regarding mite immigration - the "mite bomb" theory (bees from high mite colonies emigrating to collapsing colonies), or the "robbing" theory (natal robbing bees return home with mites from collapsing colonies). Potential host-parasite effects to bee behavior, as well as important management implications both for Varroa treatment regimens and breeding Varroa resistant bees are discussed.

Survey-derived best management practices for backyard beekeepers improve colony health and reduce mortality.

Honey bee colony losses in the US have exceeded acceptable levels for at least a decade, leaving beekeepers in need of management practices to improve colony health and survival. Here, an empirical Best Management Practice (BMP) regimen was tested, comprised of the top four management practices associated with reduced colony mortality in backyard beekeeping operations according to Bee Informed Partnership Loss and Management survey results. Seven study locations were established across the US, and each location consisted of ten colonies treated according to empirical BMPs and ten according to average beekeeping practice. After 3 years, colonies treated according to empirical BMPs experienced reduced Varroa infestation, viral infection, and mortality compared to colonies managed with Average practices. In addition, BMP colonies produced more new colonies via splits. The colonies under Average practices were given chemical Varroa treatments only once per year, and thus spent more months above economic threshold of 3.0 mites/100 bees. Increased time spent above the economic threshold was significantly correlated to both increased viral infection and colony mortality. This study demonstrates the cumulative effects of management and colony health stressors over months and years, especially the dire importance of regular Varroa monitoring and management.