Honey bees (Apis mellifera, L.) provide critical pollination services to many US crops, but decades of high colony loss rates have strained beekeepers’ ability to provide sufficient colonies for crop production. In a national survey of colony losses for the 2015-2016 season, beekeepers reported losses averaging at 37.4%, and that the parasitic mite Varroa destructor was a leading cause of mortality. Survey results were used to create empirical best management practices (BMPs) to reduce colony loss rates. Best practices were the top four practices which correlated to significant reductions in winter colony loss. This set of BMPs was tested on 140 colonies in 7 locations across the US, compared to average beekeeping practices. At the end of 3 years, apiaries managed according to BMPs exhibited reduced Varroa loads, which resulted in reduced fall viral loads and reduced winter mortality. However, colony loss rates still exceeded rates that beekeepers have deemed acceptable.

A prominent factor affecting colony health and mortality in the BMP study was Varroa. After identifying Varroa treatment as a preventative measure, the effects of Varroa management were evaluated in non-experimental apiaries. Citizen scientist beekeepers participating in the Sentinel Apiary Program provided Varroa samples and Varroa management information. Out of 192 Varroa treatments applied to 155 apiaries over 2 years, only 45 treatments resulted in reduced Varroa loads. Common hypotheses of factors affecting Varroa population growth failed to explain the rapid increases in Varroa loads experienced by beekeepers in critical fall months. Finally, a more novel explanation for rapid increases in Varroa load was explored: horizontal transmission of mites between apiaries. Colonies that were visited by non-natal bees experienced larger increases in Varroa loads than unvisited colonies, but not as a result of visitation to or from high mite colonies. High mite colonies in the landscape represent a threat to nearby colonies, and cooperative Varroa management is likely to mediate colony losses resulting from Varroa. This dissertation supports the critical need for proactive, cooperative Varroa management to improve colony health and reduce mortality.