Pathogenic associations with winter colony loss in Canada

Canadian beekeepers report that high pathogen/parasite infestation levels, poor queen quality and severe weather conditions are the leading causes of elevated wintering losses. In order to replenish annual losses or maintain their operations, beekeepers in Canada face a unique and difficult situation for purchasing new queens or package bees. Scarce local supply drives local producers to import approximately 300,000 queens and packages each year, predominantly from foreign sources. This large-scale importation of stock may contribute to the introduction of undesirable pathogens or genetics, and supply bees that have not been selected to survive and prosper in northern temperate climates, thereby influencing wintering success.

Honey bees act as a host for a multitude of pathogens and parasites. Nevertheless, the interactive effects that many of these pathogens, endoparasites and ectoparasites have on colony wintering success remains poorly understood. In order to better understand these interrelationships, we studied colony health and wintering success as a part of an ongoing national-scale study. In 2016 and 2017, we sampled 1025 and 520 colonies, respectively, across five Canadian provinces. During each experimental year (May through April), we collected pre-winter phenotypic data (fall colony weight and cluster size), and samples for pathogen analysis (Nosema spp., Lotmaria passim, DWV-A, DWV-B, BQCV, SBV, and phoretic loads of Varroa destructor) from colonies in all locations to investigate the main drivers of colony winter mortality. We were also able to study colonies wintered outdoors, as well as those wintered inside specialized wintering facilities. Although winter mortality was statistically similar between 2016 and 2017 (15% and 13%), preliminary results indicate that there is a difference between years with respect to the impact that individual pathogens and colony phenotypes have on colony winter survival. Nevertheless, in both experimental years, levels of DWV-A, fall phoretic mite loads, as well as fall colony weights and cluster sizes exhibited significant influences on colony winter survival outcomes. Additionally, we will illustrate the distribution and abundance of colony pathogens across honey bee populations in Canada.