Abstract
Many pathogens and parasites have evolved to overwhelm and suppress their host’s immune system. Nevertheless, the interactive effects of these agents on colony productivity and wintering success have been relatively unexplored, particularly in large-scale phenomic studies. As a defense mechanism, honey bees have evolved remarkable social behaviors to defend against pathogen and parasite challenges, which reduce the impact of disease and improve colony health. To investigate the complex role of pathogens, parasites and social immunity behaviors in relation to colony productivity and outcomes, we extensively studied colonies at several locations across Canada for two years. In 2016 and 2017, colonies founded with 1-year-old queens of diverse genetic origin were evaluated, which represented a generalized subset of the Canadian bee population. During each experimental year (May through April), we collected phenotypic data and sampled colonies for pathogen analysis in a standardized manner. Measures included: colony size and productivity (colony weight, cluster size, honey production, and sealed brood population), social immunity traits (hygienic behavior, instantaneous mite population growth rate, and grooming behavior), as well as quantification of gut parasites (Nosema spp., and Lotmaria passim), viruses (DWV-A, DWV-B, BQCV and SBV) and external parasites (Varroa destructor). Our goal was to examine: 1) correlations between pathogens and colony phenotypes; 2) the dynamics of pathogens and parasites on colony phenotypes and productivity traits; and 3) the effects of social immunity behaviors on colony pathogen load. Our results show that colonies expressing high levels of some social immunity behaviors were associated with low levels of pathogens/parasites, including viruses, Nosema spp., and V. destructor. In addition, we determined that elevated viral and Nosema spp. levels were associated with low levels of colony productivity, and that five out of six pathogenic factors measured were negatively associated with colony size and weight in both fall and spring periods. Finally, this study also provides information about the incidence and abundance of pathogens, colony phenotypes, and further disentangles their inter-correlation, so as to better understand drivers of honey bee colony health and productivity.
Highlights
Honey bees are an essential component of modern agriculture
Israeli Acute Paralysis Virus (IAPV) was only detected by quantitative PCR in a very small portion of the samples (3%, 1% and 2% in the fall of 2016, spring and fall of 2017, respectively); IAPV abundance data was not included in the analyses
Nosema spp. and L. passim levels were assessed once during each experimental year, in the fall, prior to any disease treatment being applied to the colony
Summary
Honey bees are an essential component of modern agriculture. In Canada alone, beekeepers produce approximately 40,000 tonnes of honey per year and provide $4–5.5 billion CDN in pollination services [1,2]. The worldwide economic value of pollination is estimated to be €153 billion [3], to which honey bees are the principal contributing species. Every year beekeepers struggle to remain profitable, a task made more difficult by the numerous pest/disease pressures on the Canadian honey bee population. One of the key factors affecting colony loss is the presence of multiple pathogens and parasites, as well as the background levels of their infective agents within hive substrates (i.e., wax, pollen, honey). Honey bee health is challenged on many fronts, resulting in multifaceted and dynamic causes of colony mortality
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