Honey bee stressor networks are complex and dependent on crop and region.

Honey bees play a major role in crop pollination but have experienced declining health throughout most of the globe. Despite decades of research on key honey bee stressors (e.g., parasitic Varroa destructor mites and viruses), researchers cannot fully explain or predict colony mortality, potentially because it is caused by exposure to multiple interacting stressors in the field. Understanding which honey bee stressors co-occur and have the potential to interact is therefore of profound importance. Here, we used the emerging field of systems theory to characterize the stressor networks found in honey bee colonies after they were placed in fields containing economically valuable crops across Canada. Honey bee stressor networks were often highly complex, with hundreds of potential interactions between stressors. Their placement in crops for the pollination season generally exposed colonies to more complex stressor networks, with an average of 23 stressors and 307 interactions. We discovered that the most influential stressors in a network—those that substantively impacted network architecture—are not currently addressed by beekeepers. Finally, the stressor networks showed substantial divergence among crop systems from different regions, which is consistent with the knowledge that some crops (e.g., highbush blueberry) are traditionally riskier to honey bees than others. Our approach sheds light on the stressor networks that honey bees encounter in the field and underscores the importance of considering interactions among stressors. Clearly, addressing and managing these issues will require solutions that are tailored to specific crops and regions and their associated stressor networks. [Display omitted] • Honey bee colonies are exposed to multiple stressors all at once • Exposure to crops during pollination increases the complexity of stressor networks • Influential stressors that regulate network complexity are unmanaged by beekeepers • Stressor networks differ among crops and regions The causes of honey bee colony losses are difficult to identify and manage. French et al. find that honey bees are exposed to complex stressor networks that are crop- and region-specific. Management strategies must consider multiple stressors, as many influential stressors in these networks are currently unmanaged by beekeepers. [ABSTRACT FROM AUTHOR]