Explaining the reasons for the increased mortality of the honey bee (Apis mellifera L.) in recent years, in Europe and North America, has become a global research priority in apicultural science. Our project was aimed at determining the relationship between environmental conditions, beekeeping techniques, the epidemiological situation of pathogens, and the mortality rate of bee colonies. Dead bee samples were collected by beekeepers from 2421 colonies. The samples were examined for the presence of V. destructor, Nosema spp. (Nosema apis and Nosema ceranae), chronic bee paralysis virus (CBPV), acute bee paralysis virus (ABPV), deformed wing virus (DWV), and Israeli acute paralysis virus (IAPV).
Among the environmental and colony management factors under analysis, significant differences between apiaries with high (>10%), low (≤10%) or no losses of the colonies were only found in the case of the methods used by beekeepers to combat varroa mites. However, the epidemiological patterns in the case of V. destructor infestation and the DWV and ABPV infections highly differed. The data we obtained indicated that co-infections play a decisive role in the etiology of the significant collapse of colonies in apiaries in Poland. The main reason for this phenomenon can be described as strong infestation with V. destructor, followed by an intensive development of viral infections caused by DWV and (much less frequently) by ABPV. Despite a high prevalence of Nosema spp. microsporidia (with a dominant incidence of N. ceranae), a direct relationship between these parasites and the mortality rate of colonies was not proved.