Significant, but not biologically relevant: Nosema ceranae infections and winter losses of honey bee colonies

The Western honey bee Apis mellifera , which provides about 90% of commercial pollination, is under threat from diverse abiotic and biotic factors. The ectoparasitic mite Varroa destructor vectoring deformed wing virus (DWV) has been identified as the main biotic contributor to honey bee colony losses worldwide, while the role of the microsporidium Nosema ceranae is still controversially discussed. In an attempt to solve this controversy, we statistically analyzed a unique data set on honey bee colony health collected from a cohort of honey bee colonies over 15 years and comprising more than 3000 data sets on mite infestation levels, Nosema spp. infections, and winter losses. Multivariate statistical analysis confirms that V. destructor is the major cause of colony winter losses. Although N. ceranae infections are also statistically significantly correlated with colony losses, determination of the effect size reveals that N. ceranae infections are of no or low biological relevance. The analysis of a dataset collected over 15 years reveals no biological relevance of Nosema ceranae infections for colony losses; hence, N. ceranae is not considered a serious threat for honey bees in the background of Varroa destructor infestations.