Abstract – The Western honey bee, Apis mellifera, is the most important animal pollinator in agriculture
worldwide providing more than 90% of the commercial pollination services. Due to the development in
agriculture the demands for honey bee pollination are steadily increasing stressing the pollination capacity
of the global managed honey bee population. Hence, the long-term decline of managed honey bee hives in
Europe and North-America is of great concern and stimulated intensive research into the possible factors
presumably causing honey bee colony collapse. We here present a four-year study involving more than
1200 bee colonies from about 120 apiaries which were monitored for the entire study period. Bee samples
were collected twice a year to analyze various pathogenic factors including the ectoparasitic mite Varroa destructor,
fungi (Nosema spec., Ascosphaera apis), the bacterium Paenibacillus larvae, and several viruses.
Data on environmental factors, beekeeping management practice, and pesticides were also collected. All
data were statistically analyzed in respect to the overwintering mortality of the colonies. We can demonstrate
for several factors that they are significantly related to the observed winter losses of the monitored
honey bee colonies: (i) high varroa infestation level, (ii) infection with deformed wing virus (DWV) and
acute bee paralysis virus (ABPV) in autumn, (iii) queen age, and (iv) weakness of the colonies in autumn.
No effects could be observed for Nosema spec. or pesticides. The implications of these findings will be