Survival of mite infested (Varroa destructor) honey bee (Apis mellifera) colonies in a Nordic climate

An isolated honey bee population (N = 150) was established on the southern tip of Gotland, an island in the Baltic sea. After infestation with 36 to 89 Varroa destructor mites per colony, they were unmanaged and allowed to swarm. For over six years colonies were monitored for swarming, winter losses, infestation rate in the fall, and bee population size in the spring. Winter mortality rate decreased from 76% and 57% in the third and fourth years, to 13% and 19% in the fifth and sixth years. Swarming rates increased from zero the third field season to 57.1% and 36.4% in the last two years. The mite infestation on adult bees decreased during the last two years, from 0.47% in the third year to 0.19% and 0.22% respectively. Our data suggest that a host-parasite co-adaptation has occurred ensuring survival of both the host and the parasite. The mechanisms behind this co-adaptation require further study.