VARROA-JACOBSONI OUD IN COLD CLIMATES - POPULATION-GROWTH, WINTER MORTALITY AND INFLUENCE ON THE SURVIVAL OF HONEY-BEE COLONIES

The development of Varroa jacobsoni infestations was monitored in two groups of honey bee (Apis mellifera) colonies kept in SE Finland under conditions where no infestation pressure from outside apiaries was possible and in a climate with a brood rearing period of less than five months. One group of 36 colonies was infested with 5-8 mites/colony and another group of 20 colonies with c. 100 mites/colony. Mite population levels in the colonies were followed by using debris samples. Yearly mite population growth factors were calculated based on values of maximum numbers of mites in the debris per day during the brood season. The averages varied from 8.2 to 24.6 despite the short brood rearing period and an average calculated 39.8% loss of the mite population in winter. The results of chemical treatment of five colonies/group, 38 months (group 1) and 26 months (group 2) after the infestation, supported the population growth results. Equal numbers of mites were killed by the second treatment in the following autumn, both in previously treated colonies and in untreated colonies, a result stressing the need to treat all colonies in an apiary simultaneously. Close correlations were found between mite downfall levels in autumn and the number of mites on wintering bees. It is concluded that under the conditions described, untreated colonies can survive for 4-5 years from an initial infestation by a few mites.