A Reciprocal Transplant Experiment Confirmed Mite-Resistance in a Honey Bee Population from Uruguay

SIMPLE SUMMARY: In Uruguay, as in many countries around the world, the Varroa destructor mite is the main biotic threat to honey bees (Apis mellifera). Most beekeepers regularly apply acaricides to their colonies to have good honey harvests and avoid large losses, with the exception of beekeepers in the east of the country where bees coexist with V. destructor without suffering significant damage. To unravel the different A. mellifera–V. destructor relationships found in the country, a reciprocal transplant experiment was performed between the mite-resistant bee colonies and the mite-susceptible bee colonies from the east and the west of the country, respectively. The differences between the two groups of bees in the control of V. destructor were maintained in the two environments. No mite-susceptible colonies survived the winter. The behavioral resistance of bees (hygienic behavior) and reproductive aspects of V. destructor (phoretic mites/reproductive mites and mites in drone cells/mites in worker cells ratio) could explain the results obtained. ABSTRACT: In the past few years there has been an increasing interest for the study of honey bee populations that are naturally resistant to the ectoparasitic mite Varroa destructor, aiming to identify the mechanisms that allow the bees to limit the reproduction of the mite. In eastern Uruguay there are still bees resistant to mites that survive without acaricides. In order to determine if the differential resistance to V. destructor was maintained in other environments, a reciprocal transplant experiment was performed between the mite-resistant bee colonies and the mite-susceptible bee colonies from the east and the west of the country, respectively, infesting bees with local mites. In both regions, the mite-resistant colonies expressed a higher hygienic behavior and presented a higher phoretic mites/reproductive mites and mites in drone cells/mites in worker cells ratio than the mite-susceptible colonies. All the mite-susceptible colonies died during fall–winter, while a considerable number of mite-resistant colonies survived until spring, especially in the east of the country. This study shows that the bees in the east of the country maintain in good measure the resistance to V. destructor in other regions and leaves open the possibility that the mites of the two populations have biases in the reproductive behavior.