Geographical Distribution and Selection of European Honey Bees Resistant to Varroa destructor

SIMPLE SUMMARY: The parasitic mite Varroa destructor is a major challenge to honey bee populations worldwide. Some honey bee populations are resistant to the mite, but most of the commercially used stocks are not and rely on chemical treatment. In this article, we describe known varroa-resistant populations and the mechanisms which have been identified as responsible for survival of colonies without beekeeper intervention to control the mite. We review traits that have potential in breeding programs, discuss the role played by V. destructor as a vector for virus infections, and the changes in mite and virus virulence which could play a role in colony resistance. We also describe results of surveys carried out (mostly within Europe) on the presence of naturally surviving honey bee populations, on the commercial availability of mite resistant stock and on the traits considered in breeding programs. We found that there is a growing interest and awareness among beekeepers, but that there are very few commercially available resistant lines; some from breeding programs and some from naturally selected populations. The most commonly considered traits for assessing varroa resistance are linked to mite reproduction and specific hygienic behavior of the bees. ABSTRACT: Developing resistance to the varroa mite in honey bees is a major goal for apicultural science and practice, the development of selection strategies and the availability of resistant stock. Here we present an extended literature review and survey of resistant populations and selection programs in the EU and elsewhere, including expert interviews. We illustrate the practical experiences of scientists, beekeepers, and breeders in search of resistant bees. We describe numerous resistant populations surviving without acaricide treatments, most of which developed under natural infestation pressure. Their common characteristics: reduced brood development; limited mite population growth; and low mite reproduction, may cause conflict with the interests of commercial beekeeping. Since environmental factors affect varroa mite resistance, particular honey bee strains must be evaluated under different local conditions and colony management. The resistance traits of grooming, hygienic behavior and mite reproduction, together with simple testing of mite population development and colony survival, are significant in recent selection programs. Advanced breeding techniques and genetic and physiological selection tools will be essential in the future. Despite huge demand, there is no well-established market for resistant stock in Europe. Moreover, reliable experience or experimental evidence regarding the resistance of stocks under different environmental and management conditions is still lacking.