Evaluation of Traits for the Selection of Apis Mellifera for Resistance against Varroa Destructor

SIMPLE SUMMARY: Infestation with the parasitic mite Varroa destructor is a serious cause of bee colony (Apis mellifera) losses on a global level. However, the presence of untreated survivor populations in many different regions indicates that selection for resistance might lead to a long-term solution. The success of selection depends on suitable testing criteria. To be effective, results must show repeatable effects of the individual genotype and correlate with the breeding goal. As colony survival is difficult to measure, selective breeding for Varroa resistance can be based on differences in mite infestation and specific behavioral traits. In this paper we look into different definitions of mite infestation and link these with brood hygiene (pin test), brood recapping and suppressed mite reproduction (SMR). Due to the large dataset (489 colonies) from Austria, Croatia and Germany and four seasons (2016–2019), our study arrogates high representability. Repeatability analysis depicts different infestation parameters, brood hygiene and recapping data as characteristic colony traits while SMR results are very strongly influenced by environmental effects. Brood hygiene and recapping data correlate weakly but significantly with mite infestation. We therefore recommend combining them with estimates of mite population increase and brood infestation for an effective selection on resistance. ABSTRACT: Infestation with Varroa destructor is a serious cause of bee colony (Apis mellifera) losses on a global level. However, the presence of untreated survivor populations in many different regions supports the idea that selection for resistance can be successful. As colony survival is difficult or impossible to measure, differences in mite infestation levels and tests for specific behavioral traits are used for selective breeding for Varroa resistance. In this paper we looked into different definitions of mite infestation and linked these with brood hygiene (pin test), brood recapping and suppressed mite reproduction. We based our analyses on datasets of Apis mellifera carnica from three countries: Austria (147 records), Croatia (135) and Germany (207). We concluded that bee infestation in summer, adjusted for the level of natural mite fall in spring, is a suitable trait in the breeding objective, and also suggested including brood infestation rate and the increase rate of bee infestation in summer. Repeatability for bee infestation rate was about 0.55, for cells opened in pin test about 0.33, for recapping 0.35 and for suppressed mite reproduction (SMR) virtually zero. Although in most cases we observed correlations with the expected sign between infestation parameters and behavioral traits, the values were generally low (<0.2) and often not significantly different from zero.