Exploring Two Honey Bee Traits for Improving Resistance Against Varroa destructor: Development and Genetic Evaluation

SIMPLE SUMMARY: Selection of honey bees requires traits which can be easily measured in the field by beekeepers. This is also the case for traits linked to honey bee resistance against the parasitic mite Varroa destructor. We therefore describe two new trait evaluation protocols, ‘Recapping’ and ‘Solidness’, conceived to enable an easy evaluation of two putative colony resistance traits, recapping (i.e., opening and re-sealing) and solidness (i.e., amount of capped brood in a defined area) of worker brood, respectively. The hypothesis of this study is that higher levels of ‘Recapping’ and ‘Solidness’ could provide resistance to V. destructor. Repeatability and heritability of the two traits, as well as their phenotypic correlations with other colony traits were calculated, in order to investigate their potential for resistance selection. Both traits showed low repeatability between different measurements within each year. ‘Recapping’ had a low heritability and a negative correlation to hygienic behavior evaluated by the pin-test method. The heritability of ‘Solidness’ was moderate. The two traits did not show an association with V. destructor infestation levels. Further research is needed to confirm these results, as only a small number of colonies could be evaluated. ABSTRACT: For the development of novel selection traits in honey bees, applicability under field conditions is crucial. We thus evaluated two novel traits intended to provide resistance against the ectoparasitic mite Varroa destructor and to allow for their straightforward implementation in honey bee selection. These traits are new field estimates of already-described colony traits: brood recapping rate (‘Recapping’) and solidness (‘Solidness’). ‘Recapping’ refers to a specific worker characteristic wherein they reseal a capped and partly opened cell containing a pupa, whilst ‘Solidness’ assesses the percentage of capped brood in a predefined area. According to the literature and beekeepers’ experiences, a higher recapping rate and higher solidness could be related to resistance to V. destructor. During a four-year field trial in Switzerland, the two resistance traits were assessed in a total of 121 colonies of Apis mellifera mellifera. We estimated the repeatability and the heritability of the two traits and determined their phenotypic correlations with commonly applied selection traits, including other putative resistance traits. Both traits showed low repeatability between different measurements within each year. ‘Recapping’ had a low heritability (h(2) = 0.04 to 0.05, depending on the selected model) and a negative phenotypic correlation to non-removal of pin-killed brood (r = −0.23). The heritability of ‘Solidness’ was moderate (h(2) = 0.24 to 0.25) and did not significantly correlate with resistance traits. The two traits did not show an association with V. destructor infestation levels. Further research is needed to confirm the results, as only a small number of colonies was evaluated.