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

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 (h2 = 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 (h2 = 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.

Influence of model selection and data structure on the estimation of genetic parameters in honeybee populations

“…When mating was uncontrolled, estimated parameters and projected variances matched well. Under controlled mating, we detected a bias, but still, in most cases the projected variances provided a better description of the genetic estimates under the QM and WM than comparing them to and , respectively ( Guichard et al 2020 , 2021 ). Consequently, the projected variances and should also be used in the numerator of heritability definitions when comparing parameter estimates across models.…”

“… Guichard et al (2020 , 2021 ) used both the QM and WM to estimate genetic parameters in honeybee populations and received estimates for , and . At first glance, it might appear possible to use the system of linear Equations (4a) , (4c) , (6a) , and (6b) to retrieve estimates for , and .…”

“…As a remedy, several recent studies used simplified honeybee models for parameter estimations, attributing the entire genetic component of the phenotype either to the queen (queen model, QM) or to the worker group (worker model, WM), while ignoring the other caste ( Andonov et al 2019 ; Facchini et al 2019 ; Guichard et al 2020 , 2021 ): or …”

“…These simplified models lead to increased numeric stability of the estimation procedures, also when the data quality is poor ( Willam and Eßl 1993 ; Guichard et al 2020 , 2021 ). They are easily justified for traits that are clearly attributed to only one caste ( de Graaf et al 2020 ; Facchini et al 2021 ).…”

“…For these traits it is to date unclear, how parameter estimates that were obtained with the simplified models QM and WM compare to estimates using the complete model CM. Sometimes, from the QM is directly associated with from the CM, although there exists a general sentiment that likely also captures some worker effect information due to the close relationship between a queen and her workers ( Guichard et al 2020 , 2021 ). The same holds for and .…”