Selecting bees for mite non-reproduction (MNR)
Selecting bees with traits that prevent Varroa reproduction (MNR, mite non-reproduction) has been the aim of numerous studies conducted over several years in different bee populations and generations.
Our programme used nine generational lines to investigate the existence of bees that prevent Varroa from producing fertile offspring and the possibility of breeding them, without investigating the underlying causes. MNR values in both mother queens and their offspring were assessed over several years using a specific protocol.
Queens in the daughter colonies were always mated with drones from the same line, except in groups 1 and 5, where drones from two different colonies were used for instrumental insemination. Among the 56 daughter colonies studied, 39% (n = 22 colonies) and 51% (n = 29 colonies) showed higher MNR values than their mothers. In some lines, all colonies headed by daughter queens had higher MNR values than their mothers, while in other lines all had lower values. Daughters typically improved the trait when mothers had low MNR scores and performed less well when mothers had high scores.
MNR improvement in daughters ranged from 0 to 100%, but mother colonies with high MNR scores tended to show lower scores in the next generation. Across the years of selection, there was no consistent improvement in this trait, although colonies with high MNR values were identified each year without being associated with particular lines. No association was found between MNR values in daughter queens and those in their respective mothers. The aim of selection and breeding programmes is to fix desirable traits within a population so that offspring predictably express the same traits (Dekkers 2012).
In this study, we were able to show that colonies with high MNR levels can be found in every generation. We also showed that colonies with low MNR levels can be improved in the next generation by crossing them with high-performing drones. However, we also demonstrated that these desirable traits vary greatly among daughter colonies despite identical mating conditions (using drones from the same source). This high variability makes it very difficult to maintain lines with high MNR scores. It also means that costly, time-consuming assessment of MNR traits in daughter colonies is necessary in every subsequent generation.
A low MNR value can be improved in the next generation, but a high MNR value is highly variable in the next generation and therefore not stable. In other words, MNR is not a trait that can be established in a breeding population simply by combining maternal and paternal colonies. Across several lines, we showed that a steady increase and consistent improvement are not achievable (Fig. 1), making stabilisation at high values difficult. Consequently, each generation must be tested for MNR in a large number of daughters to identify the few “high-MNR” colonies that continue to express this trait.
A high MNR value does not guarantee a high value in the next generation. Improved, high-value colonies are possible, but can only be found by screening the colony families in every generation.
Adapted from The selection traits of mite non‐reproduction (MNR) and Varroa sensitive hygiene (VSH) show high variance in subsequent generations and require intensive time investment to evaluate - Lina Sprau et al. Apidologie (2024) 55:68 © The Author(s), 2024
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