Descriptive Analysis of the Varroa Non-Reproduction Trait in Honey Bee Colonies and Association with Other Traits Related to Varroa Resistance

SIMPLE SUMMARY: Honey bees, Apis mellifera, are currently facing drastic colony losses, some of which lies partially on the infestation by a parasite, varroa destructor. To maintain healthy honey bee colonies breeding for resistance to this infestation seems promising. However, measuring resistance is a tedious task as it is a complex process. One can consider a honey bee colony resistant to varroa infestation when the parasite fails reproducing and thus its population does not grow in the colony, this is called mite non reproduction. In this study we dissected the performance of a scoring method for mite non reproduction and compared it to other known methods to measure resistance. Although the mite non reproduction method is preferred for field studies because of its relative simplicity it appears to lack correspondence with other methods and it should be interpreted cautiously because it is variable. Even though resistance to varroa infestation is desired for breeding decision it should be done carefully when the way to measure such resistance is using the mite non reproduction method. ABSTRACT: In the current context of worldwide honey bee colony losses, among which the varroa mite plays a major role, the hope to improve honey bee health lies in part in the breeding of varroa resistant colonies. To do so, methods used to evaluate varroa resistance need better understanding. Repeatability and correlations between traits such as mite non-reproduction (MNR), varroa sensitive hygiene (VSH), and hygienic behavior are poorly known, due to practical limitations and to their underlying complexity. We investigate (i) the variability, (ii) the repeatability of the MNR score, and (iii) its correlation with other resistance traits. To reduce the inherent variability of MNR scores, we propose to apply an empirical Bayes correction. In the short-term (ten days), MNR had a modest repeatability of 0.4, whereas in the long-term (a month), it had a low repeatability of 0.2, similar to other resistance traits. Within our dataset, there was no correlation between MNR and VSH. Although MNR is amongst the most popular varroa resistance estimates in field studies, its underlying complex mechanism is not fully understood. Its lack of correlation with better described resistance traits and low repeatability suggest that MNR needs to be interpreted cautiously, especially when used for selection.