Results of four generations of selection for Varroa Sensitive hygienic behavior in honey bees

Varroa destructor remains one of the most critical threats to honey bee health and colony survival worldwide. As conventional control methods face increasing limitations due to acaricide resistance and environmental constraints, selective breeding for resistance traits has emerged as a sustainable alternative. Among these traits, Varroa Sensitive Hygiene (VSH) represents a key behavioural mechanism by which worker bees detect and remove mite-infested brood. In this study, the expression of VSH was monitored over four consecutive years (2021–2024), using Suppressed Mite Reproduction (SMR) as a quantitative indicator of VSH performance. The SMR trait was measured in a breeding population where queens were instrumentally inseminated with semen from a single drone to control the maternal and paternal genetic contribution. A steady increase in mean SMR scores was observed, rising from 22.1% in 2021 to 41.0% in 2024, along with greater phenotypic variability and the appearance of high-performing colonies, several reaching the maximum score of 100% in the final two years. The proportion of colonies with no detectable VSH expression (0% SMR) declined over time. While no statistically significant differences among years were detected, the temporal trend suggests that the selection strategy is promoting the accumulation of resistant phenotypes. No significant genetic line effect was found across all years; however, a significant difference emerged in 2024, with line 2024_GL17 showing particularly high SMR values compared to line 2024_GL15 (p = 0.02). Mean heritability for SMR was 0.30. These results confirm the potential for targeted breeding programs to enhance VSH expression in honey bee populations.