# Evaluation of 10-Year Selection for Virus Resistance in a Mass Breeding Program

## TL;DR

This study shows that selective breeding for virus-resistant honey bee queens can significantly reduce viral infections in bee colonies over time.

## Contribution

The study demonstrates that targeted breeding for queens with suppressed in ovo virus infection can improve virus resistance in honey bees.

## Key findings

- The proportion of virus-free egg-producing queens increased significantly over a 10-year period.
- Breeding from virus-free queens increased the likelihood of producing virus-free offspring.
- The prevalence and intensity of several honey bee viruses decreased over time.

## Abstract

Honey bees continue to face health threats from viral infections. Most virus-control efforts focus on reducing Varroa mite infestations, which spread viruses within honey bee colonies. However, some honey bee queens possess the heritable trait suppressed in ovo virus infection, which means these queens naturally produce drone eggs free from honey bee viruses. This provides the opportunity to directly select for virus resistance. In this study, we used data collected within an ongoing mass breeding selection program (2015–2024) in Flanders. This data includes drone egg samples collected from bee colonies and analyzed for four honey bee viruses. Hereby, we evaluate the incorporation of this trait within a selection program. Our results show that the number of virus-free egg-producing queens increased significantly over time. Breeding from these queens further increased the likelihood that their offspring would also produce virus-free eggs. In parallel, both the presence and intensity of several virus infections decreased over time. These findings show that targeted breeding, together with targeted mating, for virus-free egg-producing queens can successfully improve virus resistance in managed honey bee populations and that, in the future, this trait should be implemented together with Varroa-resistant traits in selection programs.

Viral infections pose a major threat to honey bee health. While viruses are typically controlled indirectly through efforts of attaining Varroa resistance, the heritable trait suppressed in ovo virus infection (SOV) provides a direct avenue for selecting virus resistance. This study evaluated the potential of this trait using data collected within an established mass breeding selection program. Drone egg samples collected from honey bee colonies in Flanders (2015–2024) were screened for four viruses to determine the queen’s SOV status. Queens are classified as SOV-positive if no viral particles are detected in their sample, and as SOV-negative if genomic material from at least one of these viruses is present. The proportion of SOV-positive queens significantly increased over time, regardless of maternal background, and targeted breeding from SOV-positive maternal lines enhanced the likelihood of producing SOV-positive offspring. Simultaneously, the prevalence and viral load values of several viruses decreased over time. These findings demonstrate that selective breeding for SOV-positivity can improve virus resistance in managed honey bee populations. There is even a potential to raise the SOV trait occurrence by incorporating targeted mating within selection programs. Therefore, future research should focus on the combined selection for SOV through targeted breeding and mating, alongside Varroa-resistant traits.