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A Decade of Varroa Resistance Findings

Posted March 27, 2026

DISCLAIMER: The blog below is generally created from the research listed in the “Referenced Materials” section. This is not my research. All credit is given to these studies and researchers who are helping beekeepers better understand how we can improve our management practices.

Over the last three decades, numerous selection programs have been initiated to improve the host–parasite relationship and to support honey bee survival in the presence of the Varroa destructor parasite without the need for acaricide treatments. Although resistance traits have been included in the selection strategy of some commercial beekeepers, it has not been possible to globally solve the V. destructor problem. Encouragingly, studies in the last 10 years are providing deeper insights into the connections between Varroa resistant traits and virus tolerance growing in free living honey bees around the globe and found amongst colonies in long-term selection programs at the USDA’s Baton Rouge facility and multiple research centers in Europe.

Below are the research articles (see Referenced Materials section for specific studies) used for this brief summary on the topic. I have listed them by year to highlight the relatively resent learnings. Even in the last few years some commercial beekeepers and large beekeeping clubs have also jumped on board promoting their own efforts and methods in this area. After 50 years of trying to solve the Varroa problem through chemicals it seems growingly obvious that we should return with support and understanding from research to the honey bees and see how they have evolved to solve the problem themselves.

“We cannot improve beekeeping by moving farther and farther away from bees’ natural tendencies.” ~ George de Layens (1834-1897)

2014 – Virus Tolerance and Resistance

The honey bee ectoparasitic mite, Varroa destructor, has a world-wide distribution and inflicts more damage than all other known apicultural diseases. However, Varroa-induced colony mortality is more accurately a result of secondary virus infections vectored by the mite. This means that honey bee resistance to Varroa may include resistance or tolerance to virus infections.

2019 – Virus Tolerance and Natural Survival

Caged adult bees and laboratory reared larvae, from varroa-free colonies, were inoculated with Deformed Wing Virus (DWV) and Acute Bee Paralysis Virus (ABPV) in a series of feeding infection experiments, while control groups received virus-free food. Virus infections were monitored using real-time polymerase chain reaction (RT-qPCR) assays in individuals sampled over a time course. In both adults and larvae the DWV and ABPV infection dynamics were nearly identical between mite resistant (MR) and mite sensitive (MS) groups, but MS adults suffered significantly higher mortality than MR adults. Results suggest virus tolerance, rather than reduced susceptibility or virus resistance, is an important component of the natural survival of this honeybee population.

2021 – Traits and Local Population Evolution

The near-globally distributed ecto-parasitic mite of the Apis mellifera honeybee, Varroa destructor, has formed a lethal association with Deformed wing virus, a once rare and benign ribonucleic acid (RNA) virus (uses ribonucleic acid as its genetic material instead of DNA). In concert, the two have killed millions of wild and managed colonies, particularly across the Northern Hemisphere, forcing the need for regular acaricide application to ensure colony survival. However, despite the short association (in evolutionary terms), a small but increasing number of A. mellifera populations across the globe have been surviving many years without any mite control methods. This long-term survival, or Varroa resistance, is consistently associated with the same suite of traits (recapping, brood removal and reduced mite reproduction) irrespective of location.

Study results suggest that resistance is a sequence of events that generate these key traits (increased recapping, brood removal and mite infertility) rather than a single trait. It was found that the enhanced expression of these three key traits is common among resistant populations. This independent occurrence of the key traits within colonies across the world could be an example of parallel evolution, because while the recapping and removal behaviors predate Varroa, they have been co-opted to control Varroa, recapping is rare trait in mite-naive colonies but occurs at low and high levels in susceptible and resistant colonies respectively. Similarly, other traits such as brood suppression of mite reproduction, or DWV tolerance may complement those within this framework. There is also likely to be a mite element to resistance which could be illuminated by further studies into the coevolution of A. mellifera and Varroa. As resistance is a population level trait rather than a single colony trait, a resistant colony becomes vulnerable if moved out of its population and could collapse if a sudden influx of mites occurs due to excessive (40–60%) brood removal. This may explain why resistant colonies moved out of their population typically do not survive.

2022 – Commercial Benefit of Varroa Sensitive Hygiene Selection

The term “Pol” in Pol-line honey bees refers to a selective breeding program developed by the USDA. A primary goal of the program is to breed honey bees that are resistant to varroa mites, a significant threat to bee health. It was also initiated to create bees that commercial beekeepers would find beneficial for their operations and sales. Results show that under migratory beekeeping conditions, Pol-line honey bees have significantly enhanced colony and queen survival when compared to a standard commercial Italian stock. Additionally, the productivity of Pol-line colonies appears equivalent to that of Commercial honey bees, as evidenced by comparable honey production and population sizes among surviving colonies. This improved performance is associated with lower Varroa levels, and concomitantly reduced titres of DWV-A, DWV-B, and chronic bee paralysis virus (CBPV). Notably, the multiplicative effect of Varroa levels on colony survival is significantly weaker in Pol-line colonies, leading to better survival prognoses at all but the highest infestation strata. An ostensible explanation for this is the constant removal of infested brood by Pol-line workers, as is characteristic of VSH behavior, and thus a subsequent dampening of Varroa growth dynamics and viral transmission. Consequently, Pol-line colonies that received only one mite treatment demonstrated survival rates empirically equivalent to Commercial colonies receiving two. Indeed, the benefit of additional mite treatments was generally greater for Commercial colonies, as the majority of Pol-line colonies naturally maintained Varroa levels below the recommended treatment threshold. This has considerable implications both for improving colony survival, and reducing the escalating need for acaricide use.

Incorporating the VSH trait into commercial stocks appears to engender both Varroa-resistance, and favorable colony productivity. Furthermore, it find that DWV-A, DWV-B, CBPV, and black queen cell virus (BQCV) titres are inferior predictors of colony mortality when compared to Varroa levels alone.

2025 – Selective Breeding – Varroa

Suppressed Mite Reproduction

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 behavioral mechanism by which worker bees detect and remove mite-infested brood. The expression of VSH was monitored over four consecutive years (2021–2024), using Suppressed Mite Reproduction (SMR – Suppressed mite reproduction explained by the behaviour of adult bees 2004), which is a trait in honey bees that allows them to limit the reproduction of the Varroa destructor mite, 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. Mean heritability for SMR was 0.30. These results confirm the potential for targeted breeding programs to enhance VSH expression in honey bee populations.

Low Varroa Growth/Multi Gene Trait

Honey bees (Apis mellifera) bred for resistance to the parasitic mite, Varroa destructor, were examined for potential Varroa resistance mechanisms following bidirectional selection for low (resistant) or high (susceptible) Varroa population growth (Low Varroa Growth LVG and High Varroa Growth HVG, respectively) based on mite fall in colonies at two different time points. Hygienic and grooming behavior rates in LVG colonies were significantly higher than those in HVG colonies for two out of three generations of selection, indicating that behavioral resistance to the mite increased. For the third generation, grooming start time was significantly shorter, and grooming intensity more frequent in LVG bees than in HVG bees. Cellular immunity was increased as well, based on significantly higher haemocyte concentrations in non-parasitized and Varroa-parasitized LVG bees. Humoral immunity was increased with Varroa-parasitized LVG bees, which had significantly higher expression of the antimicrobial peptide gene, hymenoptaecin 2. In addition, antiviral resistance may be involved as there were significantly lower levels of deformed wing virus (DWV) in Varroa-parasitized LVG bees. While selection for LVG and HVG bees was solely based on Varroa population growth, it appears that behavioral, cellular, and humoral mechanisms were all selected along with this resistance. Thus, LVG resistance appears to be a multi-gene trait, involving multiple resistance mechanisms.

Suppressed Ovo (Egg) Virus Trait

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. SOV is a trait in honey bees that indicates the ability to prevent virus infections in eggs. This trait is heritable and helps reduce the virus load in bee colonies, contributing to their overall health and survival. A 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.

2026 – Selective Breeding – Brood

Unhealthy Brood Odor Sensitivity

Varroa-resistant honey bee breeding programs have developed as a promising and sustainable long-term strategy to control Varroa mite infestations in managed colonies. These breeding programs drive the coevolution of hygienic bees and Varroa mites, however the impact of such coevolution on bee and mite viral dynamics remains poorly understood. A study investigated how Varroa-resistant traits influence the tripartite interaction among honey bees, Varroa mites, and viruses. The tripartite interaction is a complex relationship between honey bees, their gut microbiota, and the pathogens they encounter. This interaction influences the health and behavior of the bees, as the gut bacteria can help bees recognize nestmates and defend against diseases, while pathogens can disrupt these beneficial relationships. Two apiaries were established: one in Greensboro, North Carolina, consisting of high sensitive and low sensitive unhealthy brood odor (UBO) colonies, and another in Stoneville, Mississippi, consisting of Pol-line and Commercial colonies. Worker bees and Varroa mites were collected from each colony throughout the beekeeping season and screened for 7 viruses. Hygienic selection significantly reduced the Varroa mite infestation level and influenced the dynamics of viruses in worker bees and Varroa mites. Specifically, titers of Varroa-associated viruses were significantly reduced in worker bees and in mites collected from hygienic colonies. Additionally, hygienic selection altered the co-occurrence patterns and correlations among multiple critically important viruses in mites and worker bees. These findings highlight the value of selective breeding as an effective strategy for improving honey bee health and colony survival and shed light on the complex tripartite relationships between honey bees, Varroa mites, and viruses.

Cold Storage

This study evaluated the impact of a cold storage strategy to decrease bee brood production, and increase mite treatment efficacy, in a commercial Italian bee stock and mite-resistant Russian and Pol-line bee stocks, from prior to cold storage in August until the start of the commercial pollination season the following February. For each year of two years, thirty new bee colonies (10 colonies per stock) were either placed in cold storage (5 °C, darkness, 18 days) starting mid-August or left outdoors, and all hives subsequently treated with a thymol-based varroacide. Colony brood area, adult bee mass, hive weight, internal temperature and CO₂ levels were monitored during the experiments using periodic hive assessments as well as sensors. Honey bee workers were sampled at different points and evaluated for bee health biomarker gene expression (vitellogenin) as well as virus levels of DWV-A and DWV-B. Cold storage effectively halted brood production but differences in brood levels between groups disappeared within two months, with no long-term impact on population size, mite levels, virus loads, or daily hive weight change. Bee stock was the dominant factor influencing outcomes: mite susceptible Italian colonies had higher mite densities, higher DWV loads, lower vitellogenin expression and higher rates of hive weight loss than Russian or Pol-line colonies. In the study mite-resistant honey bee stocks offered more effective control, reducing mite loads by over 65% compared to the susceptible stock, across both years and both treatment groups of the study, and they have the potential to support honey bee health by reducing reliance on chemical treatments in beehives.

Conclusion

Research, selective breeding, and a growing understanding and appreciation of honey bee biology shows promising results in the fight against Varroa. It would seem that we are just now catching up to what are the possibilities already innate within the honey bee. As much as evolution is bantered around it seems we truly don’t understand the genetic treasure the bees have gained through their thousands of years of evolutionary survival experience of fighting pests and pathogens we aren’t even aware of. So, let’s give the bees a chance to take back control and show us they can protect themselves if can just help them to have an environment that allows them to live within their “natural tendencies”.

Referenced Materials

Apiary Services

Layens Spring Inspections

Resistance Against Varroa and Tropilaelaps

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