0. Introduction: A Question That Cannot Be Reduced to an Acronym

Objective To set the context for this article: to show that varroa resistance cannot be reduced to an acronym or a single trait, but represents a colony's capacity to durably slow the growth of the varroa mite population and remain viable through to overwintering.

Any beekeeper who takes an interest in breeding will sooner or later encounter terms such as VSH, MNR or DMR. These acronyms circulate in trade journals, in breeding programmes, and even in conversations at the apiary. They sometimes give the impression that certain bees are fundamentally different from others – capable of "resisting" the varroa mite through a precise, measurable and heritable mechanism. The reality is at once more nuanced and more interesting.

What the recent scientific literature has progressively shown is that no single trait is sufficient to explain why some colonies contain the spread of the parasite better than others. What ultimately matters is a dynamic: a colony's capacity to durably slow the growth of the varroa mite population over the course of the season and to reach overwintering in a still-viable state. The mechanisms contributing to this are multiple, partially interlinked, and none of them constitutes a solution on its own.

This article offers a review of current knowledge on this topic, intended for beekeepers who wish to understand what is really at stake – without undue simplification but without unnecessary jargon. It draws on several recent reference works (Guichard et al., 2020; Mondet et al., 2020; Sprau et al., 2024; Scaramella et al., 2023; von Virag et al., 2022) and distinguishes, as far as possible, between what is solidly established, what remains under discussion, and what is better characterised as a promising avenue than as a fully demonstrated conclusion.

1. Resistance, Tolerance, Resilience: Three Words for Different Realities

Objective To clarify three concepts that are often conflated and to show why this distinction has practical consequences for colony assessment and for breeding.

Before examining the mechanisms, the terminology must be clarified. In the beekeeping press and in field discussions, the terms resistance, tolerance and resilience are often used as though they referred to the same thing. They do not, and this confusion has real practical consequences.

Resistance refers to the mechanisms by which a colony actively reduces the parasite load or slows varroa mite reproduction. A resistant colony therefore does not simply tolerate infestation better: it acts, directly or indirectly, on the reproductive success of the parasite.

Tolerance describes something else. A tolerant colony can sustain a relatively high parasite load without collapsing immediately. It does not necessarily control the parasite well, but it buffers the damage more effectively – for example through better management of viral pressure or through other compensatory mechanisms that are still incompletely understood. A colony that "holds on" despite a high infestation level is therefore not necessarily a good genetic source for breeding if that survival rests on tolerance rather than on genuine suppression of parasite spread (Guichard et al., 2020; Mondet et al., 2020).

Resilience, finally, is a broader term. It can be understood here as a colony's capacity to withstand parasite pressure, to limit its functional effects – on demography, production, vitality and winter survival – and to return to viable functioning over the course of the season. The term is useful precisely because it directs attention to the end result rather than to any particular mechanism.

Why does this distinction matter at the apiary? Because a colony that survives is not necessarily one that controls the parasite well. If selection is based solely on year-to-year survival, one risks retaining – indiscriminately – genuinely resistant colonies alongside primarily tolerant ones, even though these two profiles do not carry the same long-term implications for one's own apiary or for neighbouring apiaries. Recent work emphasises precisely this point: survival alone is insufficient to characterise how a colony manages the parasite (Guichard et al., 2020; Mondet et al., 2020).

One final note on the term immunity. It sometimes appears in beekeeping discussions by analogy with mammals. It is better avoided here: the defence mechanisms of Apis mellifera against Varroa destructor do not involve adaptive immunity in the strict sense. Speaking of behavioural resistance, of colony defence mechanisms, or naming the relevant traits directly is more rigorous and less misleading.

2. The Mechanisms Under Discussion: Behaviours, Outcomes, Brood Traits

Objective To present the principal mechanisms currently under discussion – VSH, MNR, DMR, recapping and brood traits – clearly distinguishing worker behaviour, observed phenotype and level of action.

The literature today distinguishes several levels of action that must not be conflated: some terms describe a worker behaviour, others an observed outcome relating to varroa mite reproduction, and others still characteristics of the brood. It is precisely because these levels have often been mixed together that the discussion can appear confusing (Guichard et al., 2020; Sprau et al., 2024).

2.1 VSH: A Behaviour, Not an Outcome

VSH (Varroa Sensitive Hygiene) designates a targeted hygienic behaviour: workers detect infested capped brood, uncap the cell, and then remove the pupa or the mite. This behaviour acts directly where the parasite reproduces, which explains its relevance in research and in breeding programmes (Panziera et al., 2017; Guichard et al., 2020).

However, VSH remains a behaviour, not a guarantee of resistance in itself. A colony achieving a good VSH test score does not automatically display a low infestation dynamic over the course of the season. The behaviour may be present without its effect being sufficient, or without it being expressed with the same intensity under real apiary conditions. A VSH score must therefore not be equated with the actual outcome on parasite spread.

2.2 MNR and DMR: Measuring What Is Observed, Without Prejudging the Cause

MNR (Mite Non-Reproduction) and DMR (Decreased Mite Reproduction) describe, by contrast, an observed outcome: a high proportion of mites that fail to produce normal offspring in the cells examined. These terms are useful precisely because they describe the phenomenon without imposing a single cause.

A high MNR rate may reflect the effect of VSH-type behaviour, of brood traits, of factors relating to the mite itself, or a combination of these elements. This is an essential distinction: VSH names a behaviour, whereas MNR names what is observed in the cells after the fact. The two may be related, but they are neither synonymous nor systematically correlated (Eynard et al., 2020; Sprau et al., 2024).

DMR was proposed to refine the terminology further. It replaces or refines the older term SMR (Suppressed Mite Reproduction), which was judged too mechanistically suggestive, since the word "suppression" implied that a single active process was necessarily at work. DMR allows more neutral reference to a reduction in parasite reproduction, whatever its origin (von Virag et al., 2022).

2.3 SMR, MNR, DMR: Why the Terms Changed

The term SMR was long used to designate a reduction in the reproductive success of the varroa mite. It has progressively been replaced by MNR and DMR for two reasons. First, because the word "suppression" suggested a precise mechanism where several causes may in fact produce the same result. Second, because the more recent terms better distinguish what is measured – an observed phenotype – from what causes it.

For beekeepers reading older studies or breeding documents, this implies a simple watchfulness: when the term SMR appears, it is worth checking what the authors actually mean by it. Depending on the period and context, it has not always covered exactly the same reality.

2.4 Recapping: An Associated Behaviour, but One Requiring Careful Interpretation

Recapping refers to the opening of a capped cell by workers, followed by its resealing, without systematic removal of the pupa. This behaviour is regularly observed in populations or lines considered more resistant, and several studies show that it displays measurable heritability (Gabel et al., 2023).

Its interpretation remains more delicate than that of VSH, however. Depending on the study, recapping may be associated with higher resistance, but may also reflect a response to greater parasite pressure. In other words, its frequency can sometimes signal useful vigilance, sometimes heavy infestation. The link with an effective reduction in varroa reproduction is documented in some studies (Morin & Giovenazzo, 2023), but its independent weight, separately from other mechanisms, is difficult to isolate. At this stage, it therefore seems reasonable to regard it as a complementary indicator, but not as a primary selection criterion on its own.

2.5 Brood Traits: A Level That Is Often Underestimated

One of the major contributions of recent work is to demonstrate that resistance is not decided solely through the behaviours of adult bees. Characteristics of the brood itself – duration of the capped phase, chemical composition, olfactory signals or other physiological properties – can influence the reproductive success of the varroa mite.

Scaramella et al. (2023), working from naturally surviving populations in Scandinavia and France, showed that certain brood traits can contribute to reducing the varroa mite's reproductive success, beyond what would be explained by adult bee behaviour alone. This observation substantially changes the interpretation of the problem: it requires accepting that MNR or DMR can arise through several distinct biological pathways, and not solely as a by-product of VSH.

For beekeepers, this means that a resistance assessment based exclusively on the visible behaviour of workers risks missing a significant part of the picture.

3. Interlinked Mechanisms – but No Single Silver-Bullet Trait

Objective To show that the mechanisms currently under discussion may overlap, but that none of them is sufficient on its own to explain or guarantee a colony's varroa resistance.

The central point of the recent literature can be summarised as follows: the mechanisms contributing to varroa resistance are partially interlinked, but none of them is sufficient on its own to explain a colony's behaviour in the face of the parasite.

Several studies show that VSH and MNR are neither synonymous nor systematically correlated. A colony may express marked hygienic behaviour without automatically displaying a high rate of varroa non-reproduction – and the reverse is also possible (Eynard et al., 2020; Sprau et al., 2024). This does not mean VSH has no effect. In certain lines, it clearly contributes to limiting the reproductive success of the parasite. But the relationship is neither universal nor sufficient in itself.

Work on brood traits adds a further dimension: reduction in varroa reproductive success can originate from a level that VSH does not cover. And recapping, although heritable and associated with resistance in some studies, yields variable results depending on the population and assessment context (Gabel et al., 2023; Guichard et al., 2020).

The conclusion that follows is straightforward: the resistance observed in the best-performing colonies appears to emerge from a combination of mechanisms – behavioural, brood-related, arising from host-parasite interaction – rather than from any dominant single trait. It is precisely for this reason that the most rigorous current approaches move away from single-trait explanations and attach greater importance to combinations of indicators and to actual outcomes at colony level (Guichard et al., 2020; Sprau et al., 2024).

For beekeepers assessing their colonies, this has a direct implication: a colony with a good score on a single trait is not necessarily a resistant colony. What must be examined is the overall dynamic.

4. What Really Matters: Slowing Varroa Population Growth at Colony Level

Objective To shift the focus from the isolated trait to the most biologically and practically relevant criterion: a colony's capacity to durably slow varroa mite population growth and to reach overwintering in a still-viable state.

At apiary level, the most relevant criterion is not the isolated presence of a behavioural trait, but a colony's capacity to durably slow varroa mite population growth over the course of the season. This is the point that most directly links the mechanisms described above to a biologically and practically relevant outcome: less parasite multiplication, less viral pressure – in particular less DWV – and a greater chance of reaching overwintering with a colony still in functional condition.

The naturally surviving populations documented in Europe point in this direction. In Norway, colonies followed without treatment for more than 17 years demonstrated that durable survival can become established through natural selection, with reduced varroa reproductive success as the key factor (Oddie et al., 2017). In Sweden, surviving colonies studied on Gotland also displayed characteristics consistent with a more balanced host-parasite relationship, including reduced mite reproductive success (Locke & Fries, 2011). These examples do not provide recipes that are directly transferable, but they demonstrate that a different dynamic is biologically possible.

Experimental selection work confirms the value of this criterion at colony level. De La Mora et al. (2024) selected divergent lines on varroa mite population growth over three generations. Under their experimental conditions, lines selected for low parasite growth showed infestation levels approximately 90% lower than those of the opposing lines, accompanied by reduced DWV levels and better winter survival. This result is significant because it demonstrates that a criterion measured at colony level integrates several partial mechanisms more effectively than a single behavioural marker.

This perspective also helps to avoid a common confusion: equating survival with effective parasite control. A colony can hold on for a period despite heavy parasite pressure – through tolerance, through a transient equilibrium, or simply because conditions were momentarily favourable. This is not sufficient to make it a good selection model. From a rigorous beekeeping standpoint, the decisive point is therefore not only that a colony survives, but that it does so by containing varroa spread at a lower and more durable level.

For beekeepers monitoring their colonies, this directs attention towards concrete criteria: the trend in infestation level over the season, differences between colonies within the same apiary, and the capacity to get through winter without a late emergency treatment. These data do not replace behavioural tests, but they provide a more direct picture of what is actually happening.

5. What Selection Can Realistically Achieve – and Where Its Limits Lie

Objective To evaluate what selection for varroa resistance can reasonably offer today, taking into account genuine progress, methodological constraints and practical limits.

Selection for improved varroa resistance is a serious avenue. It must, however, be presented with a degree of caution proportionate to the available data – and those data are both encouraging and nuanced.

5.1 Real Progress, but Slow

Several decades of breeding programmes have produced genuine advances in understanding the mechanisms and in identifying better-performing lines. Studies on surviving Nordic populations, divergent selection work and ongoing genomic analyses all demonstrate that resistance is a heritable and selectable trait (Guichard et al., 2020; Mondet et al., 2020).

Yet these same reviews also converge on one point: despite this progress, it has not been possible to "solve" the varroa problem globally through resistance traits alone. This does not mean that selection is failing, but that it is advancing within a biologically complex, strongly context-dependent framework, and that its effects unfold over the long term rather than within the horizon of the next season.

5.2 The First Obstacle: Measurement Difficulty

The most widely discussed traits – notably VSH and MNR – are costly to measure. They demand demanding protocols, careful observation and standardised assessment conditions. A recent study on the separate selection of MNR and VSH highlighted high variance in subsequent generations and a substantial time investment for evaluation, which limits large-scale implementation in practical programmes (Sprau et al., 2024).

In practical terms, this means these markers remain difficult to use for an individual breeder or a small operation, and their value depends heavily on the resources available to measure them correctly.

5.3 The Second Obstacle: Biological Repeatability

Indicators based on varroa mite reproduction also display repeatability limitations. Results do not automatically become more robust simply by increasing the number of cells examined or by switching brood type. Assessment of DMR shows variability that does not arise from measurement error alone, but from biology itself: varroa reproduction also depends on environmental and seasonal factors and on the composition of the mite population within the colony (von Virag et al., 2022).

In other words, a good score obtained at a given point in time is insufficient to classify a colony as durably resistant.

5.4 The Multi-Trait Approach: Promise and Reality

Combining several criteria – VSH, MNR, recapping, brood traits, infestation dynamic – is conceptually more solid than focusing on a single marker. But this approach has practical limits: it multiplies evaluation time, requires consistent protocols between assessors, and demands a selection infrastructure that is not accessible to everyone.

The work of Sprau et al. (2024) illustrates this tension well: even within a resource-equipped research setting, separate selection on MNR and VSH produced variable results and required considerable investment. This does not invalidate the multi-trait approach, but it rules out presenting it as a simple and immediately available solution for any breeding programme.

Selection on varroa mite population growth, as explored by De La Mora et al. (2024), offers a particularly interesting avenue here: rather than measuring complex behaviours, it focuses on the overall outcome, which integrates the effect of several mechanisms. The results are encouraging, but still require confirmation at larger scale and in varied contexts.

5.5 Selection Does Not Take Place in a Vacuum

In areas with high apiary density, the environment can slow or obscure selection progress through two routes: open mating with drones from unselected colonies, and re-infestation with varroa mites from neighbouring apiaries. This does not make selection impossible, but it makes results harder to interpret and reinforces the value of mating stations, breeding networks and a coordinated approach to colony health management at the local level.

5.6 What Selection Can Reasonably Promise

The most defensible position today is the following: selection is a genuine lever, but its effects unfold over the long term. It can progressively improve the average robustness of colonies within an apiary or a programme, reduce treatment dependence, and contribute to populations better adapted to the parasite pressure of their environment. It is not, however, a short-term substitute for rigorous colony health management.

Presenting selection as the promise of bees that would no longer require treatment would be to promise more than the literature currently supports. It would also be potentially dangerous: colonies receiving insufficient treatment during a transition phase can collapse and become sources of re-infestation for neighbouring apiaries.

6. Practical Implications: What All This Changes at the Apiary

Objective To translate the findings of the scientific discussion into useful reference points for colony assessment, practical selection and colony health management at the apiary.

After concepts and research findings, it is time to return to the apiary. What does this discussion actually change for the beekeeper?

Assess the dynamic, not a single trait. If you wish to identify the most promising colonies, monitoring the trend in infestation level over the season is often more informative than a one-off test on a single behaviour. Colonies that maintain a lower infestation level at the end of summer than comparable colonies in the same apiary are interesting candidates, regardless of which combination of mechanisms contributes to this.

Collective selection is more robust than isolated selection. The choice of environment also matters. A beekeeper wishing to select colonies is not working in a vacuum. In areas with high apiary density, open mating, frequent re-infestation or widely varying health management practices from one apiary to another, correctly interpreting results becomes more difficult. For more interpretable selection, it is therefore preferable to work in as coherent an environment as possible – ideally within a coordinated local network.

Survival alone is not enough. A colony that survives winter has not necessarily done so because it controls varroa well. The infestation level before and after overwintering, the visible viral pressure and colony vigour in spring provide essential complementary information.

Be wary of claims without data. A line presented as "resistant" should ideally be documented by repeated monitoring over several seasons and under conditions comparable to your own. A good result on a single test is insufficient to establish durable resistance under field conditions.

Collective selection is more robust than isolated selection. Programmes combining several apiaries, several assessors and several years generally produce more solid results than purely individual approaches. For a beekeeper, joining an assessment or selection network improves the quality of observations and reduces the risk of incorrectly interpreting a particular case as general evidence.

Colony health management remains indispensable during the transition. Even within an approach oriented towards resistance selection, monitoring infestation levels and applying necessary treatments remain indispensable in the short term. Reducing treatments too early, before documented resistance has actually become established in the apiary, puts not only your own colonies at risk, but also neighbouring apiaries.

7. Conclusion: Between Resistance, Resilience and Beekeeping Management

Objective To restate the essentials: no single trait is sufficient, the colony must be understood as a functional unit, and selection must be understood as a long-term lever to be integrated with rigorous beekeeping management.

The discussion on varroa resistance can no longer be reduced to the opposition of two acronyms or to the search for a single trait. Recent work converges towards a more demanding but also more useful reading: VSH describes a worker behaviour, MNR and DMR describe an observed outcome relating to mite reproduction, and these elements may overlap without being equivalent or systematically correlated (Guichard et al., 2020; Sprau et al., 2024; von Virag et al., 2022).

One of the most important advances of recent years has been shifting the focus from the isolated trait to the colony as a functional unit. What ultimately matters is a colony's capacity to durably slow varroa mite population growth and to maintain sufficient viability over the course of the season and through overwintering. This perspective is further reinforced by work demonstrating that certain effects on parasite reproduction can originate from the brood itself, without being fully explained by adult bee behaviour (Scaramella et al., 2023).

No silver-bullet trait exists today. Recapping, VSH, MNR or DMR are useful elements for research, for breeding programmes and for comparative colony assessment. But none of them alone allows a beekeeper to explain or guarantee durable resistance at the apiary. The resistance observed in the best-performing colonies appears to emerge from a combination of partial mechanisms that vary according to line, population and context (Guichard et al., 2020; Sprau et al., 2024; Scaramella et al., 2023).

Selection therefore remains a credible and necessary avenue, but must be presented in measured terms. The available studies demonstrate both its value and its limits: costly measurements, high variability, sometimes low repeatability, and the persistent difficulty of linking a one-off test to actual survival at the apiary. The most defensible position is to regard selection as a long-term lever, to be integrated with rigorous beekeeping management, rather than as a promise of a simple or immediate solution.

Faced with Varroa destructor, the challenge is not to find an "immunised" bee, but to better understand how certain colonies contain the parasite more effectively, buffer its effects more successfully and navigate the season more durably. It is in this space – between behavioural resistance, brood traits, partial tolerance and colony resilience – that the most useful discussion is now taking place, both for science and for beekeeping.

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See also:

• Varroa Resistance
• "Resistant" Bees to Varroa destructor
• Practical Guide: 4.7 Colony Assessment and Selection
• Development and Dynamics of Bees and Varroa over the Year
• What Wild Colonies Teach Us

 

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