0. Introduction: From Biology to Practice

Objective To present the biological mechanisms by which certain colonies slow varroa reproduction, and the concrete methods that allow these to be evaluated — as a complement to the article on conceptual issues and the limits of selection.

Understanding varroa resistance requires starting from a concrete point: the biology of the parasite. It is from this biological cycle that the various windows of action available to a colony derive, and it is from these windows that the behaviours and traits which breeding seeks to exploit emerge.

This article complements The Colony Facing Varroa: Resistance, Resilience and the Limits of Selection, which addresses the conceptual issues (resistance vs. tolerance vs. resilience), interactions between mechanisms and long-term perspectives. It focuses here on the how: how the parasite reproduces, what mechanisms the colony can deploy at each stage, and how these mechanisms can be assessed at the apiary.

1. The Varroa Cycle: Why Capped Brood Is the Critical Point

Objective To establish the biological foundations: the varroa reproductive cycle defines two windows of intervention for the colony — one in the capped brood, the other during the phoretic phase on adult bees.

Outside the brood, the varroa mite lives in the phoretic phase on adult bees: it feeds there and awaits the opportunity to enter a cell at the moment of capping. All reproduction takes place in the capped brood. After capping, the foundress female lays successively one male egg and then female eggs. She feeds on the pupa, and her fertilised daughters leave the cell at the bee's emergence. One complete cycle in a worker cell takes approximately twelve days.

This biological cycle defines two windows of intervention for the colony: one in the brood (acting on parasite reproduction before or while it is taking place), the other on adult bees (acting on the varroa mite during its phoretic phase, between two reproductive cycles). The known resistance mechanisms operate at one or the other of these two stages — or at both simultaneously.

One practical implication of this cycle deserves emphasis: any broodless period — whether resulting from swarming, a winter brood break, or a beekeeping intervention — interrupts varroa reproduction and mechanically reduces the growth of its population. This dynamic is exploitable, both to optimise treatment efficacy and as a complementary lever within an integrated management approach.

2. Mechanisms Active in the Brood

Objective To describe the principal mechanisms by which the colony acts on varroa reproduction in the capped brood, distinguishing worker behaviours, observed phenotypes and brood-specific traits.

2.1 VSH: A Behaviour Targeted at Infested Brood

VSH (Varroa Sensitive Hygiene) is the best-documented mechanism in this window of action. Workers detect infested capped brood, uncap the cell and remove the pupa or the mite. This behaviour acts directly where the parasite reproduces, which explains its relevance in breeding programmes (Panziera et al., 2017; Guichard et al., 2020).

It must nonetheless be recalled that it is a behaviour, not a guaranteed outcome. A high VSH score in a test does not automatically translate into a favourable infestation dynamic over the full season. Its effect depends on the intensity with which it is expressed, on the assessment conditions, and on the combination with other mechanisms.

2.2 Varroa Non-Reproduction: An Outcome That Can Arise from Several Sources

MNR (Mite Non-Reproduction) and DMR (Decreased Mite Reproduction) describe an outcome observed in the cells: a high proportion of mites that produce no viable offspring there. These terms have progressively replaced the older SMR (Suppressed Mite Reproduction), whose formulation implied a single active mechanism, whereas several causes can produce the same phenotype.

A high MNR rate may reflect the effect of VSH — but also of brood-specific traits. Scaramella et al. (2023), working from naturally surviving populations in Scandinavia and France, showed that brood characteristics — chemical composition, olfactory signals, physiological properties — suggest 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 changes the interpretation of the problem: varroa non-reproduction can emerge through several distinct biological pathways, and an assessment based exclusively on visible behaviours risks missing part of the picture.

2.3 Recapping: A Complementary Indicator, Requiring Careful Interpretation

Recapping refers to the opening of a capped, infested cell by workers, followed by its resealing without systematic removal of the pupa. This behaviour is heritable and has a documented association with certain resistance levels (Gabel et al., 2023). Its interpretation remains delicate, however: its frequency may signal useful vigilance, but may also simply reflect a high parasite load. It is a complementary indicator, not a primary selection criterion on its own.

2.4 The Duration of the Capped Phase

A shorter capped phase mechanically reduces the number of possible reproductive cycles for the parasite in each cell. The Asian honey bee (Apis cerana), the varroa mite's natural host, displays characteristics in this respect that are less favourable to parasite reproduction than those of the European honey bee. In Apis mellifera, the range of variation exploitable through selection on this trait is difficult to establish, and its practical value for applied breeding remains limited to date.

3. Mechanisms Active During the Phoretic Phase

Objective To present the less frequently discussed mechanisms that act on the varroa mite during its phase on adult bees, between two reproductive cycles in the brood.

Less often highlighted than mechanisms active in the brood, the behaviours that act on the varroa mite during its phoretic phase nonetheless constitute a real lever, still insufficiently exploited in breeding programmes.

Grooming behaviour — a bee removing a varroa mite from its own body or that of a nestmate — exposes the parasite to a risk of falling or being damaged. It is well documented in Apis cerana and observed to varying degrees depending on the line in Apis mellifera. Its measurement remains difficult and selection methods based on this criterion are not yet standardised on a large scale.

The capacity to physically damage the varroa mite — tearing off a leg, perforating the exoskeleton — is another mechanism observed in certain lines. Its assessment requires examination of dead mites under a stereomicroscope on the open mesh floor, which makes it time-consuming. It represents a complementary avenue at this stage rather than an established operational criterion.

These two mechanisms are difficult to clearly distinguish from one another under field conditions, and their relative contribution to the overall dynamic remains poorly quantified. They deserve mention as potential additional levers, without selection being able to rely on them with the same rigour as for VSH or MNR. For practical breeding, this means these mechanisms remain of interest, but must be read as complementary levers rather than as autonomous and sufficient criteria.

4. Assessment Methods: Strengths and Limits

Objective To review the main available assessment methods — their principles, advantages and practical limitations — to help select the most appropriate approach depending on the context and available resources.

The quality of a breeding programme depends closely on the reliability of the measurement methods. This is where one of the main current bottlenecks lies: reliable, precise and practical methods are rare. Every available method involves trade-offs between rigour, time investment and scalability.

4.1 Standardised VSH Test

Heavily infested brood is introduced into the colony to be tested; the number of varroa mites in 200 cells is counted before and after one week. The measured reduction provides direct information on VSH behaviour. The method is reliable and well described, but it requires heavily infested source colonies, a demanding protocol and an early evaluation point in the season. It remains difficult to apply at scale by an individual breeder or a small operation.

4.2 Non-Reproduction Rate (MNR / DMR)

The brood of the tested colony is examined to distinguish reproductive mites (presence of offspring) from non-reproductive ones. The method is informative, but displays non-negligible biological variability: results depend on seasonal and environmental factors and on the composition of the mite population in the colony (von Virag et al., 2022). A single good score is insufficient to durably characterise a colony, and the method requires a sufficient infestation level for the data to be statistically usable.

4.3 Pin Test (Hygiene Test)

A small area of brood is damaged — by pinning or freezing with liquid nitrogen — and the speed and intensity of cleaning by workers is measured. A good score is consistent with VSH but is not a direct predictor of it: VSH involves specific detection of the varroa mite within the cell, which cannot be reduced to general cleaning behaviour. Colonies with a low pin-test score do, however, generally also display weak VSH behaviour. The method is useful as a preliminary filter in a breeding programme, but insufficient on its own.

4.4 Monitoring Varroa Mite Population Growth (MPG)

The trend in infestation level over the season is estimated through repeated measurements: counting on adult bees (using icing sugar or alcohol), or through natural mite drop on an open mesh floor. This is the method most directly linked to the overall outcome, as it integrates the effect of all active mechanisms — whether behavioural or brood-related — without having to identify them separately. It is this approach that divergent selection work (De La Mora et al., 2024) has used to produce the most encouraging results to date. Its precision depends on the regularity of measurements and the consistency of protocols between the colonies being compared.

4.5 "Live and Let Die" Test

Treatments are reduced or stopped and it is observed which colonies maintain the lowest infestation levels. This is the method most directly relevant in terms of the final biological outcome, but also the riskiest: colonies carrying interesting genetic material but insufficiently resistant may collapse before they can be selected, and become sources of re-infestation for neighbouring apiaries. It is only justified in a context where previously documented resistance is already present in the stock, and ideally in coordination with beekeepers in the area.

5. Some Sources of Documented Resistant Material

Objective To mention, without claiming exhaustiveness, some populations and lines whose resistance characteristics are documented, situating their value and their limits for selection in a European context.

Without claiming exhaustiveness, certain populations deserve mention as documented sources of genetic material displaying measured resistance levels.

The naturally surviving populations of Scandinavia (Norway, Sweden/Gotland) and certain regions of France have been studied without treatment over several generations. They display in particular a higher varroa non-reproduction rate than managed populations, and characteristics consistent with a more balanced host-parasite relationship (Oddie et al., 2017; Locke & Fries, 2011; Scaramella et al., 2023). These populations constitute valuable reference material for research and certain breeding programmes.

The VSH lines developed in the United States constitute one of the best-known examples of selection on behaviours associated with varroa resistance. Other programmes, in particular from bees of Russian origin (Primorsky), have also contributed to the search for more resistant material. These lines are sometimes perceived as less productive than other commercial lines, which has led to cross-breeding programmes aimed at combining resistance with other desirable traits. Initiatives of this type have emerged in Europe (Buckfast × VSH crosses in particular).

In Europe, several breeding programmes combine resistant material with beekeeping performance criteria: the Carnica AGT programme, Elgon bees, the Duurzame Bij project, selections in France and Switzerland, among others. These initiatives rely largely on motivated volunteers and display varying levels of documentation. Their relevance for a given beekeeper depends on the compatibility of the genetic material with local conditions and the rigour of the evaluations conducted.

One point must be emphasised: genetic material does not express itself in a vacuum. In areas with high apiary density, open mating, high re-infestation pressure or heterogeneous health management practices between apiaries, colony assessment and selection progress can be obscured. The choice of selection environment — or working within a coordinated local network — is therefore as much a part of the method as the choice of assessment criteria.

6. Conclusion: Mechanisms, Measurements and Realism

The biological cycle of the varroa mite defines two moments at which the colony can act: in the capped brood and during the phoretic phase on adult bees. The mechanisms best documented today — VSH, MNR/DMR, recapping, brood traits — operate principally in the first window. Phoretic mechanisms (grooming behaviour, physical damage) constitute additional levers, still insufficiently exploitable in breeding due to the absence of standardised methods.

No assessment method is perfect. The VSH test is reliable but costly to implement. MNR monitoring is informative but variable. The pin test is practical but indirect. Monitoring varroa mite population growth is the most directly relevant, but requires repeated and consistent measurements. The choice of method must therefore be adapted to the available resources, the scale of the programme and the objective being pursued.

What recent work suggests with some consistency is that the resistance observed in the best-performing colonies emerges from a combination of partial mechanisms — and not from a single trait. This is why an assessment based on several complementary indicators, followed over time and under comparable conditions, gives a more accurate picture than a one-off test on a single behaviour.

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See also:

• The Colony Facing Varroa: Resistance, Resilience and the Limits of Selection
• "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

 

Bibliography

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Gabel, M., Hoppe, A., Scheiner, R., Obergfell, J., & Büchler, R. (2023). Heritability of Apis mellifera recapping behavior and suppressed mite reproduction as resistance traits towards Varroa destructor. Frontiers in Insect Science, 3, Article 1135187. https://doi.org/10.3389/finsc.2023.1135187

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