Varroa-Tolerant Bees: Breeding Approaches

For over 40 years, European beekeepers have been fighting the Varroa mite. Acid, thymol, drone brood removal -- all measures that keep the parasite in check but do not defeat it. Would it not be much better if the bees could help themselves?

That is exactly what bee scientists and breeders worldwide are working on. In this lesson, you will learn which natural defence mechanisms bees can develop against Varroa, which breeding programmes are running in Germany and Europe, and how realistic the vision of "treatment-free" beekeeping actually is.

The Asian Honey Bee: The Role Model

The Asian Honey Bee? (Apis cerana) has lived with the Varroa mite for millions of years and has developed effective defence mechanisms:

• Active grooming: The bees actively clean mites off their nest mates
• Shorter capping period: The brood develops faster, limiting mite reproduction
• Recognition of infested cells: Infested brood cells are opened and the brood removed
• Encapsulation: Mites are sometimes sealed in propolis

Our Western honey bee (Apis mellifera) was only confronted with Varroa destructor in the 1970s -- far too short for natural adaptation. But some promising behaviours can be promoted through targeted breeding.

The Three Pillars of Varroa Tolerance

1. VSH -- Varroa Sensitive Hygiene

VSH? is the best-researched and most promising trait for Varroa tolerance. VSH bees can detect capped brood cells infested by Varroa and systematically remove them.

How VSH works:

1. Worker bees inspect capped brood cells (presumably via scent compounds)
2. They detect cells in which a Varroa mite is reproducing
3. The cell is opened and the pupa (along with the mites) is removed
4. The queen re-lays in the empty cell

Efficacy: In studies by USDA Baton Rouge, VSH lines showed 50-80 % lower mite reproduction. The mite population grows significantly more slowly, and the colonies require less or no chemical treatment.

2. SMR -- Suppressed Mite Reproduction

SMR? describes the phenomenon whereby in certain bee colonies, the reproduction of the Varroa mite is suppressed. The mite enters the brood cell but cannot reproduce successfully.

Possible causes:

• Bee larvae produce signalling substances that disrupt the mite's reproductive cycle
• The capping period is slightly altered and does not match the mite's timing
• The cell environment (temperature, humidity) is unfavourable for mite reproduction

Distinction from VSH: While VSH is an active behaviour of adult bees (opening cells, removing brood), SMR occurs passively within the cell. Both mechanisms can occur together and complement each other.

In practice, VSH and SMR are hard to separate, as both lead to a reduction in the mite population. Modern breeding programmes therefore often record the overall effect as "recapping" (resealing of cells) and "mite reproduction rate".

3. Grooming Behaviour

Grooming? is the active removal of mites from one's own body (autogrooming) or from nest mates (allogrooming). The bees bite or scratch the mite, which is often damaged or falls off in the process.

Identifying signs:

• Damaged mites on the monitoring board (bitten legs, holes in the exoskeleton)
• Higher number of dead mites in the debris despite low infestation

Practical significance: Grooming alone is generally not sufficient to keep a colony mite-free. It reduces the mite pressure by an estimated 10-30 % and is therefore more of a complementary trait. In combination with VSH and SMR, however, it can make the decisive difference.

Bee Races and Varroa Tolerance

Not all bee races behave the same way towards the Varroa mite. Here is an overview of the races relevant in Germany:

Carnica

The most widespread race in Germany has been selected for decades for gentleness, honey yield, and low swarming tendency. Varroa tolerance was long not a primary breeding goal but is increasingly being incorporated. Individual Carnica breeding lines show promising VSH behaviour.

Buckfast

The breeding line founded by Brother Adam (Buckfast Abbey) is known for its adaptability. Buckfast breeders started early to integrate Varroa tolerance as a breeding goal. The open breeding philosophy (crossing in different subspecies) facilitates the introduction of tolerance genes.

Dark Bee (Apis mellifera mellifera)

The original European honey bee is experiencing a renaissance. Proponents argue that the race adapted to northern European conditions shows naturally higher grooming behaviour and more robust health. However, the data on Varroa tolerance is still limited.

Breeding Programmes in Germany

AGT -- Working Group on Tolerance Breeding

The AGT? (Arbeitsgemeinschaft Toleranzzucht) is the largest German network for Varroa tolerance breeding and coordinates the breeding work of over 150 apiaries.

Methodology:

• Annual performance testing at test apiaries
• Measurement of natural mite drop (as an indicator for tolerance)
• Recording of hygienic behaviour (pin test, freeze test)
• Since 2018 also VSH testing with standardised methods
• Breeding records via Beebreed

Results: The AGT reports measurable progress: colonies from the tolerance breeding programme show on average 15-25 % lower mite reproduction rates than unselected colonies. Individual top colonies achieve significantly higher values.

Beebreed

Beebreed? is the central European breeding value database for honey bees, operated by the Bee Research Institute Hohen Neuendorf.

• Over 170,000 test results from bee colonies
• Calculation of breeding values for honey yield, gentleness, swarming tendency -- and increasingly also Varroa tolerance
• Publicly accessible at beebreed.eu
• Beekeepers can specifically search for queens with high tolerance breeding values

SMARTBEES and Other EU Projects

The EU research project SMARTBEES (2014-2018) laid the groundwork for Europe-wide tolerance breeding:

• Development of standardised VSH test methods
• Identification of genetic markers for tolerance traits
• Establishment of a European network of tolerance test apiaries
• Follow-up projects build on these results

Tolerance breeding is making steady progress. We see significantly lower mite reproduction rates in selected colonies. But it is important to note: we are breeding for tolerance, not resistance. The goal is for colonies to manage with less treatment -- not with none at all.

The Reality Check

As promising as the research is -- an honest look at the current state is important:

What Tolerance Breeding CAN Do

• Reduce the mite reproduction rate by 15-30 % (in selected lines)
• Extend the window between treatments
• Make colonies more robust against mite infestation
• Reduce treatment effort in the long term

What Tolerance Breeding CANNOT (Yet) Do

• Completely replace chemical treatment
• Deliver "Varroa-resistant" bees within a few years
• Guarantee that a single colony survives

Why Progress Is So Slow

Breeding for Varroa tolerance is particularly challenging for several reasons:

1. Mating biology: Queens mate in free flight with 10-20 drones. Genetic control is more difficult than with mammals.
2. Multi-level selection: VSH is a behaviour of the workers, but inherited through the queen -- an indirect breeding target.
3. Conflicting selection: High Varroa tolerance has historically often come with lower honey yield or higher swarming tendency. Both must be selected for simultaneously.
4. Natural recombination: After successful selection in one generation, mating with unselected drones can dilute the progress.
5. Not a single-gene trait: Varroa tolerance is based on many genes that interact in complex ways.

What You Can Do as a Beekeeper

Even without your own breeding station, you can contribute to tolerance breeding:

1. Buy Queens from Tolerance Breeders

   Source queens from AGT breeders or breeders with documented tolerance breeding values on Beebreed. Check with your regional beekeeping association for recommended breeding operations.

2. Apply Selection Pressure

   Preferentially multiply from colonies that show low natural mite drop despite a normal treatment regime. Do not use weak or mite-susceptible colonies for rearing.

3. Participate in Monitoring

   Take part in test apiaries run by your beekeeping association or regional body. Your data feeds into the breeding value estimation and helps the community.

4. Observe Vitality

   Document not only the mite infestation but also the hygienic behaviour of your colonies. How quickly do they remove dead brood? Do you find damaged mites on the monitoring board? These observations are valuable clues.

Scientific Outlook

Research is advancing rapidly. Some exciting approaches for the future:

Genomic selection: Through DNA analysis, breeders can assess which queens have the highest tolerance potential even before performance testing. This significantly accelerates breeding progress.

CRISPR and genetic engineering: Theoretically, genes for VSH or SMR could be specifically enhanced. In the EU, this is currently not possible for regulatory reasons, and ethical questions remain unresolved.

Natural selection: In some regions (e.g. Gotland/Sweden, Avignon/France), feral bee colonies have survived for years without treatment. These populations are being intensively researched -- they show that natural adaptation is fundamentally possible, but at high cost (over 50 % of colonies die in the first years).

RNAi technology: RNA interference could in the future specifically suppress Varroa mite reproduction without harming the bee. Initial studies show promising results, but market readiness is still years away.

Knowledge Check

In the next lesson, we show you how digital Varroa tracking with dashboards and forecasts helps you make data-driven decisions.