Natural varroa resistance: VSH, selection and resistant lines

Every beekeeper who now and then curses pouring acids and hanging strips dreams of the same thing: a bee that handles varroa on its own. This is not a fantasy, because such bees exist and are described in the literature. But before you throw the products in the corner, I have to put it plainly: natural resistance is years of work, not a switch you flip overnight. Anyone who treats it as an excuse to stop treating will not breed a resistant apiary, they will lose it, and along the way hand their neighbours a varroa bomb.

In this article I want to show what this resistance really rests on, why some colonies cope better than others, and how to move in that direction at home without risking your colonies. It is a natural extension of the article on varroa treatment: since we measure and treat sensibly, the next step is to breed a bee that needs it less and less.

Tolerance and resistance, and why they are not the same

First two words that are often confused, because they lead to two different strategies.

Tolerance is a colony's ability to withstand the presence of varroa without falling apart, even though the mites keep reproducing. A tolerant colony simply endures more, holds together longer, collapses more slowly. But note: such a colony still produces mites that drift onto its neighbours. Tolerance helps the colony itself, but does not solve the problem across the area.

Resistance is something more. It is a situation in which the colony actively limits varroa's reproduction, so the parasite population grows slowly or never gains momentum at all. A resistant colony stifles the problem at the source rather than merely enduring it. And this is what we care about most, because it is the only route that genuinely reduces pressure rather than passing it on to others.

In practice the two overlap, because some of the mechanisms that give tolerance also limit the mite's reproduction. You will see this in a moment.

Why a smaller and swarmy colony copes better

This is the heart of it, and it gets lost in many guides while being key to understanding the whole thing. Varroa reproduces only in sealed brood. No brood, no reproduction. And this is exactly where the advantage of two colony types comes from, the very ones we intuitively consider worse.

First, smaller colonies with less brood. This is precisely the Gotland mechanism, where the surviving colonies build smaller nests and keep less brood, giving the mite fewer cells to breed in. Fewer brood cells to colonise means slower growth of the varroa population. The numbers show it clearly: in Gotland colonies about 48 percent of female mites reproduced, against about 78 percent in susceptible colonies.

Second, and this is the underappreciated point, swarmy colonies. Swarming or supersedure creates a natural break in brood rearing. Through those few broodless weeks varroa has nowhere to reproduce, while the mites on the bees age and die without offspring. This natural broodless period can strongly stall the parasite population, exactly like the artificial queen caging I wrote about under varroa treatment. A colony that regularly gives itself a brood break gives itself a biotechnical varroa treatment for free.

In other words, the traits a production breeder fights like fire, weaker brood rearing and a tendency to swarm, are an asset from varroa's point of view. And here it gets interesting, because a conflict of interests appears.

The catch: a smaller colony means less honey

Nothing comes for free. A smaller colony with less brood and a tendency to swarm gives less honey and needs more supervision. For a production apiary, especially a migratory one, that is often unacceptable, because there colony strength on the flow and no midseason swarming are what count. That is precisely why, for such apiaries, work on true genetic resistance, above all on the VSH trait, is so important. It lets you square the circle: keep a large, productive, non-swarmy colony that at the same time limits varroa on its own.

For a hobbyist the sum looks different. If you do not live off honey, you can deliberately keep weaker, swarmier colonies, because their resistance is worth more than every extra jar. And their lower yield can be worked around by management: uniting colonies for the flow, using shared supers placed over several colonies, making splits from the surplus. In other words, a hobbyist can draw the potential out of such an apiary by pooling forces where needed, instead of fighting the bee's nature. It is a completely different philosophy from the production apiary, and both are fine, as long as you know which one you are running and why.

Four mechanisms of genetic resistance

Beyond colony size and swarming there is a purely genetic layer, that is behaviours that can be strengthened in bees by selection. These are the targets of breeding programmes.

VSH, or varroa sensitive hygiene. The best-described mechanism, popularised by John Harbo and Jeffrey Harris of the USDA lab in Baton Rouge. Workers detect by smell which sealed cell holds a reproducing mite, uncap it and remove the pupa together with the parasite. The effect is double: the mites in that cell die, and the mother mite loses her whole reproductive cycle. In colonies with strong VSH the proportion of mites that manage to reproduce at all can be drastically lower. This is the trait that interests production apiaries most, because it works regardless of colony size.

Recapping, or uncapping and resealing. Closely related to VSH. The bees open the capping over an infested cell, look in, disrupt the mite's development, and then reseal the cell. Even if the pupa survives, varroa's reproductive cycle is disturbed. Recapping is convenient in that it is easy to spot in the nest, so it is used as a simple field marker that a colony carries something of resistance.

Reduced mite reproduction. Called SMR or MNR. A high proportion of female varroa produce no fertile offspring, often precisely because the bees intervene in infested cells in time. It is measured by uncapping brood and counting how many mites sit without offspring and how many with a full litter.

Grooming, or cleaning. Bees bite and knock off the mites walking on the adults, on themselves and each other. You then find damaged, chewed mites on the floor tray, which is a useful clue in itself. A weaker mechanism than VSH, but it genuinely adds up, especially in certain lines and subspecies.

General hygiene, VSH and the aggressiveness question

It is worth separating two ideas that often blur together. General hygiene is the bees' ability to detect and remove sick or damaged brood in general, whatever the cause. Such a colony copes better with chalkbrood, foulbrood and sacbrood, because it simply clears out sick cells. VSH is a specialised subset of that behaviour, aimed specifically at cells with reproducing varroa. Studies show that colonies selected for VSH cope with chalkbrood just as well as colonies selected for general hygiene, and on top of that hold varroa much better. In other words, VSH gives you both, which is why it is so valuable.

And now the thing many beekeepers ask about, because a belief circulates that a resistant bee is a nasty bee. Where did it come from? Mostly from one extreme example: Africanized bees are at once exceptionally resistant to varroa, through strong grooming and hygiene, and notoriously aggressive. Whoever looks at that extreme concludes that resistance goes hand in hand with bad temper.

The trouble is that this association is not a law of nature, only a feature of that particular population. When you look at lines selected for hygiene and VSH in calm European stock, for example the Minnesota Hygienic bees or the VSH lines from the USA, it turns out they are perfectly manageable and not especially defensive. Review work on the payoffs and tradeoffs of hygienic behaviour, looking at honey yield, propolis, royal jelly, swarming and precisely aggressiveness, found no major negative links tied to hygiene itself. The reassuring conclusion: you can have a gentle and resistant bee at once, you just have to select for both goals deliberately, rather than assume one rules out the other.

The practical moral is this: do not confuse temperament with resistance. You check aggressiveness during inspection, and hygiene and VSH with separate tests, such as the freeze-killed brood test or an infestation assessment. Select for both, because a bee that handles varroa on its own but wrecks your apiary at every opening is a poor deal, and happily you are not forced into that choice.

Populations that already do it

We do not have to guess whether resistance is possible, because nature has shown it where bees were left to their own devices and the ones that coped survived.

The most famous example is Gotland in Sweden, where colonies were left untreated as an experiment. Most died, but a handful survived and after years form a population living with varroa without treatment, precisely thanks to smaller nests, less brood and reduced mite reproduction. Similar, independently arisen populations have been described near Avignon in France, where VSH and recapping are strongly marked, in the Russian Primorsky region, in African bees and their Africanized hybrids in Brazil, and in the Cape bee in South Africa, where grooming and recapping come to the fore.

Different populations take different routes, some through smaller nests and swarming, others through hygiene, recapping and grooming, but the conclusion is shared: the honey bee can evolve to live with varroa if selective pressure acts on it. In fairness, the other side must be added: these populations arose at the cost of enormous losses at the start, in isolation, and their traits are not always easy to move into a production apiary without losing honey yield or gentleness.

How to start at home without losing colonies

Theory is theory, but the apiary has to be wintered. A few steps you can weave into normal management.

First, select from your own best colonies. You monitor infestation anyway, so you have the data ready. Colonies that year after year hold low infestation despite the same pressure as the rest are your breeding stock. Raise queens from them, not from the worst. This is the simplest selection there is, and it works, as long as you are consistent over several seasons.

Second, learn simple tests. The VSH test consists of uncapping a portion of brood of the right age and counting how many mites sit there without offspring and how many with a litter. Add to that watching for recapping, that is the marks of resealed cells. A colony with a high proportion of infertile mites and clear recapping is a candidate for breeding.

Third, replace queens with proven lines. If you do not want to run your own breeding, reach for queens from programmes aimed at resistance. In Europe several initiatives are active, among them Arista Bee Research and programmes in the Buckfast and Carnica lines, and across the ocean the VSH lines from the USDA. Replacing the queen in a highly infested colony can turn the trend over time, as her daughters carrying the desired trait build up in the nest.

And the most important thing, I will repeat it because it is the Achilles heel of the whole idea: do not drop treatment overnight. Until your colonies prove by measurement that they hold varroa in check on their own, you treat as usual. Resistance is built alongside control, not instead of it. A naive switch to zero treatments in an ordinary apiary ends in an autumn catastrophe and a rain of mites onto the neighbours.

My view

I treat resistance as a goal you reach, not as an alternative to monitoring and sensible treatment. From the colonies that cope best I raise queens, and the ones that collapse on varroa every year drop out of breeding. It is not spectacular and cannot be done in one season, but the direction is right.

And one more thing, easy to forget: breeding for resistance only makes sense on a scale wider than a single apiary. As long as neglected hives shedding mites stand next door, even good genetic work will be diluted by drift. So it is a topic where it pays to reach an understanding with your neighbours and the local association, rather than fight alone.

Curiosities and controversies

Varroa does not drink blood. For decades it was taught that the mite feeds on haemolymph, the bee's blood. Ramsey's 2019 work showed this is untrue: varroa feeds mainly on the fat body, the organ that in a bee acts as liver, protein store and part of the immune system. This changes the picture of the damage. The mite not only weakens, it strikes exactly what the winter bee needs most: its reserves and its immunity.

The treatment-free movement and varroa bombs. There is a sizeable group of beekeepers raising bees entirely without treatment, counting on natural selection. The other side accuses them that untreated, collapsing colonies become varroa bombs: when they die, their mites spread by robbing and drift onto properly managed apiaries nearby. This is one of the hotter disputes in beekeeping, because it touches the question of where your freedom ends and your neighbour's problem begins.

The small-cell myth. A popular theory held that shrinking the worker cell from about 5.4 to 4.9 millimetres limits varroa by shortening the brood development cycle and giving the mite less time to breed. It sounds logical and many believed it, but successive studies mostly failed to confirm it. It is a good example of how a neat theory can live a life of its own despite a lack of evidence.

By treating, we hold back evolution. There is a bitter logic in it: as long as we rescue every colony with a treatment, we do not let natural selection act, so the bee never learns to cope on its own. Advocates of the live and let die approach argue that we keep bees on a drip. The problem is that the price of that evolution is mass losses at the start, which no ordinary apiary can afford. Hence the sensible compromise: select from the best rather than slaughter them all.

Sources and further reading

• Harbo J. R., Harris J. W. Work on the VSH and SMR traits, USDA-ARS Baton Rouge. The basis of modern selection for resistance.

• Locke B. (2016). Natural Varroa mite-surviving Apis mellifera honeybee populations. Apidologie 47: 467-482. A review of naturally resistant populations, including Gotland.

• Locke B., Fries I. (2011). Characteristics of honey bee colonies in Sweden surviving Varroa destructor. On the Gotland colonies, smaller nests and reduced mite reproduction.

• Leclercq G. et al. (2017). Review of the payoffs and tradeoffs of hygienic behaviour. On the relationship between hygiene, honey yield, swarming and defensiveness.

• Arista Bee Research, materials on selecting for VSH and breeding programmes in Europe (aristabeeresearch.org).