Varroa Resistant Honey Bees

Q. What is the ambition or purpose of the website?

A. To help beekeepers understand that there is an evidenced based alternative to being locked into the annual or bi-annual Varroa treatments as the majority (72%) of UK beekeepers are currently doing (Valentine & Martin 2023). The website is a place where beekeepers can access the scientific evidence, advice and examples of beekeepers already managing Varroa resistant bees. 

Q. I have monitored the mites in my colonies, and I have one colony that has high mite numbers. I don’t want to use Varroacides. What should I do?

A. There are various biotechnical options for Varroa control depending on the size of the colony and the time of the year. The National Bee Unit’s booklet ‘Managing Varroa’ has good detail on biotechnical controls www.nationalbeeunit.com. If it is the active beekeeping season, your colony is strong, and the weather forecast is for a week of good weather then a shook swarm can be very effective. On a weak colony it can be too traumatic. Other biotechnical controls to consider are drone brood trapping, using an artificial swarm for swarm control, Queen frame trapping. Some of these techniques are more time consuming than others and the suitability will vary depending on the time of year. Once mites are under control consider re-queening or uniting from one of your better colonies. 

Q. Are Varroa resistant colonies small, swarmy and unproductive?

A. Varroa resistant traits can be found in all sorts of colonies in the UK. Resistant colonies vary in colour, size, swarminess, productivity, thriftiness etc. There is no reason or evidence why they should have undesirable traits if beekeepers are involved in selecting the colonies to raise Queens and drones from. The Varroa resistant colonies on the island of Gotland, Sweden are small while those found in Cuba are prolific and very productive, while those in North Wales are in between. 

Q. I only have 2 colonies and I don’t want to lose them. Can you suggest how to minimise the risks of treatment-free beekeeping?

A. There can be risks involved in going treatment-free. Mite numbers need to be monitored and managed if they are getting high, if you are to reduce the risk to your colonies. Look closely at your colonies for any of the traits associated with Varroa resistance (bald brood, recapping behaviour, cannibalism, bee antennae & immature mites on the monitoring board). If your colonies exhibit these behaviours, then you are off to a good start especially if the mite fall is low. If not, then follow the suggestions on reducing your Varroa treatments while increasing monitoring your mite levels or try to buy bees/ queens from a local beekeeper who has colonies that are resistant or collect a swarm from an area that free-living colonies appear to persist.

Q. Could it be the mites that are different rather than the bees, in treatment-free colonies?

A. There is currently no good scientific evidence to support this idea.  In Varroa the timing each egg is laid, and the number of eggs laid has not changed in over 20 years. This is despite strong selection pressure in worker brood to reduce the number of eggs laid since the 4th and 5th offspring almost always die before the pupae emerges. The mite haplotype is the K (Korean) type that dominates almost all Varroa populations now.   

Q. How long does it take for the European honeybee to adapt to Varroa?  

A. This is a very difficult question to answer. When the population was never treated in Cuba [European honeybees] and South Africa [Apis capensis & Apis scutellate] it took around 5-7 years for widespread resistance to appear. A similar time period was observed in Hawaii [European honeybees]. If beekeepers help in the transition (Westerham Beekeepers Association) by removing colonies with high mite numbers and selecting from those that have resistant traits e.g., lower mite counts it takes around 5 years. Much depends on whether the bees are allowed to adapt.  Treatments stop adaption. You can stop Varroa treatment straight away if you collect swarms from persisting free-living colonies or buy mated queens/splits from a beekeeper that has not treated for at least 6 years. But ensure you monitor your mite levels during the first year to confirm resistance. 

Q. Surely it takes hundreds of years for honeybees to change their behaviour?

A. This is a common misperception since it all depends on the scale of the change and pressure applied. Honeybees already have developed a complex hygienic behaviour to deal with wax moth, chalkbrood and AFB. This behaviour allows the bees to detect and remove dead and diseased larva and pupae. Work from Prof Marla Spivak’s group has uncovered the role of different workers in the process and even changes in brain chemistry of specialised detector worker bees. Thus, the honeybees already have the mechanism but just need time to learn to associate the unique smell coming from mite-infested cells with the mite’s presence (Mondet et al., 2021). The ability of colonies to detect infested pupae will be variable. In free living colonies the selective pressure is extreme and only those best adapted survive. Even then it takes around 5 years for resistance to become widely establish in the population.       

Q. Why are unmanaged/feral colonies resistant to Varroa?

A. They are exposed to natural selection whereby the susceptible colonies die off, as many did when Varroa arrived.  Resistant colonies repopulate areas gradually from swarms and drones are or may be an important source of genetic fitness, but more research is needed to confirm the role of the drones.

Q. What is a high mite count from natural drop or sugar shake / alcohol wash?

A.  Interpretation of mite drop /sampling should be carried out in conjunction with the beekeeper monitoring for hygienic behaviours and  used in isolation to reach conclusions on "susceptibility" versus "resistance" of a colony.  Westerham experience: (see real example) Natural mite drop, from a National colony, of below 5 per day through the season would indicate sufficient hygienic behaviour to confer varroa resistance.  5-10 per day, resistant-->susceptible and over 10 per day = susceptible. In experiments comparing resistant to susceptible colonies, resistant colonies had on average below 5% of infested adult workers in September. Any colony with over 10% of infested adult workers in September should be regarded as high. At other times of the year, you would expect lower infestation levels. 

Q. What's the difference between the effect of DWV type A and B?

A. DWV-B is more transmittable than DWV-A. This is because unlike DWV-A, DWV-B can replicate within the Varroa mite. Both strains can kill a colony, with some evidence suggesting DWV-B is more virulent, but more studies need to be conducted.

Q: How does DWV impact the colony due to the chewing out of infected pupae? 

A. It has been elegantly shown under lab conditions that chewing out or cannibalism of the pupa does result in spreading of DWV among the workers (Posada‑Florez et al., 2021). However, the impact of DWV is not as great as in developing pupae. Although over a longer timescale DWV levels are lower in Varroa resistant populations than treated populations (de Souza et al., 2020). This lowering will be caused by the reduction of Varroa mites in the resistant colonies. It is also important to realise DWV is circulating in all hives via natural viral transmission pathways.

Q. Why don’t people make a vaccine against DWV?

A. Simply its far too costly (look at covid) and would only work for a limited amount of time.

Q. Why should commercial beekeepers consider treatment free?

A. Reduced labour time and miticide costs.  Selling resistant bees into a growing market.  Produce a sustainable honeybee not dependent on increasing amounts of treatments to keep them alive.  There is also the possibility of mites becoming resistant to varroacides, making control more difficult.

Q. Do you lose genetic diversity by selecting for Varroa resistant traits?

A.  Yes, if using instrumental queen insemination from single colony sourced drones.  It is very unlikely for openly mated queens. Prof Tom Seeley found that the entire nuclear genetic diversity in honeybees can be recovered after a bottleneck event in just a few reproductive cycles. 

Q.  Are Resistant traits lost with openly mated queens?

A  Westerham Beekeeper and Foragers Bee & Honey Co experience:  We have found strong heritability between mother and daughters. Influencing the local drone population also helps. 

Q. Is resistance genetic or learnt?

A. The new odour coming from mite infested cells must first be learnt, but then it becomes hardwired into the queens potentially via epigenetic mechanisms. 

Q. What are the genetics / alleles involved?

A. These are currently unknown and could prove very difficult to work out, since resistance becomes a behavioural change i.e., knowing what the new odour means. Identifying sets of linked genes involved in behavioural differences is notoriously difficult. Although several scientists are looking for SNP’s (single nucleotide polymorphisms) that could indicate resistance.

Q. What do you think of UBOs?

A. Unhealthy Brood Odours (UBOs) are one of the early attempts to identify the odour coming from an infested cell. It’s based on several long-chained alkenes commonly found inside all beehives. The nice idea is the synthesised mix of compounds are sprayed onto cell caps and the speed of uncapping is used as a measure resistance. More recently a major study from a French team (Mondet et al., 2021) found several unique compounds coming from Varroa infested cells. Unlike UBO’s these do not naturally occur in honeybee colonies and the team have strong support that these are the key compounds used by workers to detect infested cells. 

Q. Why has reduced mite fecundity long been associated with resistant colonies?

A. We now know that this is caused by workers in resistant colonies detecting and removing infested pupa. Although the mother mite can escape to reproduce again, she loses all her offspring from the cell. Female varroa mites can live for up to one year but only preform 2 to 3 reproductive cycles with their  20-25 eggs. The more a mother mite’s reproduction is disturbed, the fewer eggs she has. These soon run out and she becomes an infertile mite. Therefore, increased detection and removal leads to reduced mite fecundity.    

Q. Do you get a resistance halo effect in other nearby colonies?

A. Although there are no formal studies to answer this question circumstantial observations do appear to support the idea. In the UK we have measured recapping in colonies in two areas nearby a long-term resistant beekeeper and in both cases many of the colonies appeared resistant. So if you have a  beekeeper with Varroa-resistant bees nearby that may mean your colonies are well on the way to becoming resistant.  

Q. Will I get into trouble from my association or the National Bee Unit (NBU) if I want to stop treating regularly.

 A. Whereas in the past treatment-free beekeepers were in the minority and shouted down if they mentioned that at meetings, and told they must treat by the NBU, things are changing. The evidence both from long-term treatment free beekeepers and recent scientific evidence has clearly demonstrated that it is possible to manage full sized, productive, calm colonies without the need to treat for Varroa (see beekeeper examples on advice page). The NBU have suggested if beekeepers employ an IPM approach and treat colonies only when required they don’t mind if Varroa mites are controlled by the bees  or conventional treatments.   

Q. Can I just stop treating for varroa and what checks, if any, should I do? 

A. You can just stop treating but it is not recommended. There is a high risk of losing your bees within 2-3 years if they are not Varroa resistant, which is unlikely. It would be less risky to increase your Varroa monitoring, while reducing your use of Varroacides or biotechnical controls. It is also worth taking a close look at frames of brood for signs of cannibalism and recapping behaviour to assess whether your bees might already be showing signs of resistance.

Q. Why do most beekeepers and associations encourage varroa treatment and why do you suggest going against their advice? 

A. When Varroa first appeared in the UK there were lots of colony losses. Chemical Varroacides were used to reduce losses and have been the standard advice for many years. It is your decision on how you manage Varroa in your colonies. There is strong evidence that Varroa resistant bees are present in many parts of the UK (Valentine & Martin 2023). If your bees have these abilities, then you have the option of going treatment-free. Like most things in beekeeping, whatever your approach, not everyone will agree with your choice! 

Q. What evidence do you have to support the claim that bees can become naturally varroa resistant?

A. In almost all the southern hemisphere (Africa, Brazil, Central America, Mexico, and the Caribbean Islands) naturally varroa resistant populations have arisen (see research page), some several decades ago. In the UK a small but increasing number of beekeepers have demonstrated that it is possible to keep honeybees without treatment (see examples on advice page). Likewise in  Europe and the USA increasing numbers of beekeepers are switching to treatment free beekeeping.    

Q. - Will I lose all my bees? 

A. Bee colonies die for lots of reasons and Varroa resistant bees are not invincible. Varroa mites spread viruses such as DWV that can kill colonies and so all beekeepers aim to have colonies with low Varroa populations. How this is achieved is up to you. You can use Varroacides, biotechnical methods or keep Varroa resistant bees.

Q. If I see Varroa, what should I do? 

A. Most colonies on the UK mainland have Varroa mites. If you see Varroa, is it because you went looking for them on a floor insert or on drone brood? If so, well done for spotting them. As long as numbers are low, don't panic. If you noticed Varroa mites on adult bees or on the comb during a routine inspection, then you might have high mite numbers and need to check if they need further monitoring or treatment.

Q. Why is it the use of local bees/colonies is encouraged for Varroa resistance/non treatment? 

A. Varroa resistance has been found in a wide range of bee populations across the world and so it is not just one type of bee that has this ability. Local bees are recommended as they hopefully have adaptations that will help them do well in your area. A major European study (COLOSS) came to that conclusion. So ideally work with your local bee population and select the best from them. If the local population includes long-lived free-living colonies, then the genes for Varroa resistance could already be there as this a very common way of  building up your Varroa-resistant colonies (see examples on advice page).