3rd Honey bee pests, diseases and poisoning revision post: the lifecycle of Varroa destructor and monitoring & treatment techniques

B3 a) describe the life cycle and natural history of Varroa destructor & b) how does it develop within the colony? 

Varroa is the number one killer of colonies in the UK, so it’s really worth studying. Whereas European/American foul broods are dreaded but fairly rare diseases, unless you live in Australia or a remote island, varroa mites will be in your hive – whether you notice them or not.

Varroa destructor was originally a parasite of the Asian honey bee, Apis cerana. Through the movements of humans it has spread in recent decades to the Western honey bee, Apis mellifera, which unlike Apis cerana has not yet evolved any natural defences against it.

A female varroa mite – © Crown copyright 2010 “Courtesy The Food and Environment Research Agency (Fera), Crown Copyright”

The life cycle of varroa mites has two stages: a reproductive stage inside sealed brood cells and a phoretic stage as a parasite on adult bees.

An egg-carrying female mite will enter an uncapped cell with a larva inside and bury herself under the larval food, where the bees (and beekeepers) can’t detect her whilst inspecting. She uses specialised tubes to breathe during this time. A smell is given off by larva ready to be capped; female mites sense this earlier than adult bees and receive a cue to enter cells just before they are ready for capping (mites enter day 8 after the egg is laid, while worker & drone cells are capped on day 9).

Immature mites on pupa

Immature mites on pupa. Courtesy The Food and Environment Research Agency (Fera), Crown Copyright

About four hours after the cell is capped, the female mite will emerge from under the larval food and begin feeding on the developing honey bee larva, creating a lesion which her offspring can feed from as they develop. She then lays a series of eggs – first a male egg, then 4-5 female eggs, laid at regular 30-hour intervals. It takes about 6-7 days for a female egg to reach adulthood. The young mites hatch and mate with each other within the cell, obtaining energy to do so by feeding on the developing honey bee larvae. You’ll be pleased to find out that mites mate on their own faeces, which give off a pheromone smell enabling them to find each other in the dark of the cell.

The male mites cannot survive outside the cell, and they die after copulation since their mouthparts are modified for sperm transfer rather than feeding. Therefore the females must be fertilised before the bee emerges from the cell. The success rate of producing new mature fertilised female mites is about 1.7 to 2 in worker brood, but increases to 2-3 in drone brood as drones take 3 days longer to develop.

Once the honeybee larvae emerges, the young female mites crawl out too and spend some time feeding phoretically on the backs of adult honeybees, before they can carry out the cycle again by hiding within an uncapped cell. Their flat shape allows them to squeeze between overlapping segments of a bee’s abdomen to feed and helps them avoid removal by grooming bees. During the summer female mites live for about 2-3 months, during which time, if brood is available, they can complete 3-4 breeding cycles.

Varroa damage

Varroa damage: normal bee on left, bee carrying several phoretic mites in the centre, deformed bee on the right. Courtesy The Food and Environment Research Agency (Fera), Crown Copyright

c) How is it spread to other colonies?

  • Foragers carrying mites drifting into, or robbing from, other colonies
  • Drones carrying mites visiting other colonies
  • The beekeeper moving brood frames between hives or transporting infected colonies to new areas
  • Varroa mites have been found on flower-feeding insects such as certain species of bumble-bees, scarab beetles and flower-flies. Although the Varroa mite cannot reproduce on these insects, its presence on them may be a means by which it spreads short distances and finds new honey bee hosts.

B4. a) describe the various methods which beekeepers can use to detect the presence of the Varroa mite (and pyrethroid resistant Varroa) in their colonies and monitor the degree of infestation. 

Looking out for deformed wings and adult bees carrying mites, and waiting to treat until you see these signs, is not enough. Colony collapse due to a severe varroa infestation can take only a few weeks, and may affect even strong colonies, so do not assume that levels will remain low because so far no mites have been spotted.

Testing for pyrethroid resistance

Varroa in some areas (most places in the UK) have developed resistance to pyrethroids, which are the active ingredients of the widely used varroacides Apistan and Bayvarol. The presence of such mites can be identified by resistance testing, but the first obvious sign is likely to be the collapse of colonies after pyrethroid treatment fails to control mite infestation.

It is quite straightforward to test for pyrethroid resistance. A sample of live bees should be removed and put in a jar with a strip of Apistan fixed in the lid and mesh at the bottom. The jar is left in the dark for some hours over a piece of card and then the number of mites that have fallen through the mesh are counted – this represents the susceptible population of mites. The bees in the jar are then killed by shaking vigorously in soapy water and the washings filtered through a strainer. Any more mites coming off in the rinsing water are the resistant ones. A percentage resistance can then be calculated, e.g. if 6o fall down initially and another 40 subsequently, the population is 40% resistant. (Info from my correspondence course tutor, Pam Hunter).

In a severe infestation, there will be a poor brood pattern, and some larvae will die, while many of those that survive will have stunted growth and deformed wings.

Below are a couple of monitoring methods beekeepers can use to help monitor how bad infestation is.

  • Drone trapping

Varroa mites prefer to breed in drone brood. Drones take 24 days to develop whereas workers take 21, so drones give the mites time to fit in more breeding cycles. The mites identify the drone brood by its different smell, which is a result of the more protein rich diet fed to drones.

In England drone trapping can be used as a method of varroa monitoring and/or control during April, May and possibly June. To do this put a drawn super frame into the brood box, to encourage them to build drone comb in the gap underneath, or a frame with drone foundation or drawn drone comb, depending on the size and strength of the colony. Put the super frame at the side of the brood nest, not the middle.

Once the drone comb is sealed, cut it off the super frame. You can do this 2-3 times during the summer. Before you destroy the drone brood you can uncap some to see how many mites are inside.

Uncapping drone brood to check for varroa. Courtesy The Food and Environment Research Agency (Fera), Crown Copyright

Put your uncapping fork in deep, right into the neck of the drones. Fork out a hundred drones and count how many larvae have mites to get an idea of mite numbers in the hive: 5% infestation is light; between 5-10% light control techniques like drone trapping can be used; at over 25% infestation is severe and high-efficiency methods like Apiguard will be needed. Note that younger varroa mites are pale coloured and the mites move rapidly away from the light, so rotate the uncapping fork to ensure that you see all mites present.

  • Varroa board monitoring

Draw a grid with several squares on your monitoring board and smear it with vaseline, then put it under your open mesh floor for a week. The grid pattern makes it easier to count mites and the vaseline ensures the mites stick to the vaseline and aren’t blown off when you inspect the board. A week later, count the number of mites and work out an average daily drop count. If there is a lot of debris which makes it hard to count mite numbers, put the debris in a jar with a lid before mixing with methylated spirits and shaking vigorously: once the debris settles, the wax, propolis and other debris will sink, the mites will float.

To assess how bad the problem is based on the daily drop count, the season and the type of hive must be taken into account – UK colony collapse thresholds would be a daily drop of 6 in May, 10 in June, 16 in July and 20 in August. The Beebase varroa calculator is a helpful tool which can tell you how bad the problem is based on the time of year.

Varroa damaged brood

Varroa damaged brood. Courtesy The Food and Environment Research Agency (Fera), Crown Copyright.

b) give a detailed account of Integrated Pest Management

An integrated pest management strategy should be used throughout the year – this means using monitoring mite levels at regular intervals and using a variety of control techniques. Using several different control methods during the year makes it harder for the mite population to reach harmful levels. Good husbandry techniques, such as regular brood inspections, laying out apiaries to minimise the effects of drifting, and making sure your feeding techniques do not encourage robbing, are a starting point to help ensure healthy bees.

c) what treatments for Varroosis is permitted in the UK. Give a detailed account of how to carry out four forms of treatment.

  • Shook-swarm

I have a blog post explaining how a shook-swarm works, ‘A successful shook swarming‘. Ideally this is carried out in early spring, in late March – early April. The bees are shaken onto new foundation frames and all the old brood comb, containing lots of mites taking advantage of the new spring brood to breed, is burned. Sugar syrup is fed so the workers can draw out new comb quickly. This is a helpful non-chemical anti-varroa treatment because a large percentage of the mites are destroyed, followed by a short break in the queen laying while new comb is drawn out, which further cuts down on mite reproductive cycles.

  • Icing sugar

The advantages of this method are that it’s cheap and easy to do. It can also be done with supers on, unlike thymol based treatments like Apiguard which might taint the honey with their smell. The icing sugar works in two ways – by reducing the electrostatic charge by which the varroa cling to adult bees and by inducing the bees to groom. A flour dredger or a honey jar with holes punched in a lid work well. Work in pairs to do the treatment, with one person holding out each frame horizontally and another person dusting the sugar over each side.

As the treatment doesn’t kill mites, but only knocks them off, it is only any good in a hive with an open-mesh rather than a solid floor. Since it only affects phoretic mites clinging onto adult bees, which only make up about 30-40% of the mite population, it is a low efficiency treatment and generally only reduces mites by about 20-30%. This may sound good, and is better than nothing, but really an 80% effective treatment (such as Apiguard or oxalic at the appropriate times of the year) is needed to have any real effect on mite numbers.

You cannot rely on sugar dusting alone to keep varroa levels down; if you do your colonies are likely to die. This is true generally of varroa control: you cannot rely on one treatment alone, but should use several different methods throughout the year.

Apiguard, a natural thymol based treatment, can be given in August once your supers have been removed (otherwise your honey will stink of thyme). Starting Apiguard in August allows the hive to produce several generations of healthy bees before going into the winter. Two 50g treatment packs are given, one initially and the second 10-15 days later. Small colonies or nucleuses can be given a half dose.

Apiguard

The treatment works because the worker bees dislike the heavy thymol scent. They start removing the gel to clean the hive and remove the foreign smell, distributing it round the colony and killing off varroa mites in the process. Both adult mites and developing mites inside capped cells are affected, but honey bee larvae are safe. Tape up your varroa monitoring board whilst treating so the fumes stay in the hive. Apiguard should be done while the weather is still warm, as it is most effective – 90-95% effective – in the optimum conditions of an external ambient temperature of more than  15°C and active bees. This is because distribution of the Apiguard gel depends on the bees transporting it round the hive during the process of hive cleaning, and this activity increases as the external temperature rises.

  • Oxalic acid 

This treatment can be carried out once either in December or January whilst brood levels are either non-existent or low. It works by damaging the proboscis of the varroa mites, preventing them from sucking haemolymph from the host bees. Oxalic acid can be purchased pre-mixed in a sugar solution, which is the safest method.

Choose a bright and warm winter’s day when the bees are loosely clustered, so that as they move inside the cluster they distribute the chemical onto the mites. Put a varroa monitoring board over the mesh-floor, as it feels good to count the number of dead mites dropped onto it over the next few days; sometimes I’ve counted over a hundred in a week. Warm the product slightly until it’s lukewarm, remove the hive roof and crown-board, and trickle 5ml over each seam of bees. Do this very quickly to avoid chilling the bees too much. If the colony has been treated before and still has the same queen, it is unwise to use it again as it may harm the queen.

Printer-friendly pdf version.

Is varroa your most feared bee pest? What treatments do you use?

References:

About Emily Scott

I am a UK beekeeper living in Ealing, west London. I have been keeping bees in the Ealing Beekeepers Association’s local apiary since 2008 and created this blog as a record for myself of my various beekeeping related disasters and - hopefully - future successes. Busy taking the British Beekeeping Association module exams too!
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31 Responses to 3rd Honey bee pests, diseases and poisoning revision post: the lifecycle of Varroa destructor and monitoring & treatment techniques

  1. Chris Slade says:

    It was Varroa jacobsoni that was discovered in 1906. It was thought to be the pest infesting our bees until about 10 years ago when Denis Anderson noticed that ‘our’ Varroa is a different shape to V. jacobsoni and therefore a different species. He named this new species Varroa destructor. It is found on Apis cerana on different islands to Vj. Look at Dave Cushman’s web site for more details.

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  2. Up till now I have been reading about Varroa in a very piecemeal fashion, I really appreciate this clear account.

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  3. Chris Slade says:

    FERA points out, correctly, that shook swarming is not suitable for weak colonies. I suggest (but have not yet tried!) that, for Varroa control rather than EFB, you shake your strongest hive first and, instead of destroying the comb, transfer the brood to a weaker hive to build it up. You will, of course, transfer some Varroa with it but in 3 weeks, when all the transferred brood has hatched and the colony is stronger, you can shake that one. The last one of the series can have its brood destroyed.

    I’m wondering whether it would be worth giving a follow up dose of thymol a week after the shake in order to get rid of phoretic mites. The bees should have settled down by then and so won’t abscond but the new brood won’t be ready for sealing so all the mites will be exposed to the thymol fumes.

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    • Emily Heath says:

      Yes, we had a weak colony this year and decided to do a Bailey comb exchange rather than a shook-swarm in the end. We then had so many queen troubles, causing breaks in brood, that I suspect that factor alone was enough to keep numbers down.

      Your transferring brood idea is intriguing. I think the recommendation is to avoid leaving the shook-swarm too late, so that the colony has a chance to build up before the main nectar flows. But if your main priority is bee health rather than honey production, that probably doesn’t matter so much.

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  4. Barny says:

    Another very comprehensive blog. German scientists wanted to understand how Apis Cerana managed to coexist with the mites and brought some back to Germany for further study. A blunder at the lab allowed the bees to escape and we are now in this situation. A lesson to be learnt there I think. We now know that Cerana are better at grooming each other than A. Mellifera.
    I think that the research into icing sugar dusting shows that it is rather less effective than you think.
    The FERA literature also talks about Queen trapping where she is restricted to a single frame but I do not know of any beekeeper who does that.
    For exam purposes you will need to know the importance of the mites as the vector for fatal diseases. Varroa does not cause colony collapse but the viruses they transfer does.

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    • Emily Heath says:

      Cerana also have smaller colonies and swarm more often, suggesting that doing an artificial swarm could be a good anti-varroa technique.

      The 20-30% efficiency rate was given by Alan Byham, one of the NBU’s bee inspectors – unless he meant that was the rate for the phoretic mites only? I will leave it in and see if my tutor corrects me when I send my answers over.

      I have attended a talk on queen trapping. I understand the idea but it does seem a bit fiddly, and not something the more experienced beekeepers I know do.

      Expect there will be questions on the viruses to come in further papers, I have six to answer and return to my BBKA tutor, this is only the first one.

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    • Emily Heath says:

      Also – thanks very much for all your feedback! Really appreciate it.

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  5. Extremely interesting, Emily, even for a bee layman…

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  6. Hi Emily,
    I think the Apiguard treatment might have caused problems in my hive. I have written about it here: varrora & apiguard. But only the bees really know what is going on in there.
    Hope your bees are good.
    Roger

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  7. Alex Jones says:

    Thanks for providing this detailed knowledge on a significant problem. Has there been methods designed to trap mites like one can do with baiting a mousetrap with cheese to eliminate mice?

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    • Emily Heath says:

      Hi Alex, there are traps for some bee pests, like small hive beetles, but not for varroa.The difficulty is that varroa spend all their time feeding on either adult or juvenile bees, so any trap or treatment has to be able to affect them without troubling the bees.

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  8. silver price says:

    Two of the methods (alcohol wash and ether roll) for detecting and quantifying varroa from honey bee colonies require the beekeeper to obtain a sample of live bees so that the varroa mites can be collected directly from the bodies of the bees. In this way the beekeeper can estimate the level of infestation for the entire colony.

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  9. las artes says:

    Up to 85% of the mites in a colony are in capped brood cells and not visually detectable. Varroa mites are more attracted to drone brood than worker brood, so look there first. Sample about 100 cells. Locate a patch of drone cells in the purple eye pupal stage. Slide the prongs of a de-capping fork along the comb face and into the protruding drone cappings. Pry upward and remove the pupae. Carefully examine the bodies and the interior of the cells for mites.

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  10. Very comprehensive, well illustrated post. Thank you.

    Have you heard anything about HopGuard® on your side of the pond? ( http://www.betatechopproducts.com/products/varroa-mite-control ) It was just approved in Michigan this year and seems to be less fraught with cautions than the other chemicals while claiming similar effectiveness.

    Locally we are hearing “treatment-free” and “survivor stock” a lot. A few beeks seem to have been successful just leaving the bees alone after a few years of heavy losses and making splits of surviving colonies. With only two hives of our own we may again end up with no bees if we go that route. Unless they all die this winter.

    Always look on the bright side of life.

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    • Emily Heath says:

      Thanks. I’ve read a lot of posts by US beekeepers using Hopguard (e.g. Rusty at Honey Bee Suite), but haven’t come across it in the UK. Maybe it’s not approved here yet.

      I’ve only got a couple of hives, so can’t really afford to try the leaving bees alone method. Additionally, in London there are so many drones about I’d have to artificially inseminate if I wanted to try and preserve a particular gene line – not something I’m confident trying! It might be practical for country beekeepers with a lot of space away from other colonies and money to afford huge losses, but not in an urban environment.

      Ah well, if varroa didn’t exist we wouldn’t get the warm pleasurable feeling of finding lots dead on the monitoring board after treatments!

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  11. Emily, I take it there are beekeepers in the UK attempting to breed up lines of Varroa resistant bees…and your fair isle is of course the home of the Buckfast Bees. How widely available are the resistant strains and how “effective” are they? Great post, well constructed. I will link your page from mine!

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    • Emily Heath says:

      Thanks for all your nice comments! Buckfast bees are available and a few people I’ve met in London use them. New Zealand queens are more commonly purchased. Emma and I let our queens mate with local drones.

      You might be interested in a post from a varroa workshop I attended earlier this year: https://adventuresinbeeland.wordpress.com/2012/05/15/national-bee-unit-varroa-workshop-part-1-know-your-enemy. Our local bee inspector was asked how effective the ‘varroa resistant’ strains are, and this was roughly what he said:

      An “interesting idea”, but in his opinion no-one has successfully done it yet. He believes good husbandry methods, combined with requeening each year with a resistant-bred queen from specialist breeders, produces the resistant effect. It’s not sustainable when queens are mating with local drones from all over the place, and therefore resistant bees are going to take a long time to do significant good for beekeeping.

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  12. cindy knoke says:

    I am so glad I found your blog! You will be helpful for me with my bees!

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  13. Hello, this is a very comprehensive summary of life of a varroa mite. We would like to add that there is a new way how to treat varroa and that is by use of a thermosolar therapy as the mite can´t survive higher temperatures. We are developing the method for over 10 years now.

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    • Emily Scott says:

      Thanks, the thermosolar hive sounds interesting. Can you tell me what effect raising the temperature of the hive has on the brood, what is the brood survival rate?

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      • Thank you for your question Emily, we understand that beekeepers worry about brood health. But there is no need for worry as temperatures between 40 and 47°C for relatively short time is absolutely save for bees. Young bees not only survive completely, but their health is in very good order. It is deadly only for varroa mites. It is best represented by amount of honey bees gather. There are approximately 10 kg more honey in Thermosolar hives than in classical Langstroth hives at the same stands. If thermotherapy is dangerous for bees, than the result would be oposite.

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  14. Pingback: Elephant in the Hive | The Prospect of Bees

  15. Pingback: No honey, mo’ problems – Current Topics in Biology

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