Biological control with Varroa

Synopsis : Honey bees were eradicated on Santa Cruz Island following the introduction of Varroa. This provides some useful lessons for beekeepers on the importance of controlling Varroa.


Honey bees are not native to North America. They were first introduced in March 1622 at Jamestown, Virginia. The bees did well and spread west, following the settlers. They finally arrived on the west coast, in Santa Clara, California, 231 years later in 1853. Of a dozen hives ordered by Christopher Shelton, a Santa Clara botanist and rancher, only one survived the journey from New York via Panama.

Shelton barely had a chance to enjoy his bees 1 as he was unfortunately killed when the steamboat Jenny Lind exploded in mid-April 1853.

Explosion on the steamboat Jenny Lind near San Francisco, California

His bees survived 2 and three hives derived from the original stock were auctioned for $110 each. This was over 20 times the price of hives on the east coast at that time and equivalent to over $4200 today 3.

Californian Channel Islands map

Bees were in demand and they continued to spread – both as feral swarms and as farmers established apiaries to help pollination and for honey production. Having reached the California coast they were then spread to the nearby islands. Bees were transported to Santa Cruz, the largest of the eight Channel Islands near Los Angeles, in the 1880’s. They flourished, but did not spread to the other Channel Islands.

Field station, nature reserves, pigs and bees

Santa Cruz Island is 250 square kilometres in area and lies ~35 km south of Santa Barbara. It is one of the four Northern Channel islands. There is a long central valley lying approximately east-west and the rocky mountainous land reaches 740 m. It has a marine temperate climate; the average low and high temperatures are 9°C and 21°C respectively and it receives about 0.5 m of rain a year. It is a good environment for bees.

From the 1880’s to 1960’s Santa Cruz Island was farmed – primarily for wine and wool, and from the 1940’s for cattle – but, after period of university geology field trips and the establishment of a field station on the island, in 1973 it became part of the University of California’s Natural Reserve System (UC NRS).

In the late 1970’s the Stanton family sold their ranching business on the island to The Nature Conservancy who subsequently bought additional land on the eastern end of the island.

Santa Cruz Island is now jointly owned by The Nature Conservancy, National Parks Service, UC NRS and the Santa Cruz Island Foundation and much of the island is used for scientific research and education.

But what about the bees?

Good question.

As a nature reserve and research station, the presence of non-native species causes a potential problem. Why go to all the expense of managing a remote island research centre if all the same species are present as on the mainland?

The Nature Conservancy therefore initiated a programme of eradicating non-native species. It took 14 months to eliminate the feral pigs, using a combination of trapping, helicopter-based shooting and the release of sterilised radio-tagged pigs to locate the stragglers 4.

But getting rid of the bees took a bit longer …

Save the bees, or not

Why get rid of the bees? Surely they weren’t doing any harm?

The introduction of any non-native species upsets the balance (if there’s ever balance) in the ecosystem. The introduced species competes directly or indirectly with those native to the area and can lead to local extinctions.

Jonathan Rosen has described 5 how honey bee swarms, through occupying tree cavities previously used for nesting, probably played a major role in the extinction of the Carolina parakeet.

Pining for the fjords … a stuffed Carolina parakeet (nailed to its perch)

Competition between honey bees and native pollinators has been well studied. It is not always detrimental, but it certainly can be. Furthermore, it is probably more likely to be detrimental in a small, isolated, island ecosystem. For example, studies showed that the presence of honey bees dramatically reduced visitation of native pollinator to manzanita blossoms on Santa Cruz Island.

As part of the larger programme of non-native plant and animal eradication on Santa Cruz Island plans were drawn up in the late 1980’s to eliminate European honey bees. The expected benefits were to:

  • eliminate competition with native bee species (and presumably other non-bee pollinators, though these rarely get a mention 🙁 )
  • reduce pollination of weed species (some of which were also non-native to Santa Cruz Island)
  • facilitate recovery of native plant species that were reliant on native bee pollination
  • provide a ‘field laboratory’ free from ‘exotic’ honey bees in which comparative studies of native pollinators would be possible

Killer bees

After the plans to eradicate Apis mellifera were approved an additional potential benefit became apparent.

There were increasing concerns about the spread of Africanised honey bees which had recently reached Santa Barbara County. Although there was reasonably compelling evidence that swarms could not cross from the mainland (e.g. none of the other Northern Channel Islands had been colonised by bees) there were concerns that the Santa Ana winds might help blow drones from the mainland.

Had these drones arrived they might mate with the non-native but nevertheless local queens resulting in the spread of the dominant genes for defensiveness and absconding. The resulting swarmy, aggressive Africanised bees would cause problems for visitors and scientists working on the island (as they have for visitors to Joshua Tree National Park).

Aerial view of Santa Cruz Island

Although the introgression of African honey bee genes was used as further justification for the eradication it’s not clear whether drones could actually cross 30-40 km of open sea 6.

As an aside, there’s a current project – the amusingly named Game of Drones – running on the Isles of Scilly investigating whether drones can cross the sea between St Agnes, Tresco, Bryher, St Mary’s and St Martin’s. These are, at most, 11 km apart (northern most tip of St Martin’s to most southerly point of St Agnes) but the individual islands are only separated by 1-2 km. I would be surprised if drones could not cross that distance (at least with a strong following wind).

Killing bees

Adrian Wenner and colleagues set about exterminating the honey bees on Santa Cruz Island (Wenner et al., 2009). The process started in 1988 and ended in 2007, and was divided into four phases:

  1. 1988-1993 – location and elimination of feral colonies
  2. 1994-1997 – biological control and colony demise
  3. 1998-2004 – monitoring residual honey bee activity
  4. 2005-2007 – confirmation of the absence of honey bees

None of this is ’beekeeping’ – actually it’s the exact opposite – so I don’t intend to dwell in much detail on the work that was conducted. However, the ’94-’97 phase includes some sobering lessons for beekeepers which are worth discussing.

By the end of phase 1 the team had identified the existence (if not the location) of at least 200 colonies and eliminated 153 of them.

Remember, none of these were managed colonies in hives. They were all feral colonies occupying natural cavities in trees or rocks etc. Each colony was found using painstaking bee lining techniques similar to those described in Thomas Seeley’s book Following the Wild Bees.

Once located, nests were destroyed with methyl chloroform and the cavity sealed to prevent it being reoccupied.

Some colonies could not be accessed; in these cases acephate-laced sucrose-honey syrup baits were used. This organophosphate has delayed toxicity for bees, allowing foragers to return to the colony which in due course dies. This approach had been partially successful in eliminating Africanised bees on the mainland (Williams et al., 1989), but baits needed to be be monitored to avoid killing the other insects they attracted.

The scientists also deployed swarm traps (aka bait hives) and destroyed any swarms that moved in.

Together these interventions reduced honey bee numbers significantly – as monitored by regular observations at pollen- or nectar-rich plants – but did not eradicate them.

Let there be mite

Heavy rains in January ’93 washed out roads on Santa Cruz Island, thereby severely limiting travel around the island. In addition, the previous removal of cattle had resulted in the near-uncontrolled growth of fennel which now formed dense, impenetrable thickets.

Bee lining became impossible and the scientists had to invent more devious strategies to eliminate the residual feral colonies.

The approach they chose involved the introduction of Varroa.

Varroa was first detected in the USA in 1987 (in Florida) and became widespread over the next 5-8 years. Up until 1994 the honey bees on Santa Cruz Island were free of the ectoparasitic mite.

It was likely that they would have remained that way … there was no beekeeping on Santa Cruz Island and the location was too remote for bees to cross from the mainland (see above).

Varroa was already known to have a devastating impact on the health of honey bee colonies (Kraus and Page, 1995). It was also known that, other than its native host Apis cerana (the Eastern honey bee), Varroa did not parasitise other bee or wasp species (Kevan et al., 1991).

These two facts – host specificity and damage inflicted – suggested that Varroa could be used for biological control (‘biocontrol’) on Santa Cruz Island.

Biological control

Biological control or biocontrol is a method of controlling pests using natural mechanisms such as predation or parasitism.

The pest could be any living thing – from animals to bacterial plant diseases – present where it’s unwanted.

On Santa Cruz Island the pest was the honey bee.

In other studies (covered in a previous post entitled More from the fungi 7 ) biocontrol of Varroa has been investigated.

Control of the pest involves the introduction or application of a biological control agent. The key requirements of the latter have already been highlighted – specificity and damage.

Biological control works well when the specificity is high and the damage is therefore tightly targeted. It can be an abject failure – or worse, it can damage the ecosystem – if the specificity is low and/or the damage is widespread.

The cane toad was introduced to Australia to control infestations of greenback cane beetle (a pest of sugar cane). Cane toads were introduced in 1935 and rapidly spread. Unfortunately, cane toads can’t jump very high and so singularly failed to control the greenback cane beetle which tends to 8 stay high up the cane stems.

Female cane toad (not jumping)

But it gets worse; cane toads have a very catholic diet and so outcompeted other amphibians. They introduced foreign diseases to the native frogs and toads and – because of the poisons secreted from their skin – harmed or killed predators that attempted to eat them.


Vertebrates are usually poor biological control agents as they tend to be generalist feeders i.e. no specificity.

But Varroa is specific and so the damage it causes is focused. The likelihood of ecosystem damage was considered low and so the mite was introduced to the island.

Introduction of Varroa

In late 1993 Adrian Wenner caught 85 foraging bees and, to each one, added a single Varroa mite. The bees were then released and presumably flew back to their colonies … taking the hitchhiking mite with them.

Adult mites – the dark red ones you see littering the Varroa tray after you treat with Apivar – are mated females.

Due to their incestuous lifestyle a single mite is sufficient to initiate a new infestation.

The mated adult female mite parasitises a honey bee pupa and produces a series of young; the first is male, the remainder are female. You’re probably reading this before the 9 pm watershed so I’ll leave it to your lurid imagination to work out what happens next (or you can read all the sordid details in Know your enemy).

The presence of honey bees – determined by successful swarm trapping or field observation at likely sites – was then regularly monitored over the next four years.

Swarm numbers remained largely unchanged until 1996 and then dramatically decreased.

Numbers of new swarms on Santa Cruz Island 1991 – 2005. Varroa introduction indicated.

It’s worth noting that during ’94-’96 over 70 swarms were found in natural sites or bait hives. There must have been a significant number of established colonies in 1993 to produce this number of swarms.

But, from 1997 it all stopped … only a single swarm was subsequently found, in a natural cavity in 2002.

Monitoring and confirmation of eradication

From 1998 to 2004 the scientists continued to actively monitor the island for honey bees, focusing on 19 areas rich in natural forage. Although honey bees were found – in decreasing numbers – there were too few to attempt bee lining to locate their colonies.

At the sites being monitored, bees were detected 9, 7, 4, 2 and 1 times respectively in the 5 years from 2000 to 2004. After that, despite continued monitoring, no more honey bees were detected.

The final phase of the project (’05-’07) confirmed the absence of honey bees on Santa Cruz Island.

Whilst, as a scientist, I’m a firm believer that ’absence of evidence does not mean evidence of absence’, as a beekeeper I’m well aware that if there are no scout bees, no swarms and no foragers (when I search in likely places) then there are no honey bee colonies.

Lessons for beekeepers

I wouldn’t have recounted this sorry tale – at least from a beekeeping perspective – unless I thought there were some useful lessons for beekeepers.

There are (at least) three.

The first relates to Varroa resistance, the second to Varroa transmission in the environment and the last to ‘safe’ levels of Varroa. All require some ‘arm waving guesstimates’ 9, but have a good grounding in other scientific studies.

Varroa resistance

There wasn’t any.

At a very conservative estimate there were at least 20 colonies remaining on Santa Cruz Island in 1995. I say ‘conservative’ because that assumes each colony generated two swarms that season (see graph above). In studies of other natural colonies only about 75% swarm annually, meaning the actual number of colonies could have been over 50.

The numbers – 20 or 50 – matter as they’re both much higher than the number of colonies most beekeepers manage (which, based upon BBKA quoted statistics, is about 5).

Whether it was 20 or 50, they were all eliminated following the introduction of 85 mites. Colonies did not become resistant to Varroa.

This all took a few years, but – inferring from the swarm numbers above – the vast majority of colonies were killed in just two years, 1994 and 1995. This timing would fit with numerous other studies of colony demise due to mites.

Wenner estimates that only 3 colonies survived until 2001.

Leaving small numbers of colonies 10 untreated with an expectation that resistance – or even tolerance (which is both more likely and not necessarily beneficial) – will arise is a futile exercise.

I’ve discussed this before … it’s a numbers game, and a handful of colonies isn’t enough.

Varroa spread

Wenner doesn’t elaborate on where the foragers were captured before he added the mites. If I was going to attempt this I’d have chosen several sites around the island to ensure as many feral colonies as possible acquired mites … let us assume that’s what he did.

However, with 85 mites piggybacking on returning workers, and somewhere between (my guesstimated) 20 to 50 colonies, I think it’s highly likely that at least some colonies received none of this ’founding’ mite population.

Yet almost all the colonies died within two years, and those that did not subsequently died with no further intervention from the scientists. We don’t know what killed off the last surviving colonies but — and I know I’m sticking my neck out here – I bet it was the mites.

This is compelling evidence for the spread of Varroa throughout the island environment, a process that occurs due to the activities of drifting and robbing.

If a neighbouring apiary to yours has mites some will end up in your hives … unless you are separated by several kilometres 11.

The transmission of mites in the environment is a very good reason to practice coordinated Varroa control.

One mite is all it takes

But, just as I’ve argued that some colonies may have received none of the founding mites, I’m equally sure that others will have acquired very small numbers of mites, perhaps just one.

And one mite is all it takes.

Without exceptional beekeeping skills, resistance in the bee population or rational Varroa control 12 there is no safe level of mites in a colony.

The more you prevent mites entering the colony in the first place, and the more of those that are present you eradicate, the better it is for your bees.

Here endeth the lesson 😉


It’s worth noting that island populations do offer opportunities for the development of Varroa resistant (or tolerant) traits … if you start with enough colonies. Fries et al., (2006) describes the characteristics of the 13 surviving colonies on Gotland after leaving about 180 colonies untreated for several years. I’ve mentioned this previously and will return to it again to cover some related recent studies.


Fries, I., Imdorf, A. and Rosenkranz, P. (2006) ‘Survival of mite infested (Varroa destructor) honey bee (Apis mellifera) colonies in a Nordic climate’, Apidologie, 37(5), pp. 564–570. Available at:

Kevan, P.G., Laverty, T.M. and Denmark, H.A. (1990) ‘Association of Varroa Jacobsoni with Organisms other than Honeybees and Implications for its Dispersal’, Bee World, 71(3), pp. 119–121. Available at:

Kraus, B. and Page, R.E. (1995) ‘Effect of Varroa jacobsoni (Mesostigmata: Varroidae) on feral Apis mellifera (Hymenoptera: Apidae) in California’, Environmental Entomology, 24(6), pp. 1473–1480. Available at:

Wenner, A.M., Thorp, R.W., and Barthell, J.F. (2009) ‘Biological control and eradication of feral honey bee colonies on Santa Cruz Island, California: A summary’, Proceedings of the 7th California Islands Symposium, pp. 327–335. Available as a PDF.

Williams, J.L., Danka, R.G. and Rinderer, T.E. (1989) ‘Baiting system for selective abatement of undesirable honey bees’, Apidologie, 20(2), pp. 175–179. Available at:



  1. Remembering these were for honey and wax production, not enjoyment!
  2. They weren’t on the boat.
  3. Which makes my nucs look like bargains.
  4. Hello friend … bang!
  5. Unfortunately behind a paywall … a bargain at $1 for 6 months.
  6. Let alone then still have the strength to mate.
  7. An entirely predictable pun if you’ve ever seen Paul Stamets talk.
  8. Sensibly.
  9. You’d be disappointed if they didn’t.
  10. Say below a few hundred.
  11. And UK apiary density is usually too high to have that luxury of isolation.
  12. I’ve done a lot of beekeeping and consider the last of these to be the most reliable and certainly easiest to achieve … and it’s what I do.

32 thoughts on “Biological control with Varroa

  1. Ihor Pona

    I see a hidden talent waiting to emerge if keeping bees does not provide a smile or two.
    Love those chuckle insertions.
    my thanks for a balanced update,

  2. Steve Donohoe

    Thanks David, for an interesting article with important lessons for beekeepers – especially beginners with 2 hives who listen to ‘treatment-free’ evangelists. It’s hard enough to keep bees alive in the early years, but for those who don’t treat their bees, it’s probably close to impossible.

    1. David Post author

      Hi Steve

      There are some associations where the ‘evangelists’ are in the ascendancy and now influence – or determine – the training programme and mentoring schemes. If they can achieve no disease-related losses for winter after winter then all well and good … but I suspect they cannot. If their beginners can achieve similar no/negligible disease-related losses then I would be delighted … but, again, I suspect they cannot.

      I would like to see some sort of independent verification of colony losses by treatment free beekeepers averaged over several years (one is not enough) and – while they’re at it – perhaps they could also try and validate the losses quoted by the national associations every winter. Those from 2021/22 were very, very strange. With 84% of beekeepers reporting no losses at all and about 16-17% overall losses then something doesn’t quite add up. Did those reporting losses lose everything? Did those who reported losses run very large numbers of hives, whereas the 84% were all 1 – 2 hive owners?



      1. Archie McLellan

        Hi David
        I seem to remember the cut off date for the BBKA winter losses survey this year was 1 April. At that point I had no winter losses for the simple reason that I hadn’t opened any hives. I wrote to Diane Drinkwater about this, pointing out that I did have winter losses, but after 1 April. As I got no reply, I filled in the survey as specified. It was obviously going to produce nonsense and it wasn’t my job to try to fix that.

        Great post, great story this week. Many thanks.

        1. David Post author

          Hi Archie

          Those surveys could be very useful if they were properly controlled and validated. It is important information. If the figures could be trusted individual (large) associations or counties could compare their performance and see if more emphasis on Varroa management was needed in training. For smaller associations the ‘noise’ from small numbers of managed colonies and/or one or two very good or very poor beekeepers might make this impossible.

          But instead we get a single headline figure and some additional – though perhaps even less trustworthy – data on the cause of the losses. In the ’21/’22 results wasps were reported to cause as many losses as DWV (which also equalled Varroa). Weather-related problems caused more losses than the combined impact of DWV and mites. Almost 40% of reported losses were due to queen problems.


          Drill down further and it gets even messier … 33% of beekeepers didn’t treat in midwinter (that I can believe). Of those that did, as many used Apivar as OA (19% vs. 18%) and 13% used Apiguard. In midwinter?

          Wales reported only 1% of colonies were lost.

          As you remember, here in Scotland we’re asked to report our winter losses in June … winter is defined as the period between the start of July and the end of May 😉


  3. Andrew

    What an incredibly interesting post.

    It reminds me of the current situation with Ash dieback, where some are advising to leave woodlands intact to encourage growth of potentially resistant strains – and the forestry guys I am working with who tell me that is hocus and there is no evidence for resistance – they are all going to die.

    Then we look at herbicide resistance and how the wonders of natural selection and Monsanto have now led to the emergence of superweeds such as pigweed in the US – glyphosate resistance through atypical pathways (this is a scary read ..

    `Fingers crossed the current work in RNAi pesticides leads to something…

    1. David Post author

      Hello Andrew

      The entire ash dieback story is farcical. As I understand it we continued imports when it was known to be widespread in European nurseries.

      Resistance to overused/misused herbicides – just like to misused antibiotics – will necessitate us finding new solutions all the time. Better that we at least attempt to use the things we have got now a bit better. Apistan is little use, but could be if we restricted its use for 4 out of every 5 years, alternating it with other miticides.

      Ho hum!


  4. Janet L. Wilson

    When I started beekeeping here in Canada 15 years ago, there was much ballyhoo about finding feral colonies…which everyone hoped/assumed were survivors of Varroa infestation and would be the foundation of breeding programs focused on raising up a honey bee that was meaningfully resistant to Varroa.

    Alas, it was discovered that ALL feral colonies in Canada had perished, and all the “feral” colonies were simply escaped swarms from managed colonies, not resistant at all. And now we all know escaped swarms return to haunt us in the form of mite bombs as the colonies collapse under their own mite loads.

    Randy Oliver has demonstrated beyond doubt that the chance of any hive surviving untreated is vanishingly small, and when he has felt he found hives able to overwinter untreated, that quality cannot be fixed in subsequent generations.

    Years when nearby beekeepers were treatment-free saw mite levels in my hives explode thanks to robbing and drift…bees drift far more than most beekeepers realize. Sadly, one year a research project went ahead with a treatment free based project, unmindful/uncaring about the impact of their high mite levels on area colonies. Research that imperils bee health should be conducted outside the range of other colonies….or provide support and compensation for negative impacts.

    There is no question bees cannot be healthy unless assisted with Varroa control. The Varroa genome is much, much more nimble than the bee genome! We desperately need more and safer methods of Varroa control, and gene therapy undoubtedly offers the best hope of control and possibly even eradication.

    1. David Post author

      Hello Janet

      There are studies here of the genetic relatedness of ‘feral’ swarms and colonies managed by local beekeepers and – surprise, surprise – they are very close 😉

      I’ve been meaning to write more about this and will when the new genetic studies are published on some of the more recently identified feral wild bees apparently flourishing for years without intervention by beekeepers (though the apiary density in the same area is extremely high … my estimates suggest there are ~50-100 hives within flying range of some of these ‘isolated wild bees’).

      I need to read up a bit about Varroa genetics … the brother/sister matings limits genetic diversity considerably until late in the season when you get high mite numbers and multiple occupancy of cells (at least, that’s my understanding).

      I’m going to continue with Apivar/whatever/OA applied rationally to colonies regularly monitored for the moment. Gene therapy would be nice, but I expect to have hung up my hive tool long before it’s available.


  5. Steve Riley

    An excellent note out this week from Prof Stephen Martin’s team on the world’s largest European honeybee varroa resistant population consisting of 220,000 colonies, found in Cuba. The second largest is here in the UK in north west Wales with 500 colonies. Interestingly, high uncapping / recapping levels of worker brood (50%+), to interrupt mite reproduction, are a key feature across the geographies – something we are also seeing in our colonies in the south east.

    1. David Post author

      Hello Steve

      I knew that Stephen was looking in Cuba and pleased now to see the paper. I don’t know anything about the Welsh example and can’t access the BBKA article. I’m not aware of any peer reviewed papers on the Welsh bees in the scientific literature.

      It’s notable that Cuba certainly, and possible North Wales, and several of the previously reported ‘resistant/tolerant’ populations of honey bees are in isolated locations. Cuba bans imports, Gotland is an island, and I believe the same applies to the colonies studied in France. There may be other examples, and perhaps the geography of North Wales (hills etc.) also provides a degree of isolation.

      As a virologist, what I’d really like to see are good data on DWV levels in emerging brood. The primary cause of colony damage are the viruses that Varroa transmits, and with Varroa still present, albeit at reduced levels, I would expect virus transmission to be continuing. High(er) levels of DWV are known to be detrimental in terms of foraging, winter bee longevity etc. How do these factors compare in untreated colonies with treated colonies? Of course, in a closed/isolated population there will be pressure on the virus to reduce its pathogenicity as well.

      Evolutionary adaptations often come at a cost … for example, colony productivity (Yves le Conte’s colonies were significantly less productive – about 50% if I remember correctly). Are untreated colonies more (or less?) susceptible to other pathogens? What are the levels of stress indicators in the colony? Are antimicrobial peptide levels elevated?

      It’s an interesting topic and one that deserves more research and less anecdote.


  6. Geoff Spry

    Timely post David, considering the arrival “outbreak” of Varroa in Australia.
    Authorities trying to contain it by quarantine and destroying affected hives but I worry that it will be too late; hard to stop drifting and robbing.
    I have not read how it got past their strict import quarantine measures.

    1. David Post author

      Hi Geoff

      I have a half-written post on the situation in New South Wales … Is it all over, down under? I was waiting for the situation to stabilise and for a slightly more up-to-date account of how the control measures were succeeding (or, I too fear, not succeeding). There’s been some good discussion on this on the Bee-L mailing list, including from some providing advice to the Australian authorities.

      I’ve not seen a compelling account of how it got in, but I’ve seen lots of speculation …


  7. Amanda Millar

    I find all your articles excellent, informative, entertaining and the links are so helpful. As an ecologist and conservationist as well as a beekeeper I found this one particularly thought provoking and do hope more beekeepers read and digest it. So sad about the parakeet; as a UK bat worker I wonder what affect swarms have on displacing our tree dwelling bats. I know a bat roost behind my soffit moved out when wasps took up residence 2 years in a row and have not returned. Keep up the good work.

    1. David Post author

      Hello Amanda

      Interesting thought about bats … I’d not considered that before. Swarms favour quite large natural cavities of about 40 litres. I’m sure bats would like these as well, but they could presumably also cope with much smaller spaces. The ‘bat boxes’ I’ve seen for sale are probably only a litre in volume, perhaps less (but perhaps they’re wholly unsuited to bats?!). The number of free-living colonies is relatively limited, largely because of Varroa. If they do get established in a tree or roof space they often succumb to disease within 18-24 months.

      Of course, that’s probably not much consolation to the bats if they were usurped.

      I’m pleased you enjoy the posts.


      1. James

        Just as an aside regarding bat houses (about which I know far more than I expected to, having had to deal with them as part of a planning application), they come in all sorts of sizes, depending on the type of bat expected to inhabit them. The ones you suggest are around a litre in volume area probably the smallest I’ve seen. We have some wall-mounted boxes that are probably closer to six litres in volume and another built on stilts that is the size of a large dog kennel (which was one year home to a nest of European hornets).


  8. Steve Riley

    Prof Stephen Martin’s team have looked at the Welsh bees (12 years without treatments) and samples from other resistant colonies in the UK. They examined 14,802 worker brood cells between 2017-2019, including a comparison against “varroa naïve” colonies from the Isle of Man.

    “Why do varroa resistant colonies survive Deformed Wing Virus?” is a good question and one we have had to understand in our own project at Westerham Beekeepers. High recapping rates manage the varroa population through the season starting with the first brood in January. Lower levels of mites means less “injecting” of the virus into larvae/pupae. Prof Stephen Martin has shown the lower virus loads at a presentations at the BBKA Spring Convention in April 2022 and to the BBVA in the same month. Would be useful to see the work published for broader education. Nb; DWV levels are higher for varroa susceptible managed colonies through the season, as their mite loads grow quickly when unchecked by the bees.

    As you say, these are interesting topics. As a virologist, why don’t you reach out to Professor Stephen Martin to tap into his team’s work..

    1. David Post author

      Many thanks for the Apidologie paper which I’d not seen.

      There’s considerable variation in DWV levels in brood, even in colonies with unrestricted mite replication. However, where mites are well controlled DWV levels remain at a ‘safe’ level for much of the season, and sometimes longer. Comparison would need to be made between a number of uncapping/recapping colonies, and equivalent numbers of managed and unmanaged colonies, ideally for several months. Of course, to complicate things (because of the influence of the local environment) these colonies should be in a similar location. I’ve got genetically related colonies 15 miles apart that perform very differently (and consequently have different mite infestation levels).

      Let’s hope Stephen and co. are doing this (or something similar). That, coupled with good data on overwintering survival and other measures of colony performance would be very interesting.


  9. Benjamin Donnachie

    Interesting article, as always! I was also going to suggest Prof Stephen Martin’s work which suggests that by themselves honeybees can adapt to varroa and it’s the presence of DWV that causes the issue.

    He gave an interesting talk with the BBVA a few months ago, well worth a watch – – along with an article in July’s Beecraft magazine “Natural Varroa resistance: the end of chemical control?”

    1. David Post author

      Many thanks Benjamin … I’ll have a look at the BBVA presentation. We’ve known DWV is the problem for a long time which makes tolerance and resistance very different outcomes. As I said in an earlier response to a comment, the DWV levels and other indicators of colony health/performance are important. We know from Tom Seeley’s work that bees can adapt to cope with Varroa, but ‘his’ bees are probably wholly unsuited for beekeeping; too small, too swarmy, too unproductive, too susceptible to collapse if co-located with other colonies in the same apiary etc.


  10. David Longo

    Thank you for this and so many other interesting posts. So it is now 20 years since the last swarm was found on Santa Cruz island, after 85 varroa mites were introduced 9 years prior. I suppose that we should assume that the varroa mites have also perished on Santa Cruz island, given that varroa mites are not known to parasitize other bee or wasp species? Such high specificity and damage seem to indicate that varroa are simply too good at what they do? I realize that varroa and its native host Apis cerana (Eastern honey bee) evolved together, and that Apis mellifera (Western honey bee) is at a severe evolutionary disadvantage by comparison. Maybe that’s the extent of it.

    But I still try to analogize it to help me understand. I’m thinking of this loosely in two scenarios: (1) Maybe something like the introduction of “new” viruses into the New World from European explorers–the Europeans had coexisted with the viruses for a very long time, but the native populations in the New World had never encountered those viruses before and thus perished in huge numbers. Or an extremely deadly virus (i.e., ebola) and humans–if an outbreak occurs, the virus will devastate the human population and essentially impede its ability to infect other humans. That is, if the virus kills all of its hosts, there won’t be any more hosts for it to use to propagate. (2) Conversely, if a virus naturally evolves to be more transmissible but less fatal, then that is the evolutionary success story for the virus–most hosts survive and this in turn means the virus survives.

    It seems that the former scenario may be thought of as (very loosely analogous to) what occurred with varroa and Apis mellifera on Santa Cruz island, and that the latter scenario may be thought of as (very loosely analogous to) what occurred over millennia (?) with varroa and Apis cerana in Asia?

    My poor attempts at analogies are likely flawed, of course. But I’m just trying to wrap my brain around whether it is possible for Apis mellifera to reach a point of being able to coexist with varroa without being entirely (?) dependent on human intervention to survive. Or for varroa to dial it down a few notches so that it doesn’t kill all of its hosts. It also makes me wonder how Apis mellifera survived at all after varroa were introduced, but for humans.

    Anyway, sorry for the rambling comments. Your post really got me thinking.

    Looking forward to your next post…. _Dave (Virginia, U.S.A.)

    1. David Post author

      Hello Dave

      There are some reasonably well documented accounts of co-existence of Varroa and honey bees. Some of the best are by Thomas Seeley. A search for his name here will turn up discussion of quite a few of these examples. There are others as well, including Stephen Martin’s studies on Cuba mentioned elsewhere in these comments.

      Evolution to allow co-existence generally requires:

      1. time – it’s dependent upon chance mutations occurring and being selected (in the host and parasite)
      2. population size – if something is rare, it’s more likely to occur within a larger population
      3. compromise – it’s likely that other changes will also occur, perhaps even linked to those that contribute to co-existence, and not all of these may be beneficial

      So, it’s a numbers game – with two hives it’s extremely unlikely to happen, with 200 it might, with 2 million it probably will. Seeley’s studies of the Arnott Forest bees show that those co-existing with Varroa form small colonies that swarm very frequently (swarming leaves behind the mite-infested brood) and are physically well-separated. In crowded apiaries they perish. With swarm control they develop unsustainable mite numbers …

      However, it’s worth noting that there are probably multiple solutions to the co-existence problem e.g. behavioural adaptation might include colony size/swarminess (Arnott Forest bees) or uncapping/recapping (Cuba).

      It took 50 years for myxomatosis to reduce from a ~100% case fatality rate to about 50% in Australia. This involved an increase in resistance of the rabbits and a decrease in virulence by the virus. In the latter, dozens of mutations have been mapped that account for this changed phenotype. The population size this selection occurred within was huge – essentially the entire population of rabbits in Australia. The damage to the rabbit population in the first few years was enormous. I’m not aware of changes in the rabbit behaviour, but presume there are some.

      Without mite management most colonies will die. We know this. It’s reinforced every year when beekeepers either fail to treat or treat irrationally. Sure, there are some resistant (or tolerant) bees already but – as noted above – many are in relatively isolated areas, or closed environments. Would these traits dominate outside a closed environment? Would beekeepers accept the losses? Beefarming would certainly be even more economically precarious (and it already is). What might the consequences be in terms of pollination, to say nothing of local honey production?

      What are the benefits? Are the bees healthier? More productive? Better at pollinating? Calmer?

      Of course, it would be better all round if there were no Varroa (and I’m writing from a location where my colonies are mite-free). But, for 98% of the UK and the majority of the beekeeping world Varroa is an ever-present reality. Without vaccination measles is a devastating virus (going back to your example of colonisation of the New World) but few people seriously think we would be better off without measles vaccination … both measles and Varroa are examples of pathogens that have jumped species due to man’s activity (measles from rinderpest in cattle during domestication) and mites due to a) co-location of A. mellifera and A. cerana, and b) moving hives.

      I currently cannot see a way to get from our current situation where treatment is needed for most bees by most beekeepers to one where no treatment is needed at all … without going through an intermediate and probably unacceptable situation where the honey bee population is devastated. Perhaps I’m wrong? If the bees were healthier (not just because the beekeeper says they are healthier, but in measurable terms, both molecular and in terms of colony performance), as or more productive etc etc. then perhaps the pain would be worthwhile.

      That’s almost a post in itself … apologies 😉


      PS I meant to also add … Varroa only survives for hours outside the hive. I think we can be certain that there is no Varroa remaining on Santa Cruz Island.

  11. david teanby


    Thank you for the excellent article.
    How were the original bees transported to America (and Australia)?. Several months on a sailing ship, no forage or opportunity to fly.
    Were they confined in a skep and fed honey for the duration ?
    Slightly off piste, but I am studying for Module seven and the spread of species comes up.

    1. David Post author

      Hello David

      I think you should be doing your own homework, don’t you?

      Didn’t they have Fedex or DHL? 😉

      In the 17th Century it took about 2 months to get to the New World. Hives were transported on the rear decks of the sailing ships. If the weather was cold the bees wouldn’t have been flying much (I assume). Some Canadian beekeepers keep their hives indoors overwinter (for months). Australia would have been a bigger problem because it took 6 months to get there and, even if they’d set off in the cold, it would be getting warmer …

      I think this is another example of how resilient bees are … and the ingenuity of some of the early settlers.


  12. Lisa Parsons

    Thank you whoever is putting this out there. Because I keep losing my honeybees every year. Not much to some. I have lost alot oh money thur the years on my honeybees. This is so HELPFUL of you. THANK YOU 😊

    1. David Post author

      Hello Lisa

      I think this might explain why you’re losing them … to stop losing them perhaps start by reading Rational Varroa control. It explains the basics of why you treat, when you treat and what you choose to treat with.


  13. Paul Dosen

    Hi David,

    Beautiful article as always, it would be great if every treatment free beekeeper on the planet could have read it, however I’ve tried for the last 10 years reasoning with them, contacting some of the most vocal activists of the treatment free movement (Michael Bush and Solomon Parker) and I simply cannot get through to either of them. They back-peddle, twist my words, justify their positions using irrational concepts and methodologies and when pressed for evidence they shout back claiming they do not have to prove anything. So, I’ve given up on treatment free beekeepers and instead have decided to focus on the science of beekeeping, the science of animal husbandry and insuring that new beekeepers have the education and the resources to make informed decisions as to the health and well being of their colonies. Great article once again and I look forward to the next one.


    1. David Post author

      Hello Paul

      I’m not anti-treatment-free beekeeping. If I could do it, I would (assuming colonies were strong, healthy, productive etc.). However, I’m either not a good enough beekeeper (quite possible) or I keep bees in a region with too many other colonies sharing the environment (almost certainly). What I am certain about is that it is not a strategy that should be attempted by an inexperienced beekeeper (and there are some associations that now advocate this on their training courses). There’s already lots that can go wrong, so adding a potentially high disease burden and/or attrition rate of the colonies, is foolhardy.

      I’d love to see some independent and validated statistics on survival rates of well-managed (which, to my mind means treated for 98% of the UK’s bees) and treatment-free colonies.

      I’m not holding my breath 😉



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