Varroa Mites 101
Varroa Mites 101
The past, present, and future of the varroa mite & the European honeybee
Today, we’re talking about Varroa Destructor, which is a tiny mite that is basically a tick for bees. We’ve spent a lot of time talking about bees, bee hives, bee biology, and wild pollinators, but we haven’t spent much time talking about the one thing every beekeeper fears most, varroa destructor.
Varroa Mite actually didn’t exist in the US until 1987. The Varroa bee mite was first discovered by A.C. Oudemans in 1904, as a parasite of the Asian honey bee, Apis cerana. In the late 1940s, through the movement of the western honey bee, Apis mellifera, colonies into and out of Asia, Varroa mite became established on honey bees first in Africa and then in Europe. Quickly, it spread around the world. When it was detected in the U.S. in 1987 Mexico and Canada quickly closed their borders to U.S. bees.
It is now known that at least five species of Varroa mites can be found in the tropics and Dr. Denis Anderson, an Australian researcher, named the specific mite (Korean in origin) that is damaging worldwide populations as Varroa destructor.
Upon their arrival to the US and quick destruction of hives across the country, pyrethroids, aka insecticides that also kill mice have been the primary solution basically across the globe. Obviously, this has some negative consequences for everything else where the bees live, right? The basic idea here is to poison the bees but to poison the varroa mites more than the bees. This worked for a few years, but then quickly the mites became resistant, and we switched to thymol as an alternative. Again, this worked for a few years, and then guess what? It wasn’t effective anymore.
The latest new solution— organic acids! Go to any beekeeper classes, and they’ll tell you about using a spectrum of different tools for mites, not to rely on one type of solution, but they’ll all fall into a chemical like formic acid or oxalic acid. Now, here’s the thing. Varroa destructor is a terrible, terrible thing for honeybees because they haven’t evolved with it, there’s no healthy relationship between them where the mite can live sustainably with the bee without killing it. That would be the goal of a parasite is to not kill off its host, right?
Asian honeybee colonies are fine with varroa mite infestation. European honeybees, the ones we rely on to pollinate basically everything, have not evolved with any mites. Most bee colonies have a parasitic mite that’s evolved with them, european honeybees do not. So, needless to say, they’re least equipped to deal with mites.
Varroa mites are tiny red-brown external parasites of honey bees. Although Varroa mites can feed and live on adult honey bees, they mainly feed and reproduce on larvae and pupae in the developing brood, causing malformation and weakening of honey bees as well as transmitting numerous viruses.
Colonies with low infestation generally show very few symptoms, however, as the mite population increases symptoms become more apparent. Heavy Varroa mite infestations can build up incredibly quickly and cause scattered brood, crippled and crawling honey bees, impaired flight performance, a lower rate of return to the colony after foraging, a reduced lifespan, and a significantly reduced weight of worker bees. Colony symptoms, commonly called parasitic mite syndrome, include an abnormal brood pattern, sunken and chewed capping, and larvae slumped in the bottom or side of the cell. This ultimately causes a reduction in the honey bee population, supersedure of queen bees, and eventual colony breakdown and death. It’s basically assumed in traditional beekeeping today that this is not an if, but a when.1
Now what’s particularly interesting is how varroa has played out across the globe— like I said, it was a novel parasite across most of the globe, which meant we got to see in real-time how different management methods impact its mortality rate.
Unsurprisingly, given the honeybee’s prominence for pollinating most of our food, a lot of research has gone into understanding varroa. One of the main reasons why varroa has been so hard to stop is specifically due to how we’re treating the majority of our honeybees here in the US— specifically around our crops. The practice of migratory beekeeping, in which beehives are shipped great distances to meet pollination demands, say, almond farming, increases the dispersal of ALL honeybee pests and parasites, including our good friend varroa.2
It is everywhere, but we can see how different genetic mite varieties share their genetic codes, accelerating their ability to evolve and share stronger genetics.3 You know, in case some varroa survive a chemical treatment, those genetics can be sure to quickly spread across the country. It’s not enough that we’re breeding them poorly and then just chemically treating them, which isn’t really working long-term, but we’re also accelerating how quickly that’s not working by making sure the genetics are traveling as quickly as possible.
People have started to recognize that these chemicals are not a long-term fix. There are a few methods that have been getting attention, some of which show more promise than others. The first I want to talk about is using plant extracts. In one particular study, a number of different plant extracts were utilized, one of which is pretty common here, garlic. I also bring up garlic because of the plants tested, it was the only one that was 100% effective in lab tests while also showing no health impacts on live honeybees.4
So while the garlic extract worked, it was in an enclosed chamber, so we don’t know the ability to work in a real-world setting effectively. As a beekeeper who has dabbled in using natural solutions, I can say confidently that many advertised online do not work, so finding ones that do is fairly rare. Second, exposure for honeybees was only done with adults, so we don’t know the impacts on the most vulnerable bees, the larvae pupae, or their impact on the comb.5 And while this is a cool option, it’s ultimately interfering with the natural evolution of the mite with the honeybee, right?
Do we have any real documentation of bees living with mites and it not being a big deal at this point? We actually have quite a bit, despite what your local beekeeper association might say. There have been a couple of different projects that have taken place, and we’ve interviewed a few folks who talk about this specific practice of basically pushing honeybee colonies through intense selection pressure to breed resistant bees. The first example I want to talk about is our friends just a few miles south of us in Cuba. Cuba is considered to have the world’s largest population of varroa-resistant honeybees.
What’s happened in Cuba is really inspiring for a number of reasons. It wasn’t simply that the bees developed mite resistance, but they learned to live alongside the varroa. Not only did they have incredibly high rates of things like recapping worker cells after cleaning, and keeping the hives highly populated, but they also had high rates of actual removal of mites and, most interestingly, low mite fertility.
After three decades they co-evolved to be more sustainable. We’ve seen in our lifetime– or at least people in Cuba— a natural co-evolution that is ideal for both species, right? The most interesting thing, though, is that their traits and rates of recapping and so on reflect what is seen in places where varroa came from. Further, these hives haven’t dropped in their productivity as you might expect, given the added workload of cleaning to remove mites. The 220,000 colonies across Cuba still produce over 100 pounds of honey a year, which is incredibly high.6
So we have some studies on this specific tactic, but we have some alternative practices that we’ll talk about further in the next few articles of the series. I won’t go too deep, because I’m gonna save that content for a little later, but in Sweden, they did leave some bees to fend for themselves, and the thinning process was pretty extreme. Less than half survived. Significantly less than half. But they bounced back pretty quickly and were able to maintain their hives after about 5 years without any significant losses. So we CAN do it. But the real question is, are we willing?
There are currently, as of this writing, 2.7 million beehives in the US, responsible for pollinating a third of our food crops. Of the 100 crop varieties that provide 90% of the world's food, 71 are pollinated by bees. 151 million pounds of honey are produced and sold a year. Imagine losing 75% of that for 1, 2, 3, even 4 years. Regardless of peoples’ opinions on whether or not the honeybee SHOULD exist, I think at this point in time, we understand we can’t simply pull the plug on honeybees, even if it’s a temporary solution. We’ve talked a bit in the past about how one of the impacts of honeybees is spreading disease and parasites, and theoretically, this would make them, in the long term at least, reduce some of this. So even if you’re against honeybees in our food production, a mite-resistant honeybee is better in the long run for everyone involved.7
There has been work studying the impacts of naturally slowing down the varroa in the hive, helping the hives learn to recognize it and build up a tolerance to the varroa. In one study that just finished in 2021, colonies with low infestation had the drone comb on the exterior of the hive removed when full of brood. The results showed that the removal of drone brood cells as a control method improved overall hive health, and interestingly did not negatively affect colony development. Removing the drone brood cells had a 43% success rate as traps against varroa versus hives completely untreated.8
This process offers one path forward that probably doesn’t involve losing 75% of our bee stock in a year, so that’s probably a good thing. Again, depending on who you ask. That said, this likely won’t solve the varroa problem without those losses, but is a way to slow down the losses. I think the research was primarily geared at ways to reduce mite load during honey harvesting periods, where the chemical options are pretty limited if you’ve got varroa. However, this seems like a viable transitional solution, if we are willing to accept some losses in the near term.
So I wanna go back to the Swedish experiment because the research pointed out some specific details that are kind of… interesting. For example, despite the fact the bees learned to live with varroa, or at least keep them in check, some other things began to spring up. While we haven’t talked about other diseases and parasites, unsurprisingly, there are a LOT of them out there, and before varroa was the thing everyone was worried about, there were others that were considered to be equally catastrophic. Two particular bacteria and viruses stood out in these newly mite-resistant hives, Bartonella apis and sacbrood virus.9
Bombella apis was also strongly associated with early and late seasons, though equally for all colonies. Both bacteria and viruses that became prominent affect colony protein management and metabolism. Now, those bacteria come from inside the bee gut, which I think is interesting. Research hasn’t quite figured out what to make of it, and it doesn’t seem to be causing any uniquely dangerous issues, but it’s definitely worth being aware of. It’s possible, in my completely unscientific opinion, that what we’re starting to see in this case is a non-varroa natural selection and self-regulation of the hive ecosystem taking place. And I think, if and when we get varroa in check, or rather— when we accept the reality of the current situation and recognize that the treatment method isn’t sustainable as it’s being applied today, we’ll see a lot of these things spring up as bees try to reach a healthy equilibrium.
Of course, instead of addressing the root cause of varroa and our bee management practices, some researchers are instead focused on genetically modifying bees— actually not even bees, but the mites themselves. What they’re trying to do is called RNA interference. Basically, an enzyme, RNA polymerase delivers (transcribes) the DNA’s information onto RNA, stimulating genes to turn on or “express” themselves, usually by producing certain proteins.10 If the message is not delivered, the gene is not turned on. In summary, the life process that is supposed to occur never begins, because the proper message is not received. The DNA genetic information has been “silenced.”11
Where this applies to honeybees is around the viruses transmitted. Viruses produce their own messenger RNA, which hijacks the host’s cell mechanics to replicate the virus instead of the original organism or host. If this can be interfered with via RNAi, the viral RNA no longer can do its job. Because the genome of viruses is shorter and less complicated than other organisms, it becomes relatively easier to get their DNA sequenced. The company behind this particular project is called Beelogics, and they first started by working on Israeli Acute Virus Paralysis.
This was back in 2007 and the world was their oyster. Since then, they were bought out by Monsanto who was then bought out by Bayer, and then Bayer sold it off to Greenlight Biosciences, where the same technology is being applied to find, and I’ll quote them for it, “Holes in the armor of varroa by attacking their ability to reproduce.” At this point, they have yet to produce any meaningful results, and of course, we have no idea what the unintended consequences of these types of actions could have in the long-term. In the following pieces, we’re going to dive in more deeply on virus and pest management in hives, and how we can be better managers for a healthier beehive and healthier insect communities.
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Ryals, D., Medeiros, B. de, & Farrell, B. (2022). Migratory Beekeeping Facilitates Genetic Admixture in Populations of the Honeybee Parasite Varroa Destructor. https://doi.org/10.22541/au.166079895.54947645/v1
Locke, B. (2015). Natural varroa mite-surviving apis mellifera honeybee populations. Apidologie, 47(3), 467–482. https://doi.org/10.1007/s13592-015-0412-8
Meskele, D. K. (2022). Evaluating the effect of plants extracts against Varroa mites (Varroa destructors) of honeybees (apis mellifera). Chemistry and Materials Research. https://doi.org/10.7176/cmr/14-2-03
Lamas, Z. S., Solmaz, S., Ryabov, E. V., Mowery, J., Heermann, M., Sonenshine, D., Evans, J. D., & Hawthorne, D. J. (2023). Promiscuous feeding on multiple adult honey bee hosts amplifies the vectorial capacity of Varroa destructor. PLOS Pathogens, 19(1). https://doi.org/10.1371/journal.ppat.1011061
Luis, A. R., Grindrod, I., Webb, G., Piñeiro, A. P., & Martin, S. J. (2022). Recapping and Mite Removal Behaviour in Cuba: Home to the World’s Largest Population of Varroa-Resistant European Honeybees. https://doi.org/10.21203/rs.3.rs-1682242/v1
Güneşdoğdu, M., Şekeroğlu, A., & Tainika, B. (2021). Effect of using drone brood cells as traps against Varroa destructor (Varroa mite). Turkish Journal of Agriculture - Food Science and Technology, 9(6), 1226–1231. https://doi.org/10.24925/turjaf.v9i6.1226-1231.4374
Thaduri, S., Marupakula, S., Terenius, O., Onorati, P., Tellgren-Roth, C., Locke, B., & de Miranda, J. R. (2021). Global similarity, and some key differences, in the metagenomes of Swedish varroa-surviving and Varroa-susceptible honeybees. Scientific Reports, 11(1). https://doi.org/10.1038/s41598-021-02652-x
Hasegawa, N., Techer, M., & Mikheyev, A. S. (2021). A toolkit for studying Varroa Genomics and transcriptomics: Preservation, extraction, and Sequencing Library Preparation. BMC Genomics, 22(1). https://doi.org/10.1186/s12864-020-07363-7