The varroa mite, has been a thorn in the side of beekeepers (and a literal thorn, or worse, in the side of their bees) for more than seven decades, and coming up a quarter century in New Zealand. Ahead of an upcoming series on breeding bees for varroa sensitive hygiene (VSH), science writer Dave Black summarises where we've got to in breeding for varroa resistance, and why.
By Dave Black
It’s been 72 years since varroa mites managed to find a way of living with honey bees. By way of some random milestones you’re probably familiar with, varroa (the Korean ‘flavour’ that predominates) started to make its way across Europe from its Asia-Pacific origins in the 1960s, reaching Bulgaria in 1972 and Germany in 1977. Only a little later, a slightly different model left Japan in 1971 for Paraguay, arriving in Florida in 1987 and meeting the Korean type coming the other way from Europe, in North America! Having encircled the globe, they set about filling the gaps, reaching New Zealand in 2000, and Australia in 2022. Now what?
It’s personal, not.
Varroa was identified in Britain in 1992[i], the year before I started beekeeping there, and in 1996 I went to my first ‘international’ conference in Cardiff, Wales, – “Fight the Mite![ii]”. I remember at the time thinking, well, we must have learnt something from the 25 years it had been in Europe. Now, 70-odd years after the original host shift and 12 years after New Zealand’s own “Combat Varroa!” workshop (I was there too![iii]), I still wonder how little we know.
I don’t write much about varroa, it would be a full-time effort if I did. The truth is, I resent every minute spent on the subject, all at the expense of the bees I am interested in. I have known too many beekeepers that know more about managing mites than they know about managing their bees. It’s true arachnids are fascinating in their own right, and this one is a complex beast indeed. As an exercise in parasitology, it’s an almost unrivalled case study involving an unusual and undeniably successful relationship with a viral disease, one of only four such diseases ever known[iv].
Known and unknown unknowns...
It turns out four known varroa species parasitize honey bees, V. destructor, V. jacobsoni, Varroa rindereri, and Varroa underwoodi. Modern genetic analysis has shown the two Korean and Japanese ‘flavours’ (‘haplotypes’ - variants of a parent’s genetic identity) of V. destructor that went around the world are remarkably uniform, apparently near clones of the few individuals (probably counted in tens according to the geneticists) that made the jump from A. cerana originally. That many other haplotypes of varroa currently exist on A. cerana in SE Asia (at least 17 on jacobsoni and eight on destructor) leaves the possibility that more host jumps could occur in future.
When varroa are not reproducing they are what beekeepers have always referred to as ‘phoretic’, that is, being carried about by adult bees. This ‘phoretic’ phase is more correctly called ‘dispersal’, as they are not merely being carried around (true phoresis), but are actually feeding too. Their nutritional demand is remarkably high. Unable to feed from a host, varroa start to die after six hours, half will die in 18 hours, almost all
(95%) within 36 hours[v]. A rediscovered study by two German scientists from 1981 pointed out that varroa do not digest all the proteins they imbibe when feeding. Instead, these are used unaltered by the female and found in newly constructed eggs[vi], a phenomenon now confirmed as ‘kelptocytosis’[vii](‘kepto’ – theft). It is a remarkable metabolic economy, and partly explains both why these mites must select the fat bodies of nurse bees at particular stages of their life cycle[viii],[ix], and their rapid reproductive success.
Until recently little was known about what goes on during the dispersal phase. In 2022 Zachary Lamas in his PhD thesis ‘Feeding behaviour and distribution of Varroa destructor on adult bees of Apis mellifera’ used two main techniques to study the matter. Lamas used microspheres (Fluospheres – commercially available, solid or porous coloured spherical particles with diameters of one micron that have various medical applications) to trace the transfer of material between mite and bee by injecting them into the body and watching where, and in who, they ended up. He also marked a lot of bees, as they emerged, with a system of coloured thorax dots that allowed him to tell their age and whether they had been parasitised by mites. Painstaking work, thousands of bees, many colonies, for months. Clearly varroa are promiscuous feeders, some individuals changing host almost daily while others used the same bee for 15 days. In the midst of a global pandemic there was never any doubt about the outcome, any viruses present would thrive.
Varroa Vector Viruses.
When varroa arrived in New Zealand Deformed Wing Virus (DWV) had never been found here[x], [xi],[xii]. The same was true in Britain, in 1992[xiii]. In Britain (and elsewhere) early tests linked colony deaths to mostly Acute Paralysis Virus (APV) and/or Kashmir Bee Virus (KBV), or maybe some combination of the few viruses known at the time. By the mid-1990s the role of DWV in the collapse of colonies was becoming quite clear. In the last 50 years DWV, once virtually unknown, has become the most widespread, viral insect pathogen in the world.
During this time it has never been possible to stop varroa infesting hives, so beekeepers needed to find ways to destroy the mites without harming their bees. The treatment and techniques available are mostly ones repurposed from treating larger animals, and, as in larger animals, residues, resistance, and resourcing the work were, and remain, a problem. At the same time, seeing that some species and strains of honey bees aren’t vulnerable to varroa, it didn’t seem unreasonable to try and select and breed stains of European honey bees that could resist or tolerate the infestation.
A rock and a hard place...
After 40 years of trying around the globe[xiv] and, it has to be said, spasmodic and isolated gains, it’s probably worth asking why breeding for varroa resistance is so difficult. To summarise 22 pages of (I think) impartial expert thought, the current issues seem to be (reviewed in Guichard et al[xv]): we can’t or don’t measure accurately the traits we think contribute to better survival, don’t understand the environmental conditions that result in them being used, wrongly generalise from untested observations about naturally surviving colonies, and can’t consistently predict to what extent a trait can be inherited.
As well, understanding the trade-offs that might be likely and scaling the solution to use in the field hasn’t been tried. The collaborative infrastructure that might make that possible doesn't exist and there are no national funding schemes that might support a Genetic Improvement Programme, let’s say, for up to a decade. The Catch-22 is that, to be successful, trials tend to be specific, local, and time-limited, which means the results aren’t necessarily replicable and comparable to anywhere else. If I was to put it provocatively, not a wild goose chase, more wild geese chases. The question is, what do we do about it?
Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.
References
[i]Paxton, R., 1992. The mite marches on: Varroa jacobsoni found in the UK. Bee World 73, 94–100. https://doi.org/10.1080/0005772x.1992.11099122
[ii]Black, D., 1996. Living with a pest. 25 years of the varroa experience. BeyondBeeBooks, https://beyondbeebooks.substack.com/p/living-with-a-pest-25-years-of-the
[iii]Black, D., 2012. Towards sustainable management and control of Varroa destructor in New Zealand. https://beyondbeebooks.substack.com/p/towards-sustainable-management-and
[iv]Lamas, Z.S., 2022. Feeding behavior and distribution of Varroa destructor on adult bees of Apis mellifera. University of Maryland. Thesis.
[v] Garedew, A., Schmolz, E., Lamprecht, I., 2004. The energy and nutritional demand of the parasitic life of the mite Varroa destructor. Apidologie 35, 419–430. https://doi.org/10.1051/apido:2004032
[vi]Tewarson, N.C., Engels, W., 1982. Undigested Uptake of Non-Host Proteins by Varroa Jacobsoni. Journal of Apicultural Research 21, 222–225. https://doi.org/10.1080/00218839.1982.11100546
[vii]Ramsey, S.D., Cook, S.C., Gulbronson, C., vanEngelsdorp, D., Evans, J., Posada, F., Sonenshine, D., 2022. Kleptocytosis: A Novel Parasitic Strategy for Accelerated Reproduction via Host Protein Stealing in Varroa destructor. https://doi.org/10.1101/2022.09.30.509900
[viii]Kuenen, L.P.S., Calderone, N.W., 1997. Transfers ofVarroa mites from newly emerged bees: Preferences for age- and function-specific adult bees (Hymenoptera: Apidae). J Insect Behav 10, 213–228. https://doi.org/10.1007/BF02765554
[ix]LeDoux, M.N., Pernal, S.F., Higo, H.A., Winston, M.L., 2000. Development of a bioassay to test the orientation behaviour of the honey bee ectoparasite, Varroa jacobsoni. Journal of Apicultural Research 39, 47–54. https://doi.org/10.1080/00218839.2000.11101020
[x]Jacqui Todd, Brenda Ball, & Joachim de Miranda, 2004. Establishment and use of RT-PCR to diagnose and study economically important honeybee virus infections in NZ. (Identifying the viruses causing mortality of honey bees in colonies infested with Varroa destructor). Surveillance 31(4) pp23-25
[xi] Todd, J.H., De Miranda, J.R., Ball, B.V., 2007. Incidence and molecular characterization of viruses found in dying New Zealand honey bee ( Apis mellifera ) colonies infested with Varroa destructor. Apidologie 38, 354–367. https://doi.org/10.1051/apido:2007021
[xii]Lester, P.J., Felden, A., Baty, J.W., Bulgarella, M., Haywood, J., Mortensen, A.N., Remnant, E.J., Smeele, Z.E., 2022. Viral communities in the parasite Varroa destructor and in colonies of their honey bee host (Apis mellifera) in New Zealand. Sci Rep 12, 8809. https://doi.org/10.1038/s41598-022-12888-w
[xiii] Varis, A.L., Ball, B.V., Allen, M., 1992. The incidence of pathogens in honey bee (Apis mellifera L) colonies in Finland and Great Britain. Apidologie 23, 133–137. https://doi.org/10.1051/apido:19920205
[xiv]Büchler, R., Berg, S., Le Conte, Y., 2010. Breeding for resistance to Varroa destructor in Europe. Apidologie 41, 393–408. https://doi.org/10.1051/apido/2010011
[xv]Guichard, M., Dietemann, V., Neuditschko, M., Dainat, B., 2020. Advances and perspectives in selecting resistance traits against the parasitic mite Varroa destructor in honey bees. Genet Sel Evol 52, 71. https://doi.org/10.1186/s12711-020-00591-1
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