The battle against varroa is ongoing and getting more challenging, so some beekeepers and scientists are exploring how harnessing the right genetic attributes in our bee stocks might aid resistance to the mite. Science writer Dave Black explores varroa sensitive hygiene (VSH), explaining what it is, how it is measured, and learning of some research in our own backyard which hints at its worth.
There is plenty of evidence that honey bee colonies react differently to varroa mites and are therefore better or worse at surviving. Crudely, some live and some die. As the difference is likely to be hereditary, selecting the better ones and using these to reproduce more resilient colonies is an obvious strategy for preserving the stock. While it would be good to know just which feature confers a degree of protection, people have been selecting and breeding plants and animals effectively for a long time without that kind of knowledge. Even when we do know what the feature is, the odds on it being a combination of things determined by complex and possibly conflicted characteristics is pretty good. Stock improvement can take a (very) long time.
One of the traits that seems to allow some measure of tolerance for varroa has become known as ‘Varroa Sensitive Hygiene’. It’s actually a collection of ‘skills’ enabled by 100s of genes regulating chemosensory perception, behaviour, cognition, learning, and memory, something everyone is still struggling to understand after years of work and not something you can easily ‘see’.
Measuring VSH
Finding the characteristic is based on measuring population growth of mites, do the mites increase in number (bad) or decrease (good)? Worker bees do not display VSH characteristics until at least 10 days past eclosion (that’s emerging, so 31 days since the egg was laid). To see evidence of VSH you examine brood for mites after the (capped) 3-day pre-pupa stage, 4-6 days post capping. It’s important to have enough bees and brood at the right age, so in a newly established test colony it’s about six weeks before you will see (or not see) signs of VSH behaviour. It requires careful counts (generally with a microscope) lots of colonies and diligent attention to the composition of the colony, the timing of manipulations, and standardised methods. There have been attempts to streamline the work, by introducing infested brood of a specific age, or looking for uncapped and recapped cells, and it’s been quite common (but not ideal) to be looking for signs of general hygiene (dead brood removal) as a proxy for the VSH behaviour itself.
Not Measuring VSH
Although we can’t yet say what causes VSH and mite tolerance, or how it might work, scientists have been able to find some, usually non-functional, genetic features (let’s call them ‘markers’) that always seem to be associated with a VSH outcome. We’re at the point where the infrastructure for identifying these markers exists and is relatively cheap and quick (at least in ‘developed’ countries). That way we can screen colonies (really just the queens) for the marker and safely assume the ones that have it are in the ‘better’ and not the ‘worse’ category. We should also be able to screen drone-rearing colonies to maximise the probability of the marker appearing.
Marker Assisted Selection in New Zealand
The principle behind Marker Assisted Selection (MAS) in breeding has been understood since the 1930s, but the real value began to be realized in the late ‘80s as our knowledge of genomes and DNA improved. It has been successfully applied to crops, livestock, arboriculture and aquaculture, and ‘maps’ of molecular markers have been published for lots of species. There is still debate about the kind of marker to use in different circumstances. The initial iteration of the honey bee genome (Amel_v1.0) was published in 2003 (by Gene Robinson) and chromosome maps completed in 2006 (Amel_v4.0)[i], and potential candidates for markers were being sought from about 2010.
Last year Dr James Sainsbury (from Plant & Food Research), with a number of co-authors, published an Open Access paper describing the use of one marker identified by an American team in 2012[ii]. It described a method to improve stock at Coast to Coast Bees in a apiary near Hamilton. The idea was to see if using that marker, also found in New Zealand bees, to select queens would result in an appreciable reduction in varroa mite levels.
Forty colonies were all set up in December 2017, treated with Bayvarol, checked (alcohol washes), and equalised. They were requeened with queens that had been genotyped (so their ‘marker status’ was known) by testing wing clippings and formed into a ‘control’ group (marker absent) and a ‘treatment’ group (marker present). In January 2018 a monitoring round determined their initial mite status and ten weeks later the test was repeated. After that final assessment the colonies were treated with Bayvarol and Apivar for four weeks (one brood cycle) while the mites were monitored with sticky boards (Hive Doctor floors).
While initial mite levels were low at the start of the experiment the study reported a 28.5% drop in mite levels compared to the ‘control’ colonies, and no difference in colony size. The study does not, and was not intended to, show the reduction was due to VSH; it does indicate it was co-related with the marker, and no bees or researchers were harmed in the process. The marker was shown to be present in about half of 179 New Zealand queens tested. There is a lot more to do, multiple, better, markers, selected drones, perhaps markers that can be identified in the apiary, and figuring out how to select the queens without promoting in-bred stocks.
Even if the marker proves useful, we are still a few years away, and using it requires a bit of organisational rigour, but, if nothing else, speeding up the VSH selection process has the potential for many more beekeepers to reduce the number of treatments that must be applied, or to use a more benign medicine.
Dave Black is a commercial-beekeeper-turned-hobbyist, now working in the kiwifruit industry. 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] Science Vol 314, Issue 5799 pp. 578-579 DOI: 10.1126/science.314.5799.57806 [ii] Sainsbury J, E. Nemeth T, Baldo M, Jochym M, Felman C, Goodwin M, et al. (2022) Marker assisted selection for Varroa destructor resistance in New Zealand honey bees. PLoS ONE 17(9): e0273289. https://doi.org/10.1371/journal.pone.0273289
