Towards sustainable management and control of Varroa destructor in New Zealand
July 2012
If any one had told me fifteen years ago that I would still be attending workshops on varroa management in 2012 I wouldn’t have taken them seriously, but here we are sixteen years later at the New Zealand NBA North Island Workshop: Combating Varroa! In his opening address to conference Barry Foster the NBA President was keen to stress two things, the use of science, and the value of a strategic approach. It’s easy to agree.
A strategic approach.
In my opinion one of the more valuable academic papers this year has been Dietemann’s review of the “Varroa and viruses” workshop in Magglingen, Switzerland, in November 2010 (Dietemann et al, 2012). This was part of the European COLOSS programme 1and, co-authored by twenty-one eminent researchers from all over the world, presented the conclusions reached at the workshop. These authorities included ‘bee-household’ names such as Dennis Anderson, Anton Imdorf, Keith Delapane, Jeff Pettis, Peter Neumann, and Peter Rosenkranz. Of course people say that the most important paper is the one that reflects your own prejudices, and I won’t argue with that. I just think I’m right!
The article evaluates the current state of varroa control world-wide, describing what has been to date an ad hoc and uncoordinated effort, with great gaps in our understanding of the basic biology of the parasite hampering the development of enduring, sustainable control. It presents a ‘road map’, based on a consensus about a collaborative approach to filling those gaps and prioritising research. Short-term and long-term projects are identified, and the state of current approaches briefly summarized. The more urgent, short-term priorities identified are these:
Screening (develop and register) for new varroacidal compounds
Improve the formulation and application of existing, known varroacides
Finish the V. destructor genome description2
Improve and develop models of V. destructor population dynamics
Redefine V. destructor economic thresholds taking into account the effect of viruses.
These are what we should be doing now.
The success of long-term projects depend on getting the basics right, the short-term projects. The long-term projects proposed are:
Develop biological control against V. destructor (pheromones, enthomopathogens, endosymbionts)
Identify the trigger mechanisms of V. destructor reproduction
Develop V. destructor in vitro rearing method and reproduction tests
Search for V. destructor tolerant bees, identify the tolerance mechanism, and deal with the problem of narrowing genetic diversity
Understand and preserve the genetic diversity of host-parasite coevolution and local adaptations
Prepare for the putative arrival of new invasive mites, (haplotypes and species)
Eradication programmes and border protection for V. destructor
Investigate the impact of V. destructor on virus presence in honey bee populations
Understand virus transmission and virulence
These are not given a definitive order, but they need to occur in a collaborative environment to succeed. The authors recognise some ‘tools’ for achieving these are missing. Maybe they should be among the short-term projects. There are two important ones. In the first place there is a lack of standardized test procedures or ‘protocols’. What does ‘tolerant’ mean for example, how can you prove it’s there? Could it be pathogenicty in mites varying, or an environmental effect? These are now in the process of being agreed and published3. Second, methods of culturing cells for the purpose of purifying and propagating bee viruses make the study of virus epidemiology difficult.
Nestled in among the long-term priorities then, is this one, the core subject of the workshops this month. “Search for V. destructor tolerant bees, identify the tolerance mechanism, and deal with the problem of narrowing genetic diversity.” The authors of the report argue that, right now, the selection of such bees is performed blindly, or is based on secondary mechanisms of tolerance, and that the detailed knowledge of behavioural and physiological mechanisms isn’t there. Once these have been identified genetic markers can be used to identify the strains to use and target the traits effectively. They remark, of our current efforts, “The ideal solution would be the identification and breeding of bee strains tolerant [of] the parasite, but given our present state of knowledge we are not close to any such sustainable solutions.” Note the use of the word ‘sustainable’.
Around the world.
The structure of honey bee breeding programmes in Europe and North America are, to my mind, quite different, and reflect a difference in the motivation and the economy of beekeeping in the different regions. I don’t say that because I think the worse of either, they’re just different, with different imperatives. In Europe the historic cause of many projects was a concern with the conservation and promotion of highly diverse local bee strains and traditions. It’s largely amateur, under-resourced, but common, cooperative, extensive, and experienced. Mechanisms for the development of research (from highly capable institutions) into practical science applied in the field are still being worked out. In North America breeding has a different purpose, a more commercial purpose; it isn’t a hobby. Breeders are supported by government extension programmes and academic institutions and a good deal more ‘business-like’. Usually with a commercial imperative though, goes a degree of concealment, until there is a ‘product’, but it is remarkable how open these breeders have been so far. The link between research and application appears to be strong as a result of the extension programmes, and it may be these that keep the process open and ‘moving along’.
In North American the effort to breed tolerant bees has been going on for more than fifteen years. The Europeans, more nonchalant, spent a lot of time on thinking about the problem, avoiding or treating the problem, then finished their wine and began breeding about five years afterwards. Arguably A.m. ligustica was most affected anyway and migratory beekeeping was not widely practised so there was time for another glass. If you want a less flippant view of the Northern Hemisphere breeding effort a special issue of the magazine Apidologie on honey bee health carries a description of the effort from both sides of the pond (Buchler 2010, Rinderer, 2010).
The germ of the VSH initiative in New Zealand came from the realisation in 2003 that beekeepers were going to need a comprehensive ‘toolbox’ of measures in order to first delay, and then work around the looming problem of mites resistant to synthetic chemical acaricides (Taylor, 2012) In the last three years the evidence has been mounting that mites are indeed becoming resistant in particular areas. In Northland, through Auckland, into Hamilton, and if the results of a survey this autumn are to be believed, further a field in the North Island, tests have demonstrated an uncomfortably high survival rate for mites in colonies treated with Bayvarol and Apistan (Goodwin, 2012). While there may be other reasons for this, resistance has got to be high on the list. Naturally, one of the tools in the box was a varroa tolerant honey bee, and the effort to develop this has been a steep learning curve. We start with a disadvantage. While every honey bee may have the same genome natural selection has ‘switched’ bits on and off to produce the variety of phenotypes, or strains, we have now. For example, it has been assumed that a Russian bee, because it has lived with varroa longer than any other type, will give us a short-cut to a varroa tolerant strain, natural selection having already done half the work. If you can’t (or won’t) start with this bee, the argument goes, it’ll take you longer to get to the end-point. That is why ‘Russians’ are a bit part of varroa-tolerant bee-breeding in the US, and why people talk about ‘survivor colonies’. In New Zealand, unwilling to import bees for phytosanitary reasons, and with funding and logistical constraints on what could be taken on, the sequence of programmes run (as elsewhere) have not yet produced that ‘sustainable solution’, although queens with the putative characteristics have been produced using Artificial Insemination (AI) and the use of closed mating on an off-shore island. The plan now is to devolve the effort to the beekeeping community and so far two companies have begun to plan and train, and breed and test queens. It remains to be seen whether the wider NZ beekeeping community has the will or the capacity4 to assist with breeding and/or rearing, and it certainly isn’t clear to me yet how it can be organised, coordinated, prioritised and supported, and who will do it.
The science behind Varroa Sensitive Hygiene
There was no description of what ‘VSH’ is at the workshop, it probably wasn’t the place for it. I’m not really the one to do it, but I’ll give it go. The original thought underpinning the idea that hygienic behaviour could be inherited came from the work of Walter Rothenbuhler in 1964. His experimental results could be explained by a simple Mendelian inheritance to two independently assorting genes, and this idea is still widely used. VSH (as defined) is thought to have two separate components, one, for uncapping a cell and the other, for cleaning out the contents. The objective of selective breeding in this case is to produce a queen some of whose daughters have a low threshold response to performing these behaviours given the right stimulus (i.e. it doesn’t take much to persuade them to do it). As our knowledge has progressed however, we barely think of inheritance in terms of ‘genes’ any more. The word now is ‘quantitative trait loci’ or QTLs5, and we now believe there are seven or more that contribute to the behaviour (Lapidge, 2002). Researchers are gradually homing in on what ‘VSH’ actually is. Oxley et al identified six QTLs that affect these ‘task thresholds’, three with a general effect, two specifically for uncapping, and one for removal. At a gene level, four involved in olfaction, learning and social behaviour were proposed (Oxley, 2010). And the ‘good news’ doesn’t stop there! The precursor to VHS was SMR, Suppressed Mite Reproduction. A very direct approach to tolerance SMR is interested in traits that larvae possess and which affect the ability of the mite to find and reproduce in honey bee cells. These may be chemical or physical cues; kairomonal or synomonal effects6. A team working in a German university (including Cornelia Gebner, also of Otago) investigated bees from a Swedish island and believe they have identified three QTLs which together moderate female mite reproductive success. Cleverly, they used drones and recommend drones for marker assisted selection. It is very early, but it could be a significant finding as it would be a simpler thing to select for, and in the future could have some bearing on the control of other potentially pathological varroa haplotypes7.
Clearly all this complexity makes selection and breeding a good deal more challenging, particularly when you start from different, or limited, phenotypes, but, if the loci can be identified and marked8 it makes the business of identifying the trait you wish to transfer, and checking that it appears in the offspring, much easier. Using the science has the capacity to overcome the difficulty we have in starting with a restricted pool of bee strains. I think it does however put it beyond the ability of the amateur, even a long-lived one. A problem that will be harder to solve is the degree to which honey bees rely on intra-colonial genetic diversity to respond to everything from disease resistance to foraging behaviour and comb construction. Honey bees have a very high rate of chromosome recombination which helps, but the importance of multiple mating and a diverse group of sisters can not be under emphasised. A variety of VSH strains are required, and there is even a case for the conservation of non-VHS strains.
The future beckons?
If we return to Dietemann’s ‘road map’ for the sustainable control of V destructor and apply it to the situation here we can see that some basic important things remain undone, while others are probably beyond our grasp. The market for acaricides in New Zealand is very small, and it’s hard to see how much we can do about formulation or registering new synthetic acarcides.
The development and local application of organic or ‘biotechnical’ control measures could be a valuable avenue to research. Knowledge of the genomes of honey bee and varroa is already well advanced. In my view it is the last two points that are most important; Improve and develop models of V. destructor population dynamics, and; Redefine V. destructor economic thresholds taking into account the effect of viruses. I think the equivocal guidelines that resulted from an absence of a local model encouraged a ‘go it alone’ attitude which can’t of helped our need to delay the appearance of resistance by correctly timing treatments. We now understand that synergistic effects, like endemic virus levels, can have a great effect on the damage done by varroa mites, and it alters the predictions of the models. While a survey of NZ virus presence was done in 2001/2002 the follow-up work was abandoned. There is no doubt things have changed since the advent of Deformed Wing Virus (DWV), but we have no idea how9. Does all that mean we should abandon work on VHS bees? No. There is no value in loosing the knowledge and achievement to date. I would estimate though, that the production of VHS queens as a commercial proposition is, at best, five years away, and much longer before they make an appreciable contribution to varroa management in the general beekeeper population. Creating the extra effort to further this work will be challenging. In the meantime something must be done to deal with the problem, especially if acaricide resistance is proceeding faster, as it appears to be. Maybe it’s time to reprioritise, and systematically apply science and forethought to achieve our goal.
References.
Behrens, D. et al. (2011) Three QTL in the honey bee Apis mellifera suppress reproduction of the parasitic mite Varroa destructor.
Buchler, R., Berg, S., (2010) Breeding for resistance to Varroa destructor in Europe. Apidologie 41: 393-408.
Dietemann, V., et al, (2012) Varroa destructor: research avenues towards sustainable control. Journal of Apicultural Research (IBRA) 51(1): 125-132.
Goodwin, M. (2012) New Zealand NBA Combating Varroa Workshop, Hamilton.
Lapidge, K.l., Oldroyd, B.P., Spivak, M., (2002) Seven suggested quantitative trait loci influence hygienic behaviour of honey bees. Naturwissenschaften, 89: 565-568.
Oxley, P.R., Spivak, M., Oldroyd B.P., (2010) Six quantitative trait loci\influence task thresholds for hygienic behaviour in honeybees (Apis mellifera). Journal of Molecular Ecology 19(7): 1452-61.
Rinderer, T.E., Harris, J.W., Hunt, G.J., de Guzeman, L.I., (2010) Breeding for Varroa resistance to Varroa destructor in North America. Apidologie 41: 409-424.
Taylor, M. (2012) New Zealand NBA Combating Varroa Workshop, Hamilton.
Todd, J.H., De Miranda, J.R., Ball, B.V., (2003) Incidence and molecular characterization of viruses found in dying New Zealand honey bee (Apis mellifera) colonies infested with Varroa destructor.
Prevention of honey bee COlony LOSSes, www.coloss.org.
I think this has been done, Cornman et al, 2010, Genomic survey of the endoparasitic mite V. destructor, BMC Genomics 11:602
Neumann and Carrick, 2010. www.coloss.org/beebook
AI practitioners in NZ can be counted on one hand.
QTLs are regions of DNA linked to a trait that can be measured, and typically ones that vary continuously, like height rather than sex. These traits are the result of more than one gene (polygenic) and their distribution in a population is described by probability rather than Mendelian inheritance.
Analogous to a pheromone produced by one species, but in this case benefiting another species, or in the case of synomone, both species.
Many haplotypes (strains) of V. destructor exist. So far only the Korean haplotype has been found to be pathogenic for European Honey Bees. Others may ‘cross the species barrier’.
So-called ‘fingerprinting’ or Marker Assisted Selection (MAS). First using microsatellites, or repeating sequences of DNA base pairs, and lately by SNP, a single base pair. Think of them as ‘punctuation’ in the DNA ‘sentence’.
At the time Kashmir Bee Virus (KBV) regarded as the most significant, but it’s likely to affect the colony differently compared with DWV.