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Over the fence: Perspective on managing acaricide resistance

Dave Black

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This article was originally published in 2015

 

Everybody needs to look over the fence once in a while, especially beekeepers. Something that caught my eye recently was a study looking at weeds and glyphosate resistance, a study which itself took a glance over the palings at antibiotic resistance in hospitals. Resistance is not a phenomenon unique to beekeeping, it is universal and, at its simplest, just about how organisms adapt and evolve in their environment.

 

From our point of view, when we think about resistance, the aspect that concerns us most often is varroa mites not disappearing after we have treated the hive with a substance that usually kills them, and usually we’re talking about the efficacy of the two synthetic pyrethroids used to treat the pest, Apistan and Bayvarol. Pyrethroids have a long history and it was never in doubt that varroa mites would adapt to the treatment.

 

Pyrethroids are copies of naturally occurring esters from Chrysanthemum flowers. What we call pyrethrins were first used as insecticides in the first century in China. They were used in Europe more than 200 years ago and in commercial production by the mid-19th century. Although effective, they were expensive to produce and quickly degraded by light and air, so by 1924 chemists began making ‘synthetic’ copies which were more stable and better suited to one pest or another, or to different ‘delivery systems’. By the 60s and 70s many variants had been produced, including deltamethrin, at the time the most active insecticide ever produced.

 

Apistan and Bayvarol are different pyrethroids, called tau-fluvalinate and flumethrin respectively, which differ in the construction of one end of the molecule. One of life’s ironies (one of my favourites) finds that the ‘natural’ pyrethrins, the synthetic pyrethroids, and the environmentalist’s bête noire, DDT, are all much the same when it comes to their effect on arthropod pests. All of them affect the normal transmission of nerve impulses along the nervous system by stopping the exchange of sodium and potassium ions across an impermeable membrane through a special ‘channel’. (If you’re interested look up ‘voltage gated sodium channels’). What resistant pests have done is to modify the sodium-channel proteins so the insecticide can’t bind to the site.

Different pests have modified different proteins, but resistance is extraordinarily common; whitefly, cockroaches, fleas, lice, mosquitos, flies, aphids, thrips, and many others have all evolved resistance to pyrethroids and DDT, to a greater or lesser extent. Known as ‘knockdown’ resistance (knockdown being the excited but paralysed state caused by the nerve disruption) it was first recognised in 1951 in house flies. When we add a treatment to our hives we alter the environment in the hive, hoping that the bees can still flourish but that the things we don’t want die or can’t reproduce. In this sense varroa mites are analogous to the weeds I mentioned at the start.

 

Weeds, and mites, adapt to pesticides in various ways. Besides changing their behaviour (avoiding the chemical for instance) pests can slow or stop the chemical getting into their body, or store it up where it can’t do any damage; they can use enzymes to destroy the active ingredient, or they can change the site or process the chemical is able to disrupt. It’s important to realise these changes come at a cost, sometimes a very high cost, sometimes a very low cost, and rarely, sometimes with an added advantage. For example, a bacteria may construct a thicker ‘skin’, but it will have to use more resources to do so. As long as it faces a threat from the chemical it’s worth doing that, but in an environment where the threat no longer exists if there is extra ‘effort’ involved it’s wasted, and puts the ‘resistant’ organism at a disadvantage compared with an organism that doesn’t bother. For any particular adaption it’s important to understand the cost of that adaption if we want to understand how long-lived and pervasive that adaption will become. Complex, costly changes are easily lost when they are unnecessary; simple, cheap alterations are likely to remain.

The science looking at pyrethroid resistant mites has been able to describe in pretty good detail what genes are involved and what they do to allow make mites less sensitive to the chemical. This type of chemical and it’s mode of action is pretty well understood as it been in widespread use for a long time. Around four amino acid substitutions have been proposed in the case of varroa’s pyrethroid resistance. So far, it looks as though the metabolic cost of this adaption to pyrethroids comes at a very low cost for mites, and no differences in ‘fitness’ (for their environment) has been observed between resistant and non-resistant mites. So far, there is very little evidence that regression to a predominately non-resistant strain occurs within a reasonable period of time if pyrethroid treatments are suspended, and that also suggests the cost of carrying the adaption is relatively low.

 

There is not a lot of data about the incidence and spread of resistant mites. Beekeepers aren’t looking for resistance, even though it is simple to do so, and often the only clue is the unexplained loss of treated colonies. Ascribing the cause of the collapse to resistant mites is a long bow to draw however and so usually the cause is hidden by the many plausible reasons for a colony dying. On the large scale however, we do have a picture, and wherever we have looked we can see that the spread of ‘resistance’ replicates the same route that the original mite invasion took; a slow local spread accompanied by an occasional ‘long-distance’ hop. That’s no surprise, mites can’t survive and travel without a bee host, and so the spread of the resistant variety relies on the movement of bees and their colonies, and the same ineffective phytosanitary regimes that permitted the original spread of the pest. It’s also no surprise because that is what happened with other examples of resistance. We will have to wait for more information about the genes involved in providing resistance, but so far it does not look as though it appears spontaneously from multiple origins, it emerges once or twice, by chance, and the mites carrying the attribute spread.

 

Pyrethroid resistant mites have been reported all over Europe, the US, Israel, and parts of S. America. Resistance to coumaphos has been reported in Italy, and resistance to amitraz has been reported in Croatia, France, and the USA. In October 2009, not quite ten years after varroa was first discovered, resistant mites were reported from a hive in Auckland. In the ensuing discussion speculation suggested resistance was likely present in Northland, Waikato, and the Bay of Plenty, and maybe further south. We do not know if the necessary genetic change arose here and was selected by the prevailing pyrethroid use at the time, or whether we managed to import a resistant strain as easily as the original import had occurred. As we don’t know how varroa entered the country it’s possible the ‘gate was never closed’. However it arose, we know it will gradually spread, just as the non-resistant type(s) have. While the available evidence suggest pyrethroid resistance is not widespread amongst varroa mites in New Zealand we have to add the word ‘yet’.

 

Resistance isn’t anyone’s fault, it’s just the way Nature works, and it was not caused by the injudicious use of the chemicals used to control it; the fact is that the clock was always ticking and at some point luck ran out. Inappropriate pesticide use will ‘fix’ the problem in place though. The challenge is to understand its incidence and find the best strategy to manage resistant mites. That’s why I’m interested in looking over the fence. Everywhere we are faced with similar challenges; super-resistant weeds and antibiotic resistant hospital bacteria are two sides of the same coin.

The paper studying glyphosate resistance in Ilinois (Evans et al) used a lot of data to look at the outcome of different management practices, broadly, rotating herbicides with different modes of action (MOA) or mixing different herbicides together. These two divergent strategies are also used to manage bacterial resistance to antibiotics in public hospitals, and in both cases the finding is that mixing, not rotation, is the better (but more expensive) strategy. This appears to be true when the fitness cost of a resistant trait is low, in which case the adaption is fixed even in the absence of the selective agent (the pesticide). Mixing strategies are not reliant on the cost of fitness driving depletion. Instead, mixing depletes any resistance alleles by decreasing survival probabilities of all individuals carrying the relevant alleles. It is an expensive strategy as the pesticides for each MOA must be effective. Ineffective, low-dose mixtures can potentially increase the risk of non-target-site resistance and cross-resistance evolution. I can see very little justification for adopting the strategy as a prophylactic treatment; besides the extra cost it only increases the chance of eventually selecting for cross-resistance.

 

In the conclusion the paper points out; ”Herbicide mixtures are not a permanent solution to the problem of target-site resistance; herbicidal mixtures may delay evolution of resistance, but they do not prevent it…long-term, cost-effective, environmentally sound weed management will require truly diversified management practices… Combining chemical, cultural, physical, and biological tactics can provide cost-effective weed management while reducing reliance on herbicides.” For Evans, “We will encounter resistance evolution repeatedly in natural systems managed for human benefit. Sustainable stewardship of these systems will depend on recognising that we are always applying selective pressures, and that management responses need to grow from our understanding of applied evolution”

 

Can’t help feeling there is a lesson for beekeepers in there.

 

Evans, Jeffrey A., et al, (2015) Managing the evolution of herbicide resistance. Pest Manag Sci, (wileyonlinelibrary.com) DOI 10.1002/ps4009

 

Martin, Stephen, J., (2004) Acaracide (pyrethroid) resistance in Varroa destructor. BeeWorld 85(4): 67-69.

 

Davis, T.G.E., et al (2007) DDT, Pyrethrins, Pyrethroids, and Insect Sodium Channels. IUBMB Life, 59: 151-162.

 

Lagator, Mato et al (2013) Herbicide mixtures at high doses slow the evolution of resistance in Chlamydomonas reinhardii New Phytologist Vol 198(3): 938-945. DOI 10.1111/nph.12195

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Nice write up @Dave Black, this is exactly what we were doing with trying to combat worm resistance in sheep/lambs, 15 years ago, it wasn't just a matter of alternating treatments, but looking at pasture and grazing management water quality, monitoring (worm counts), and probably breeding but that part was out of our control at the time as we where just fattening, my point is we where looking at all aspects of what we could do to help reduce issues. I agree there is a lesson in that for beekeepers.

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