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  • Writer's picturePatrick Dawkins

Resistance Explained, Beekeepers Urged to Act

Despite beekeepers voicing concerns for several years, varroa’s genetic resistance to miticide treatments in New Zealand has not yet been proven. Dnature diagnostics and research technical director John Mackay’s lab has carried out resistance tests on hundreds of New Zealand varroa mites. He explains to Apiarist’s Advocate the methods used and how there is much more that beekeepers could, and should, be doing if they suspect miticide resistance.

For New Zealand beekeepers the problem of effectively managing varroa within their hives is intensifying. Virus loadings appear to have surged in bees in many parts of country this year, leading to unhealthy colonies and increased death rates. The varroa mite which vectors those viruses is almost certainly to blame, with some beekeepers expressing concern that mite resistance to flumethrin and fluvalinate, the active chemicals in common autumn treatments Bayvarol and Apistan respectively, could be to blame.

Despite these concerns, leading lab dnature has had only a handful of varroa mite samples supplied to them this season to test for resistance by concerned beekeepers, some in conjunction with treatment manufacturer Elanco (formerly Bayer). That is a problem, and it is beekeepers who need to get the ball rolling.

“If you suspect something has not worked, the treatment manufacturers are clamoring out to hear from you,” Mackay says.

While tests on New Zealand’s mites may not have identified any gene mutations which would signal treatment resistance so far, no such discoveries will be possible unless beekeepers take action when they suspect resistance.

“If you think, where have all my bees gone? Well, if you have 10 left, chuck them in a jar and put them in your freezer, then work out what you are going to do. You don’t have to send them to us or MPI, if you don’t want to, but you have options if you have the bees, and more specifically the varroa, in the freezer. If you just clean the hive out and carry on, then you have no options left,” Mackay says.

Two of the mite samples provided to dnature this year, one from the upper South Island and one from North Canterbury, saw the lab test about 50 mites for the three known gene mutations causing resistance – none had the mutations and therefore were unlikely to be resistant. Further sequencing of the particular gene region also failed to show any DNA differences that would suggest resistance.

How Does the Test Work?

Dnature has two types of resistance testing available to them. The commonly used probe test described overseas, which has identified traits of resistance in Europe and North America, costs about $4000 to carry out.

The second option is through a new method dnature developed using snapback high resolution melting analysis which required dnature to create reagents specific to the varroa mutations being tested for. It is a much cheaper, but time-consuming design process. Luckily, this is something Mackay has a passion for.

“Some people like doing sudoku, I like designing these technically challenging DNA assays. It is extremely weird, but it is what it is,” he quips.

Mackay describes the mutations which both tests are looking for as a “lock and key” situation, located in the sodium channel gene of the mite’s DNA. The chemical, be it in Bayvarol or Apistan, is the key, while the lock is a pocket in the mite’s genetic makeup to which the chemical slots in.

“The chemicals act against that sodium channel and stop nerve signals being transmitted, which paralyses and kills the mite.”

However, where resistance has been found overseas, it has been due to a change, or mutation, to the “lock” portion of the mite’s sodium channel resulting in the chemical “key” no longer fitting. Therefore, nerve signals continue to be transmitted and paralysis and death fails to occur – aka resistance.

An example of the make up of a voltage gated sodium channel in a varroa mite’s DNA, where the star marks where a miticide chemical might bind and act as the “key” to the channel’s “lock”, causing paralysis and death. Reference: González-Cabrera J. et al., 2013.

The first occurrence of resistance to flumethrin and fluvalinate was found in Europe in 2013, which was followed by two other mutations discovered in United States of America in 2015. Now all three mutations span both continents and it is these changes to the mite’s sodium channels that dnature has been testing for.

New Zealand Specific Mutations?

Scientists describe the makeup of DNA in terms of a long list of thousands of letters, and it is in one specific three letter sequence in the varroa mite’s sodium channel where the combination of these letters determines resistance.

“Normal, susceptible mites that will be killed by the chemical treatment have the make-up of CTG, but if they mutate to GTG, ATG or ATA then any of those last three will confer resistance. They are the only mutations that we know of, from Europe and America, that have a strong correlation and causation to varroa resistance to pyrethroids,” Mackay explains.

Could New Zealand’s mites be developing a mutation elsewhere though? Somewhere in the DNA chain entirely different to the mites found in North America or Europe?

“That is the main region it has been seen, not just in varroa but in other mites and ticks. That doesn’t rule out there being another mutation somewhere else, but it would be highly unlikely. We are looking at it though. We have sequenced a larger region of DNA this year, about 170 letters, to cover that whole span and we have not found any mutations,” Mackay says.

An example of the make up of a voltage gated sodium channel in a varroa mite’s DNA, where the star marks where a miticide chemical might bind and act as the “key” to the channel’s “lock”, causing paralysis and death. Reference: González-Cabrera J. et al., 2013.

Looking further requires sequencing larger regions or indeed the entire sodium channel gene, something dnature plan to do with their varroa samples, or in collaboration with other researchers.

Because New Zealand beekeepers, in comparison to North America, have been good at rotating between chemical families in their varroa treatments, mutations should be slower to develop here. This makes it more likely that the resistance tests currently used are focusing on the correct portion of the DNA sequence, Mackay believes.

“While we have a lot of bees, Europe and America still have plenty more. If all the mutations there are targeted to those three letters, it is hard to believe that New Zealand would be that special.”

What Now?

So, what can beekeepers do to try and determine varroa resistance or not? Besides industry coming up with funding to carry out a greater level of testing, there are three tasks which beekeepers should be doing if they suspect resistance to any of their varroa treatments: mite monitoring, Pettis tests, and freezing mites for potential testing.

Beekeepers who are not regularly mite monitoring to determine varroa counts are fighting the mite one handed, Mackay believes.

“No measurement, no management. If you don’t know what is going on, you have that hand tied behind your back.”

The Pettis phenotype test is also a valuable tool which Mackay says very few Kiwi beekeepers are using, as far as he knows. It involves subjecting a small amount of bees to the miticide in question in an enclosed space, and assessing the level of effectiveness, as detailed here or in the appendix of Goodwin & Taylor’s Control of Varroa book for New Zealand beekeepers.

“There is a bit of work to understand the Pettis test, but once beekeepers have it they would be away.”

Those methods of monitoring mites and treatments are useful tools, but at the end of the day beekeepers need to be taking action by working with manufactures of miticide products and at the very least taking mites and bee samples, not just talking about resistance.

“People fall into the habit of saying ‘the chemicals don’t work’. If you suspect that is the case, then Elanco want to hear from you and they will investigate. If you can’t produce any varroa, what can you expect them to do though?” Mackay says.

“Put them in the freezer and then work out what you are going to do. If beekeepers want to know what is happening, they need to help themselves a bit.”


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