Discussing Varroa Resistance
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By John Mackay
Varroa continues to be a challenge – both nationally (varroa-attributed losses increased in the latest Colony Loss Survey) and regionally here in Gisborne/Tairawhiti. Beekeepers have struggled to keep varroa levels down here and, as such, many participated in an online meeting in late April to share experiences. Due to the weather events and cyclone destruction, treatments have often been delayed leading to hives crashing before the varroa (and their viruses) can be knocked down to safer levels.
In this region we typically use amitraz as the varroa treatment at this time of year – counter-seasonal to many other regions. Following on from Michelle Taylor’s presentation at the Apiculture New Zealand conference last year, as well as the Martin Laas/Rae Butler MiteMonitor program, information is available as many beekeepers are measuring their mite loadings during a treatment to check that numbers are reducing as expected.
If a synthetic pyrethroid (Apistan/Bayvarol) doesn’t work as expected then often the first claim is resistance – that is, the varroa now has a genetic change that means the chemical doesn’t affect it. However, often there is the similar claim that ‘I had resistance last year but it worked well this year’. Resistance is longer-lived than a single season – in the USA and Spain it has been found the genetic resistance markers remain in varroa for some years following the last pyrethroid treatment due to wax residues ‘selecting’ for resistance1.
However, possible resistance mutations in one of the varroa targets for amitraz (Apivar/Apitraz) have now been characterised, in both French and USA populations of varroa. The authors first worked out the DNA sequences that amitraz targeted in varroa – while the general targets of amitraz are known (specific receptors for cell signalling) the specific varroa sequences responsible were not previously known. Once the DNA sequences had been identified, then the authors analysed these sequences from varroa where amitraz had worked, from those where it hadn’t been successful.
The authors found two mutations in one of the three DNA sequences analysed that were associated with treatment failure. One was associated with failure in the French varroa population, and the other mutation was associated with USA populations2. Unlike the mutations associated with pyrethroid treatment (all within a very tight region on the target gene), these two mutations were in different regions of a single target gene for amitraz. This may mean that other mutations in the gene will similarly reduce the efficacy of amitraz. Further work since this paper in 2021 has found that the USA mutation is a confirmed marker of amitraz resistance3.
What do we take from it all? The benefits of communication cannot be understated – discussion among beekeepers in a region is essential. Varroa does not care if neighbouring beekeepers compete or not – hives may collapse despite individual best attempts. A wider discussion indicated that resistance was unlikely with respect to treatments and that reinvasion was the likely pressure. That said, resistance to our commonly used synthetic treatments is now confirmed – but not currently in New Zealand. Recent data from Victoria University suggests increased tolerance to pyrethroids – but the authors did not make any claims about particular treatment performance, or lack thereof.
Monitoring varroa levels is now crucial and, if you suspect resistance, keep the contents of your sugar shake/alcohol wash. Dead men tell no tales – dead varroa can tell plenty.
John Mackay is a molecular biologist and the technical director of Gisborne-based lab dnature diagnostics and logistics, as well as a hobby beekeeper.
References 1. Residual Tau-Fluvalinate in Honey Bee Colonies Is Coupled with Evidence for Selection for Varroa destructor Resistance to Pyrethroids: https://www.mdpi.com/2075-4450/12/8/731
2. Resistance to amitraz in the parasitic honey bee mite Varroa destructor is associated with mutations in the β‑adrenergic‑like octopamine receptor: https://doi.org/10.1007/s10340-021-01471-3
3. Confirmation of the Y215H mutation in the β2-octopamine receptor in Varroa destructor is associated with contemporary cases of amitraz resistance in the United States: https://onlinelibrary.wiley.com/doi/epdf/10.1002/ps.7461