Investigating amitraz resistance in French varroa mite (Varroa destructor) populations

In total, more than 800 varroa mites from 9 apiaries in the North-East, East and South of France were tested in the laboratory to measure their sensitivity. The mortality rate of varroa mites at LC90 was on average 92%, which indicates a good sensitivity of the samples. Only two of them showed an “intermediate” sensitivity.

After the finalization of LC90 (lethal concentration 90%) laboratory assays with varroa mites from different regions all over France, including mites from the region of Auvergne-Rhône-Alpes, 105 individual mites from the 9 apiaries were sent to a French laboratory, external to Véto-pharma, specialized in molecular biology and genotyping analysis. The objective of the genetic analyses carried out by this laboratory was to study the mutation N87S at position 260 of the β-adrenergic octopamine receptor in both categories of mites.

Of the 105 individual varroa mites selected for genotyping, 53 mites had died resulting from the exposure towards the LC90 concentration of amitraz in the laboratory, and 52 mites had survived the LC90 exposure in the laboratory. The objective of the genetic analyses was to directly compare the genetic disposition of varroa mites from each category (varroa mites that survived the laboratory test versus those that died during exposure towards amitraz).

The results show that the percentage of mites carrying the new-found mutation (heterozygous* (A/G) or homozygous* (G/G)) does not differ significantly between both groups – mites that perished during the lethal concentration 90% exposure towards amitraz, and mites that survived the exposure.

In other words, mites carrying the mutation that were exposed to amitraz in the laboratory died from the exposure – just like mites that did not carry the mutation. Other (natural) causes of death for those mites carrying the mutation that died during the exposure to amitraz in the laboratory are highly unlikely, as the mortality in this group was much higher than the “natural mortality” in a control group of mites that was not exposed to amitraz, for each population.

The only difference observed between surviving and perished mites in the laboratory trial was the percentage of homozygous vs. heterozygous mites, carrying the mutation. We found a higher percentage of heterozygous mites in the sample of mites that had survived the assay.

These results indicate that a direct association between the mutation N87S at position 260 of the β-adrenergic-like octopamine receptor alone and reduced sensitivity of varroa mites towards amitraz is unlikely.

Further investigations in the form of genotyping of French varroa mites samples from different geographic origin and with different varroa treatment history are necessary to confirm these results. As of today (July 2022), we have collected additional data (as yet unpublished) which confirm these findings.

For clarification, these results do not imply that amitraz resistance in French varroa mite populations does not exist. However, our results strongly suggest that the mechanisms leading to the expression of amitraz resistance in varroa mites are much more complex compared with tau-fluvalinate and coumaphos resistance development in varroa mite populations.

As we are currently looking into amitraz resistance in French varroa mite populations and will continue to investigate this important concern of French beekeepers, we will be able to communicate more clearly about the potential risks and recommendations associated with amitraz resistance development in France in the future.

At this point in time, the evidence collected thus far clearly suggests that:

1. Amitraz resistance is developing more slowly than tau-fluvalinate and/or coumaphos resistance and in patches rather than on a wide geographical scale,
2. The reduction of field efficacy associated with amitraz resistance in varroa mites internationally is less severe compared with tau-fluvalinate or coumaphos resistance,
3. The mechanisms causing amitraz resistance are likely more complex than merely a direct link between a single nucleotide polymorphism and the physiological manifestation of resistance,
4. Data from Hernandez-Rodriguez et al. (2021)⁵ suggest that the new-found mutation N87S, which may represent one of the necessary steps in the mechanism of amitraz resistance development in varroa mites, occurs rarely or not at all in French mite populations that have not been exposed to amitraz for only one year. This suggests a significantly shorter period of amitraz resistance-reversal compared with, for example, tau-fluvalinate, which requires a reversal period of 4-6 years until the active ingredient is effective against varroa mites in the field again.

These preliminary findings indicate a positive perspective for the continued efficient treatment of varroa mite infestations with amitraz in French beekeeping operations. It is important to keep in mind that the implementation of integrated pest management (IPM) practices – combining infestation monitoring, medical treatments and biomechanical control methods, as well as the proper use of authorized treatments in compliance with their instructions for use – are essential prerequisites for effective varroa management and the preservation of the therapeutic arsenal available today.