Varroa destructor resistance to tau‐fluvalinate: relationship between in vitro phenotypic test and VGSC L925V mutation

Abstract

BACKGROUND Varroa destructor is a parasitic mite of the honey bee, Apis mellifera. Its presence in colonies can lead to a collapse within a few years. The use of acaricides has become essential to manage the hive infestation. However, the repeated and possibly incorrect use of acaricide treatments, as tau‐fluvalinate, has led to the development of resistance. The in vitro phenotypic test allows the proportion of susceptible or resistant individuals to be known following an exposure to an active substance. In Varroa mites, resistance to tau‐fluvalinate is associated with the presence of mutations at the position 925 of the voltage‐gated sodium channel (VGSC). RESULTS Here, we compared the results obtained with an in vitro phenotypic test against tau‐fluvalinate and those obtained with an allelic discrimination assay on 13 treated and untreated Varroa populations in France. The correlation between the phenotype and the genetic profile rate is found to be 0.89 Varroa mites having resistant phenotypic profile have a probability of 63% to present the L925V mutation (resistance detection reliability). However, 97% of the Varroa mites having the susceptible phenotype do not present the L925V mutation (susceptible detection reliability). CONCLUSION The L925V mutation explains most of the resistance to tau‐fluvalinate in V. destructor in the populations tested. However, other mutations or types of resistance may also be involved to explain the survival of Varroa mites in the phenotypic test. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Comparing the efficacy of synthetic Varroacides and Varroa destructor phenotypic resistance using Apiarium and Mason jar bioassay techniques

“…Research on Varroa mite resistance has conflated these terms and used them interchangeably (Table 1). For example, based on mite mortality, fluvalinate‐resistance in Varroa has been considered when mite mortality was below 70% (for 6 h), and 90% (for 24 h) 22 ; or mite mortality below 40% 23 and 80% 8 for amitraz‐resistance. Mite mortality, as it is presently calculated, indicates the ratio of dead mites to the total population regardless of the death rate in the control 66 .…”

“…9 In the absence of genetic testing, researchers and beekeepers are only able to evaluate the 'phenotypic resistance' to a Varroacide. 8,[22][23][24] Measuring phenotypic resistance will determine the rate of mortality; however, molecular analysis is required to explore and confirm existing modes of resistance in the resistant organism. Visual symptoms of resistance may also be due to changes in tolerance to a Varroacide.…”

“…36 Pettis et al 22 considered Apistan to have a high efficacy when mite mortality was >85% after a 24 h exposure period using the Mason jar method. Almecija et al 23,24 considered resistance for tau-fluvalinate and amitraz high when mortality was ≤40% using the Petri dish method; while Rinkevich 8 defined Apivar as having a low effectiveness (resistance) when mite mortality was <80%. The inconsistency in identifying the threshold for resistance presents a challenge for determining when a management intervention is required and highlights the need for standardization across methodologies, for each compound.…”

“…While Varroa may exhibit signs of resistance to a Varroacide, that resistance can be confirmed through genetic testing 9 . In the absence of genetic testing, researchers and beekeepers are only able to evaluate the ‘phenotypic resistance’ to a Varroacide 8,22–24 . Measuring phenotypic resistance will determine the rate of mortality; however, molecular analysis is required to explore and confirm existing modes of resistance in the resistant organism.…”

“…considered Apistan to have a high efficacy when mite mortality was >85% after a 24 h exposure period using the Mason jar method. Almecija et al 23,24 . considered resistance for tau ‐fluvalinate and amitraz high when mortality was ≤40% using the Petri dish method; while Rinkevich 8 defined Apivar as having a low effectiveness (resistance) when mite mortality was <80%.…”