Pyrethroid resistance in Italian populations of the mite Varroa destructor: a focus on the Lombardia region

Varroa destructorAnderson et Trueman (Acari Varroidae) is a major pest of honey bees and synthetic acaricide treatments re-main the most effective tool to contain its infestations. In 1991, pyrethroidresistance was first reported in Lombardy, and is now spread worldwide. Recently, three different mutations (L925V/I/M) occurring in the voltage-gated sodium channel have been as-sociated with tau-fluvalinate resistance. Furthermore, in the literature, indirect evidence from laboratory bioassays have indicated that high levels of esterases may be involved in mites resistant to tau-fluvalinate. This study provides an update on the actual spread of target-site resistance to tau-fluvalinate in V. destructorsamples collected in the Lombardyregion. TaqMan assays showed that mutation L925V is present in this area, however only low frequencies of this resistant allele were detected. The ma-jority of resistant mites were found in the homozygous form (11%), and onlya small fraction possessed the heterozygous geno-type (2%). Additionally, a protocol was set up to detect esterase activity directly in single mites. Slight variability was observed among different populations collected in Lombardy. Additional studies are needed to confirmthe involvement of esterases in re-sistance to pyrethroids in V. destructorand whether this can be correlated to changes in enzyme activity