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

Varroa destructor is one of the major pests against honey bees. Beekeepers can rely on different approaches to try and prevent colony collapses, and the most relevant are acaricide treatments. Pyrethroid resistance was first reported in Lombardia in 1991 and is now spread worldwide. Recently, three different mutations (L925V/I/M) occurring in the voltage-gated sodium channel have been associated with tau-fluvalinate resistance. Furthermore, indirect evidence from laboratory bioassays indicated that high levels of esterases may be involved in mites resistant to tau-fluvalinate. This study provides an update of the actual spread of target-site resistance to tau-fluvalinate in varroa samples collected in the Lombardia region. TaqMan assays showed that mutation L925V is present in this area, however only low frequencies of this resistant allele were detected. The majority of resistant mites were found in the homozygous form (11%), and only a small fraction possessed the heterozygous genotype (2%). Additionally, a protocol was set up to detect esterase activity directly in single mites to determine if metabolic resistance can occur. We observed slight variability among different populations, and piperonyl butoxide showed only very low inhibition of this activity in vitro. Additional evaluation of monooxygenases activity must also be explored in the future.