The ectoparasitic mite Varroa destructor is potentially the greatest threat to honey bee colonies of Apis mellifera worldwide. Acaricide rotation is widely accepted to mitigate development of resistance to synthetic acaricides. When a population has developed resistance to a compound, cessation of its use can result in some degree of susceptibility. This study aimed to follow changes in the susceptibility of mites to coumaphos in a population not treated with the acaricide over many generations. The LC₅₀ increase over the years from 2 to 8-fold. Considering that coumaphos had not been used for nine years in the studied apiary, the results provided here lead to reflections about the problem of the resistance phenomenon and its populations dynamics. Varroa destructor: cuando la reversión de la Resistencia a cumafós no ocurre El ácaro ectoparásito Varroa destructor constituye una de las mayores amenazas en el mundo para las colmenas de Apis mellifera. La rotación de acaricidas es una medida ampliamente aceptada para mitigar el desarrollo de resistencia de acaricidas sintéticos. Se ha observado que, cuando se deja de aplicar un acaricida a una población resistente, luego de un periodo de tiempo la población se puede volver susceptible al mismo. El objetivo del presente trabajo fue estudiar si la susceptibilidad al cumafós de una población de Varroa destructor no expuesta a dicho acaricida durante varios años, decrece a lo largo del tiempo. Los valores de CL₅₀ se fueron incrementando a lo largo de los años, este incremento fue desde 2 a 8 veces. Considerando que el apiario estudiado no ha sido expuesto al cumafós durante nueve años, estos resultados llevan a la reflexión sobre el problema de la resistencia a acaricidas de síntesis y su dinámica en las poblaciones.

Varroa destructor is the most serious parasitic mite that infests the honey bee Apis mellifera. Different treatments with systemic acaricides are applied to control, but due to the intensive use of these chemicals, resistance to organophosphates and pyrethroids has developed worldwide. Most of the resistance episodes have been registered for fluvalinate, and only in a few cases has flumethrin resistance been reported. In Uruguay, no studies on V. destructor resistance to flumethrin have yet been recorded. High infestation levels of V. destructor are continuously detected in colonies of A. mellifera after treatments with flumethrin. Hence, this study aimed to estimate the possible resistance to flumethrin of a V. destructor population from the Colonia Department. Furthermore, the LC₅₀ baseline levels for flumethrin in two Uruguayan populations were determinated. The LC₅₀ for flumethrin for the population from Colonia Department was 3.8 μg/Petri dish, which means an increase of 34.5 fold when compared to the corresponding baseline, suggesting the development of resistance. These results are the first report of resistance to flumethrin in V. destructor in Uruguay, and extend the knowledge of acaricide resistance in the country.

Varroa destructor is a parasitic mite, which is considered a severe pest for honey bees causing serious losses to beekeeping. Residual hydrolats from steam extraction of hop essential oils, generally considered as a waste product, were tested for their potential use as acaricides on V. destructor. Four hop varieties, namely Cascade, Spalt, Victoria, and Mapuche, showed an interesting performance as feasible products to be used in the beekeeping industry. Some volatile oxidized terpenoids were found in the hydrolats, mainly β-caryophyllene oxide, β-linalool, and isogeraniol. These compounds, together with the presence of polyphenols, flavonoids, and saponins, were probably responsible for the promissory LC₅₀ values obtained for mites after hydrolat exposition. Victoria hydrolat was the most toxic for mites (LC₅₀: 16.1 µL/mL), followed by Mapuche (LC₅₀ value equal to 30.1 µL/mL), Spalt (LC₅₀ value equal to 114.3 µL/mL), and finally Cascade (LC₅₀: 117.9 µL/mL). Likewise, Spalt had the highest larval survival, followed by Victoria and Mapuche. Cascade was the variety with the highest larval mortality. In addition, none of the extracts showed mortality higher than 20% in adult bees. The Victoria hydrolat presented the best results, which makes it a good compound with the prospect of an acaricide treatment against V. destructor.

This study aimed to detect mutations associated with pyrethroid resistance mechanism in different populations of Varroa destructor from Uruguay and Argentina with different degree of susceptibility to this family of acaricides. Bioassays were performed to evaluate the degree of susceptibility to flumethrin and tau-fuvalinate; then mites were studied in order to determine the presence or absence of the resistance-associated mutations by sequencing a region of the sodium channel gene. In the mites studied from Colonia, which showed resistance to flumethrin and susceptibility to tau-fluvalinate in the bioassays, L925V mutation was observed in heterozygous and homozygous state. The presence of the wild-type allele in homozygous state was identified in all mites from Treinta y Tres, in which acaricides have never been used. The mite population from Mar del Plata through bioassays were determined as susceptible to both pyrethroids, however, genotyping results showed the presence of heterozygous individuals (925 V/L) as well as homozygous wild type (925L), while homozygous resistant individuals (925V) were present at low frequency. This would suggest that this population presents a great potential to give a generalized resistance to pyrethroids. This study represents the first report of a L925V mutation in resistant mites to flumethrin from South America.