Resistance phenomena to amitraz from populations of the ectoparasitic mite Varroa destructor of Argentina.

In Argentina, Varroa destructor resistance to coumaphos has been previously reported. However, the status of mite susceptibility to other hard acaricides is still unknown. At present, high infestation levels of V. destructor are being detected in colonies of Apis mellifera after treatment with amitraz. The aim of the present study was to determine the LC(50) of amitraz in V. destructor from three apiaries with high mite density after treatment with the acaricide. The LC(50) values were 3.9, 3.5, and 3.7 µg/Petri dish for mites from three different apiaries. Significant LC(50) differences were detected between resistant and susceptible mites. LC(50) increased 35-39-fold when compared to the corresponding baseline, suggesting the development of resistance. These results are the first report of resistance to amitraz in V. destructor in Argentina and extend the knowledge according to the status of acaricides resistance in the country.

Morphotypes of Varroa destructor collected in Apis mellifera colonies from different geographic locations of Argentina.

Parasites display considerable phenotypic plasticity in life-history traits such as, body size. Varroa destructor is an ectoparasitic mite of the western honey bee Apis mellifera. Several studies have reported that in V. destructor, there is a wide phenotypic plasticity within a population of mites. However, it is unknown if there are morphologic variations in V. destructor populations affecting different A. mellifera populations. A morphometric study of V. destructor populations was conducted to provide information concerned to the relationships among parasite populations found in different geographic locations from A. mellifera colonies of Argentina. The hypothesis tested was different morphotypes of V. destructor populations parasitizing different A. mellifera populations from Argentina exist. A discriminant analysis employing eight morphologic variables revealed that it is possible to differentiate morphotypes of mites in Argentina. However, the level of discrimination detected among mites population varied according to the grouping of mite's population. Possible causes explaining the morphometric variability in the V. destructor populations were discussed.

First detection of Varroa destructor resistance to coumaphos in Argentina.

In Argentina, studies on Varroa destructor resistance to coumaphos are still unknown. At present, high infestation levels of V. destructor are being detected in colonies of Apis mellifera after treatment with this acaricide. The aim of the present study was to determine the LC50 of coumaphos in V. destructor from four apiaries with high mite density after treatment with coumaphos. The LC50's were 112, 319, 127 and 133 microg/Petri dish for mites from the four apiaries. Significant LC50 differences were detected between resistant and susceptible mites. LC50 increased 197-559-fold when compared to the corresponding baseline, suggesting the development of resistance. These results are the first report of resistance to coumaphos in V. destructor in Argentina.