First detection of strains of Varroa destructor resistant to coumaphos. Results of laboratory tests and field trials

Abstract

The susceptibility of four populations of Varroa destructor to the organophosphorous acaricide coumaphos was studied. The mites were taken from apiaries kept in Lombardy (northern Italy) and tested with a laboratory assay on paraffin wax with known concentrations of coumaphos. The susceptibility of two populations to coumaphos was close to that of susceptible populations, indicated in the literature (LC50 at 24 hours: 12.6 μg/g); a third population showed a significant, but slight increase in tolerance (LC50 at 24 hours: 29 μg/g) while the fourth population had a much higher LC50 (>200 μg/g at 24 hours). In subsequent field trials carried out in the apiary in which the latter population had been sampled, the total average efficacy of two treatments with Perizin® (the commercial formulation of coumaphos) in colonies without capped brood was 46% (28-88%).