pointed out the need for regular surveys of the susceptibility of V. destructor to the acaricides used.Bioassays that make it possible to study the susceptibility of the mites to pyrethroid and organophosphorous acaricides in the 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 (LC 50 at 24 hours: 12.6 µg/g); a third population showed a significant, but slight increase in tolerance (LC 50 at 24 hours: 29 µg/g) while the fourth population had a much higher LC 50 (>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%).coumaphos / Varroa destructor / resistance / bioassay / field trial

Different methods for evaluation of the degree of Varroa destructor infestation of honey bee colonies were tested. The methods using in vivo evaluation were the most sparing for the bees but less precise. The methods using evaluation with the killing of the bees or brood were the most precise but less sparing for bees.