VARROOSIS - DEMONSTRATING RESISTANCE OF P ARASITES TO ACARICIDES BY MEANS OF THE DETERMINATION OF MEAN LETHAL TIME METHOD

A simple method of evaluation of Varroa jacobsoni resistance to acaric ides (amitraz, fluvalinate and acrinathrin) was developed by compariso n of lethal times with a reference strain from CNEVA Sophia Antipolis. This method reproduced the field conditions of the bee colony and kep t the host-parasite relationship. The acaricide was administered eithe r as a small piece of plastic strip (Apistan(R), Apivar(R)), or by imp regnation of a piece of Whatmann paper no 3 with a known quantity of a caricide. The strip was stuck on the bottom of a Petri dish covered wi th a net. Five bees were put in contact with the acaricide for 2 hours (fig 1). After that, five mites were introduced onto the bees. The ti me for the parasites to fall off their host was determined and compare d to controls without acaricide. According to the different varroa str ains, the lethal time varied from 6-1218 min with fluvalinate. The hig hest lethal times were registered from mites coming from apiaries wher e treatments with Apistan(R) were ineffective (table I). A significant difference was noted with acrinathrin, coincident with the degree of resistance of the mites to fluvalinate. With amitraz, no difference wa s found. This simple method, named 'determination of average lethal ti me', can be applied to other acaricides and provide information on the development of resistance through time to beekeepers.