Varroosis: Demonstrating resistance of parasites to acaricides by means of the 'determination of mean lethal time' method

A simple method of evaluation of Varroa jacobsoni resistance to acaricides (amitraz, fluvalinate and acrinathrin) was developed by comparison of lethal times with a reference strain from CNEVA Sophia Antipolis. This method reproduced the field conditions of the bee colony and kept the host-parasite relationship. The acaricide was administered either as a small piece of plastic strip (Apistan(R), Apivar(R)), or by impregnation of a piece of Whatmann paper no 3 with a known quantity of acaricide. The strip was stuck on the bottom of a Petri dish covered with 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 time for the parasites to fall off their host was determined and compared to controls without acaricide. According to the different varroa strains, the lethal time varied from 6-1218 min with fluvalinate. The highest lethal times were registered from mites coming from apiaries where 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 was found. This simple method, named 'determination of average lethal time', can be applied to other acaricides and provide information on the development of resistance through time to beekeepers.