3.6.3. Bioassays to quantify the susceptibility of the Varroa mite to acaricides

Acaricide resistance represents a dramatic problem for apiculture and has been related to widespread losses of bee colonies. To reduce the impact of such losses, a prompt detection of resistant varroa population is vital and reliable methods for testing the susceptibility of the varroa mite to different acaricides are a fundamental resource, notwithstanding the possible use in basic research on the mode of action of pesticides (Milani, 1999). There is also a need to discover new varroacidal substances. For both purposes a simple and fast bioassay is necessary. A convenient bioassay was devised by Milani (1995) for the study of acaricides that are active by contact (i.e. the active ingredient contaminates the cuticle of the bees and is taken up by the mite by indirect contact). This is the case of most acaricides used currently (e.g. pyrethroids and some organic acids). The bioassay described has been used to test the activity of several acaricides including tau-fluvalinate, flumethrin (Milani, 1995), perizin and Cekafix (Milani and Della Vedova, 1996), oxalic and citric acids (Milani, 2001), as well as for the study of reversion of resistance (Milani and Della Vedova, 2002). Other acaricides that are widely used for the control of the varroa mite are air-borne and the bioassay above is not suitable. For these cases a new bioassay developed by the honey bee laboratory of the University of Udine is presented in section