DETERMINATION OF DL(50) OF AMITRAZ AND CO UMAPHOS ON VARROA-JACOBSONIOUD BY MEANS OF ANTI-VARROA(R) (SCHERING) AND PERIZIN (BAYER) ACARICIDES

The occurrence of a resistance phenomenon in Varroa jacobsoni can be s hown by comparing the LD50 of an acaricide formula over time. This wor k proposes a simple method for determining the LD50 and applies it to 2 compounds: Anti-varroa(R) (Schering) and Perizin(R) (Bayer). Tests w ere made in polycarbonate boxes which were changed each time. Temperat ure was 26-28-degrees-C, humidity 70%. Varroa mites were taken from in fested bees with a paintbrush and immobilized on a foam-rubber bed. 0. 25 ml of an acaricide solution was applied via a microsyringue. The mi tes were then replaced on the bees. Each box contained 20 infested bee s and 10 non-infested bees. Different solutions of the acaricide were tested to determine a toxicity curve and to calculate the LD50 after 2 4 h by the method of Lichtfield and Wilcoxon (Thalarida and Murry, 198 8). The mean LD50 of Anti-varroa(R) was 17.3 pg per varroa (11.7-25.6 pg per Varroa; P < 0.05). The LD50 of amitraz in the product Anti-varr oas was 2.16 pg per Varroa (1.46-3.2 pg). The mean LD50 of Perizin(R) was 98.4 pg per varroa (79.1-122.3 pg per Varroa; P< 0.05). The LD50 o f coumaphos in the product Perizin(R) was 3.15 pg per Varroa (2.53-3.9 1 pg). The experimental conditions must be accurate in order to obtain reproducible or comparable results: the area where the acaricide solu tion is deposited must be precisely defined and the reading time speci fied. Commercial products were used in this study because excipients m ay influence the toxicity of the active ingredients. This method, usin g free Varroa mites on bees is more representative of the actual field conditions than methods using isolated mites.