Changes in the response time for Varroa jacobsoni exposed to amitraz

SUMMARYVarroosis is a serious parasitic disease of the honey bee caused by Varroa jacobsoni. Varroa must be treated frequently to maintain productive honey bee colonies. In France, four products have been successively used to control varroa, then later abandoned. Amitraz (formamidine), which is currently used in France and other European countries, was studied for efficacy and the possible emergence of resistant mites. We estimated efficacy by determining the ‘mean lethal time’. Mean lethal time increased significantly between a 1995 survey (24.9 ± 1.9 min) with a reference strain of the mite, and a 1998 survey with strains of mites from three regions of France (57.6 ± 3.5, 45.5 ± 3.8, 37.8 ± 3.8 min). These differences suggest that the efficacy of amitraz has decreased.