Changes in the response time for Varroa jacobsoni exposed to amitraz

Varroosis is a serious parasitic disease of the honey bee caused by Varroa jacobsoni. Varroa must be treated frequently to maintain productive honey b ee colonies. In France, four products have been successively used to contro l varroa, then later abandoned. Amitraz (formamidine), which is currently u sed in France and other European countries, was studied for efficacy and th e possible emergence of resistant mites. We estimated efficacy by determini ng 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 ef ficacy of amitraz has decreased.