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Among various biotic stresses that tea plants face, mite attack, especially from the tea red spider mite (Oligonychus coffeae, Acarina: Tetranychidae), has been a major challenge in recent years. The difference in relative toxicity of different commonly used acaricides to O. coffeae was observed in plantations of the tea-growing region of North Bengal, India. This study indicated that the relative susceptibility values (50% lethal concentration; LC50) of O. coffeae to different acaricides varied with region. O. coffeae populations of West Terai and South Terai showed less susceptibility to all acaricides tested compared with those from the Eastern Dooars. Populations of the Central Dooars, Western Dooars, and North Terai showed intermediate levels of susceptibility to different acaricides against O. coffeae. Ethion had the lowest efficacy to the pest in all tea-growing regions in North Bengal. Effective field dosages of these acaricides were compared with LC95 values, and a significant decrease in the susceptibility of the test population to ethion and dicofal was observed. As yet, there was little change for the newly introduced acaricides like fenazaquin, fenpropathrin, and propargite, which were effective at doses lower than the recommended dose. [ABSTRACT FROM AUTHOR]