Susceptibility of twospotted spider mites ( Tetranychus urticae ) to abamectin in Midsouth cotton

Many applications of abamectin for control of twospotted spider mite, Tetranychus urticae (Koch), in Midsouth cotton have been made over the last ten years. Field-control failures and shortened residual control have recently been observed and may be an indication of resistance formation. The fast activity and relatively low costs of abamectin products have made repeat and consecutive applications more frequent as incidence of T. urticae infestations have increased in Midsouth cotton. Using a leaf-dip bioassay, studies were conducted to evaluate resistance levels to abamectin in 12 populations of T. urticae collected from the Midsouth. Two populations in Louisiana were highly resistant with corresponding LC50 values of 0.082 and 0.184 ppm and resistance ratios of 630 and 1415-fold. In contrast, one population from Mississippi was slightly resistant with an LC50 value of 0.0021 ppm and a resistance ratio of 11.1 compared with a susceptible control population. LC50 values for all colonies were significantly greater than the control population. These results demonstrate that variable levels of abamectin resistance exists in populations of T. urticae from Louisiana, Mississippi, Arkansas, and Tennessee. Implications from this study emphasize the importance of implementing integrated pest management and judicial use of acaricides for control of damaging populations of T. urticae. This is the first documentation of abamectin resistance in T. urticae in the Midsouth.