Acaricide Efficacy and Resistance in South Carolina Tomato Populations of Twospotted Spider Mite

Twospotted spider mite, Tetranychus urticae Koch (Trombidiformes: Tetranychidae) is a key pest of vegetable crops in the southeastern US. Spider mites can cause significant yield loss in tomato due to reduced photosynthetic capacity and direct feeding damage. Use of acaricides is the primary control method, but acaricide resistance is a serious concern. We sought to characterize efficacy of acaricides registered for use on tomato by conducting 2 field trials in South Carolina in 2015 and 2016. The most effective treatments were abamectin, fenpyroximate, and cyflumetofen. Bifenazate and bifenthrin had lower efficacy than other products, and acaricide resistance was a suspected cause. Therefore, 3 spider mite populations were collected from grower fields in 2017, subjected to concentration-response screening, and compared to a known-susceptible population. Probit analysis revealed that all populations were resistant to bifenthrin at levels that would likely result in field failure. All populations were resistant to abamectin, with the LC50 of 1 population above field rate. Resistance to acequinocyl and spiromesifen also was present in all populations, but LC50 values were well below field rate. Based on our results and known non-target effects of bifenthrin and abamectin on predatory mites, growers should avoid using these products for spider mite management. Poor performance of bifenazate in the efficacy study could not be attributed to resistance, although it is possible that the populations from the efficacy study were resistant and those screened for resistance were not. Many acaricides registered in tomato appear to be effective for mite management in South Carolina.