Development and Evaluation of a Rapid Bioassay for Monitoring Propargite Resistance in Tetranychus Species (Acari: Tetranychidae) on Cotton

Cotton test plots treated with the selective acaricides propargite, avermectin B1 or left untreated showed that a Tetranychus urticae Koch population can cause loss of efficacy of propargite. In contrast, test plots infested with mixed species and resistance levels of Tetranychus species were maintained closer to economic thresholds by propargite. A rapid bioassay for detection of T. urticae and T. pacificus McGregor populations resistant to propargite was developed. Comparison of the rapid with the standard leaf bioassay method using discriminating concentrations of 1,000 and 3,162 ppm resulted in the same prediction of significant levels or frequencies of propargite resistance in 86.7 and 88.7% of the replications tested. A threshold of <70% mortality with a discriminating concentration of 1,000 ppm propargite was chosen for the rapid test to identify spider mite populations that are most likely to cause field efficacy problems. The rapid propargite bioassay was used in combination with sampling of cotton leaves to show that it can be used commercially to evaluate the severity of propargite resistance in individual fields.