Relevance of Slide-Dip and Residual Bioassay Comparisons to Detection of Resistance in Spider Mites

Abstract

Field and laboratory studies were performed on spider mites (Acari: Tetranychidae) from San Joaquin Valley cotton fields to investigate grower claims of resistance to the miticide dicofol. Laboratory bioassays described field-collected susceptible and resistant Tetranychus urticae Koch. By a slide-dip bioassay there was a 5.7-fold resistance; by a residual bioassay, a 544-fold resistance. A 1,000-ppm discriminating concentration of dicofol, using a residual bioassay, was selected for detecting dicofol-resistant mites. Field miticide trials were performed at two locations; one contained a high proportion of spider mites capable of surviving 1,000-ppm discriminating concentrations of dicofol, and the other had no such survivors. Field dicofol efficacy was poor and excellent, respectively, at these two locations. Species identifications made throughout the growing season showed that the location where dicofol was very effective contained almost entirely T. turkestan; Ugarov and Nikolski. The location where dicofol performed poorly contained nearly all T. urticae . throughout the season. The field results verified the significance of laboratory findings; there is a strong resistance to dicofol residues in some spider mites in the San Joaquin Valley. Sole reliance on topical bioassays such as the slide-dip method may result in overlooking or underestimating field resistance problems such as the one described herein.