Practitioner-assessable Bioassay for Detection of Dicofol Resistance in Spider Mites (Acari: Tetranychidae)

Abstract

An inexpensive, practitioner-assessable (PA) bioassay procedure for estimating frequency of dicofol-resistant spider mites, Tetranychus spp., was developed in the laboratory and evaluated in San Joaquin Valley cotton fields during 1984–85. PA assays using a discriminating concentration of 56.2 ppm readily distinguished between resistant and susceptible mites. Paired comparisons of susceptibility of spider mites from 23 field locations, as depicted by the PA bioassay and a previously validated but labor-intensive whole-plant-residual bioassay, showed that the PA bioassay can be reliably used to detect fields in which frequency of dicofol resistance is above the critical frequency of 10%. An action threshold of 80% mortality (at the discriminating concentration of 56.2 ppm) is suggested for use with the PA bioassay; dicofol should not be used at locations where mortality in PA assays is <80%. The bioassay provides a tool for taking advantage of the observed instability and limited scope of dicofol resistance in spider mites of cotton.