Evaluation of a Petri Plate Assay for Assessment of Abamectin Susceptibility in Tetranychus urticae (Acari: Tetranychidae)

A petri plate (rapid) bioassay for monitoring abamectin susceptibility in twospotted spider mite, Tetranychus urticae Koch, populations was evaluated. Decision rules for classifying resistance were established by using abamectin-resistant and -susceptible populations. The assay was validated with field samples collected in California and Florida by comparing a leaf residual bioassay with the petri plate bioassay. The petri plate bioassay, using a discriminating concentration of 3 ppm, n = 100, and a decision rule to declare detection of resistance when mortality fell below 87%, can accurately identify abamectin-resistant and -susceptible populations. The petri plate assay is also a good predictor of the mortality in the leaf residual assay.