A bioassay for evaluation of the resistance of Tetranychus urticae (Acari: Tetranychidae) to selected acaricides

Chemical control of the two-spotted spider mite, Tetranychus urticae Koch, depends on early monitoring of acaricide resistance. Because few bioassay methods currently used to monitor acaricide resistance in T. urticae are easy to use or are able to detect multiple kinds of toxicity, a new bioassay is needed. A vial-leaf dipping (VLD) bioassay was developed using a 2-ml microcentrifuge tube coated with acaricide and a 1-cm-diameter leaf disc coated with acaricide. When T. truncatus females were exposed to five acaricides, LC50 values indicated that the VLD bioassay was substantially more sensitive than the conventional slide-dip (SD) bioassay. The VLD bioassay was used to obtain baseline sensitivity to 10 acaricides for adult females of T. urticae. The VLD bioassay was also used to assess the resistance of six field populations of T. urticae to 10 acaricides; the results showed that the adult females of T. urticae populations had different resistance to each acaricide. All the six populations were highly resistant to abamectin and exhibited sensitivity to medium resistance for the other acaricides. It is concluded that the VLD bioassay method is sensitive and easy to use; field chemical trials for control of T. urticae in China could be conducted by the acaricides rotation except abamectin.