A field colony of Tetranychus urticae (Koch) (Acari: Tetranychidae) resistant to pyridaben was selected with pyridaben successively for 20 generations to produce the PR-20 strain. Resistance and multiple resistance levels of the PR-20 strain to 15 acaricides were determined using a spray bioassay. The PR-20 strain was extremely resistant to pyridaben (resistance ratio [RR] = 240]. The strain exhibited extremely strong resistance to fenpyroximate (RR = 373) and acrinathrin (RR = 329) and strong resistance to benzoximate (RR = 84). An RR = 10–40 was observed with abamectin, fenazaquin, fenbutatin oxide, fenpropathrin, and tebufenpyrad. The PR-20 strain showed low levels of resistance (RR < 10) to azocyclotin, bromopropylate, chlorfenapyr, dicofol, milbemectin, and propargite. Synergist experiments with different metabolic inhibitors revealed that piperonyl butoxide (PBO), a mixed function oxidase (MFO) inhibitor, had the greatest effect on pyridaben resistance. PBO significantly caused pyridaben resistance in the PR-20 strain to drop to the full susceptibility level of the susceptible (S) strain. However, there was no significant difference in MFO activities measured using a model substrate between the S and PR-20 strains. These results suggest that use of certain acaricides with little multiple resistance or PBO will be useful for the management of pyridaben resistance in the field.
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Vol. 99 • No. 3