Abstract
A field colony of the Two‐spotted spider mite, Tetranychus urticae (Koch), resistant to fenpyroximate was further selected with fenpyroximate 5SC for 20 generations at a selection pressure of 30–50% mortality (designated as FR‐20 strain). Resistance and cross‐resistance levels of the FR‐20 strain to 18 acaricides were determined using a spray method. The FR‐20 strain was extremely resistant to fenpyroximate [resistance ratio (RR) 252]. The strain exhibited extremely strong positive cross‐resistance to acrinathrin (RR 196), and high levels of resistance to benzoximate (RR 55) and propargite (RR 64). Moderate levels of cross‐resistance (RR 11–40) to abamectin, fenbutatin oxide, fenpropathrin, pyridaben, pyridaben + bifenthrin and tebufenpyrad were observed. The FR‐20 strain showed low levels of resistance (RR < 10) to azocyclotin, bromopropylate, chlorfenapyr, chlorfenapyr + bifenthrin, chlorfenapyr + pyridaben, dicofol, fenazaquin and milbemectin. Synergist experiments with different metabolic inhibitors revealed that piperonyl butoxide had the greatest effect on the efficacy of fenpyroximate, followed by iprobenfos and triphenyl phosphate. In a comparative assay with detoxifying enzymes, the FR‐20 strain showed 2.5‐fold higher activity in p‐nitroanisole‐O‐demethylation, and 2.5‐ and 2.2‐fold higher activities in α‐ and β‐naphthyl acetate hydrolysis, respectively. These results suggested that enhanced activities of both mixed‐function oxidases and esterases likely contribute to the fenpyroximate resistance of the FR‐20 strain of T urticae. Copyright © 2004 Society of Chemical Industry