The toxicity of chlorpyrifos to three Iranian populations of two-spotted spider mite, collected from Isfahan (ISR), Yazd (Yz) and Guilan (GUS2) Provinces were surveyed using the residual contact vial bioassay. The bioassay results showed that resistance ratios of ISR and Yz populations were 176.90 and 9.78 fold compared to the GUS2 population, respectively. Determination of esterase and glutathione-S-transferase activity and their kinetic parameters showed that ISR population had the highest

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... cific activity and specificity constant among the studied populations. Besides, the content of mixed function oxidases in ISR population was the highest. However, synergistic effects of Piperonyl Butoxide, Diethyl Maleate and Triphenyl Phosphate showed that metabolic enzymes did not play an important role in resistance to chlorpyrifos in ISR and Yz populations and enhanced activity of esterase, glutathione-S-transferase and content of mixed function oxidases in these populations were probably due to resistance to some other acaricides. To determine the role of acetylcholinesterase insensitivity in resistance mechanisms, kinetic parameters and inhibitory effect of chlorpyrifos-oxon on this enzyme were investigated. The K m value of acetylcholinesterase was determined as 0.036, 0.04, and 0.050 mM using acetylthiocholine iodide for GUS2, Yz, and ISR populations, respectively. In addition, the insensitivity ratios of chlorpyrifos-oxon on acetylcholinesterase activity were estimated at 23.30 and 2.96 for ISR and Yz populations, respectively. These results confirmed amino acid substitutions in active site of this enzyme and also indicated that resistant population possed qualitatively altered AChE.