Resistance mechanisms to oxydemeton-methyl in Tetranychus urticae Koch (Acari: Tetranychidae)

The resistance mechanisms to oxydemeton-methyl were surveyed in two Iranian strains of the two spotted spider mite, Tetranychus urticae Koch. Bioassay was carried out on two strains, collected from Tehran and Rasht using dipping method. The results of bioassay indicated that resistance ratio was 20.47 for resistant strain. The activity of esterase and glutathione S-transferase in resistant and susceptible strains showed that one of resistance mechanisms to oxydemeton-methyl was esterase-based resistance and glutathione S-transferase. The esterase activity of the resistant strain was 2.5 and 2.14-fold higher than those of the susceptible strain for α-naphtyl acetate (α-NA) and β-naphtyl acetate (β-NA) respectively. The kinetic characteristics acetylcholinesterase (AChE) showed that the AChE of resistant strain had lower affinity to artificial substrates; acetylthiocholine and butyrylthiocholine than that of susceptible strain. I50 of oxydemeton-methyl for resistant and susceptible strains were 2.68×10-6 M and 7.79×10-7 M respectively. The results suggested that AChE of resistant strain is insensitive to oxydemeton-methyl and ratio of AChE insensitivity of resistant to susceptible strain were 3.49 and 7.8-fold to oxydemeton-methyl and paraoxon, respectively