Mechanisms of Resistance to Abamectin in Two-Spotted Spider Mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Tetranychus urticae Koch is an injurious pest of, fruit crops, crop plants as well as ornamental plants worldwide. The high reproductive potential of T. urticae and its fairly short life cycle along with the frequent application of acaricides usually required to maintain T. urticae populations below economic thresholds, facilitates the pest's resistance to acaricides, and consequently control failures. In the course of the present study, the resistance mechanisms of two-spotted spider mite, Tetranychus urticae, to abamectin was surveyed. Bioassay tests were carried out through leaf dipping in a toxic solution method using formulated abamectin (25 EC). The LC50 for resistant population could not be exactly figured out due to phytotoxicity of abamectin at its high doses (>3100 ppm), with resistance ratio to abamectin being over 3000 fold. Enzyme assays revealed that esterase activities in the resistant population were 2.14 and 1.33- times higher than those in the susceptible ones where ?-naphtyl acetate (?-NA) and ?-naphtyl propionate (?-NP) were used as substrate, respectively. Estimation of kinetic parameters and the amount of glutathione S-transferase activity also indicated the significant difference between the two populations; since enzyme activity in resistant population was 1.71 folds that in susceptible population. Also Km and Vmax values of resistant population (on CDNB substrate) were 1.43 and 1.15 times lower vs. higher than those in susceptible population, respectively. Furthermore, the results showed that the level of cytochrome P450 in resistant population was 1.37 times that in susceptible population. Considering the high resistance ratio, in addition to metabolic resistance, one of the other resistnce mechanisms very likely be the target site becoming more and more insensitive.