Multiple resistance mechanisms to abamectin in the two-spotted spider mite

The two-spotted spider mite, Tetranychus urticae, is one of the most destructive pests of various orchard trees and garden plants. Biochemical mechanisms of abamectin resistance in two T. urticae strains (PTF, 239-fold resistance; AbaR, ca. 4753-fold resistance) were investigated. The involvement of both esterase (Est) and mixed function oxidase (MFO) in abamectin resistance was suggested by synergistic bioassays, in which median lethal time (LT₅₀) values were significantly reduced by pretreatment with triphenyl phosphate (TPP) and piperonyl butoxide, respectively. Detoxification enzyme assays confirmed that Est and MFO were related to abamectin resistance as metabolic factors. Moreover, some Est bands on a native isoelectric focusing gel were specifically inhibited by TPP, implying their association with resistance. Pretreatment with verapamil in synergistic bioassays did not reduce the LT₅₀ to abamectin, suggesting that the ABC transporter is not likely involved in resistance. However, enhanced MFO and Est activities in the AbaR strain were not enough to account for the extremely high level of abamectin resistance, which suggests the involvement of additional resistance mechanisms, such as target site insensitivity.