Spirodiclofen resistance risk assessment in Tetranychus urticae (Acari: Tetranychidae):: a biochemical approach

Spirodiclofen belongs to a novel class of acaricides active against all developmental stages and female adults of tetranychid mite species such as the two-spotted spider mite Tctranychus urticae. Spirodiclofen showed no cross-resistance to organophosphates, METI's (mitochondrial electron transport inhibitors), hexythiazox and abamectin in well-characterised strains with high resistance to at least one of these established acaricides. A monitoring with spirodiclofen at a diagnostic concentration of 4 mg litre(-1) revealed full susceptibility of multi-resistant T urticae strains from Colombian roses. Artificial laboratory selection of a T urticae field strain from Italy resulted in a slow increase of resistance over 21 months and 37 selection cycles with spirodiclofen. The selected strain exhibited a 13-fold resistance and investigations of detoxifying enzymes in vitro revealed that the constant selection with spirodiclofen led to an increase in the activity of metabolic enzymes. Compared to the non-selected parental generation the selected strain showed a 1.2-fold significantly increased carboxylesterase activity detected with a purpose-designed spirodiclofen-specific substrate, I-naphthyl 2,2-dimethylbutyrate. Glutathione S-transferase activity with CDNB as substrate was also increased 1.2-fold, but monooxygenase activity measured by 7-ethoxy-coumarin O-deethylation was even 2.1-fold increased. Synergism and metabolism studies in T urticae in vivo supported the results of the biochemical assays, i.e., ester cleavage and oxidative detoxification seemed to be the major degradation pathway of [C-14]spirodiclofen in T urticae. The degradation is significantly increased in the spirodiclofen selected strain. (C) 2003 Elsevier Science (USA). All rights reserved.