Variable susceptibility in populations of Tetranychus urticae Koch (Acari: Tetranychidae) to propargite and chlorpyrifos

The two-spotted spider mite, Tetranychus urticae, is a major pest of crops and ornamental plants worldwide. It is a highly polyphagous pest that has a notorious reputation for its ability to rapidly develop resistance to commonly used pesticides. Thus, in the current study, the contact toxicity of propargite (EC ۵۷%) and chlorpyrifos (EC ۴۰.۸%) was investigated against two populations of T. urticae from Iran, Mahallat (MhR) and Karaj (KrS), using a leaf-dip bioassay. Findings showed that the LC۵۰ of propargite against MhR and KrS populations was ۵۳۳۷.۹۰ and ۱۱۶.۸۱ mg a.i/l, respectively. While the LC۵۰ of the chlorpyrifos against MhR and KrS populations was ۲۷۶۰.۸۳ and ۴۷۹.۲۵ mg a.i/l, respectively. Based on the median lethal concentrations (LC۵۰), MhR and KrS populations were considered as resistant and susceptible populations to both pesticides, respectively. MhR population was ۵.۷۶-fold and ۴۵.۷۰-fold more resistant to chlorpyrifos and propargite than the KrS population, respectively. Insecticide synergists including triphenyl phosphate (TPP), piperonyl butoxide (PBO), and diethyl maleate (DEM)) revealed the contribution of esterases, glutathione S-transferases (GST), and cytochrome P۴۵۰ monooxygenases (P۴۵۰s) to resistance. Nonetheless, the involvement of esterases and P۴۵۰s was more evident against chlorpyrifos and propargite, respectively. The activity of P۴۵۰s, GSTs, esterases, and acetylcholinesterase (AChE) was measured in susceptible and resistant populations. All enzymes showed significantly higher activity in the resistant population than in the susceptible one. Additionally, zymogram analysis of esterase showed two distinct bands in the MhR population, whereas the stronger band was absent in KrS population. These results indicate that metabolic pathways are associated with chlorpyrifos and propargite resistance in the MhR population.