Toxicological and biochemical characterizations of acetylcholinesterases in chlorpyrifos-susceptible and resistant populations of two spotted spider mite, Tetranychus urticae Koch

The enzyme acetylcholinesterase insensitivity plays a major role in organophosphate insecticides resistance in T. Urticae (Acari: Tetranychidae). In this study، toxicological and biochemical resistant mechanisms to chlorpyrifos in three populations of spider mites were surveyed. Mites were first collected in Isfahan (ISR)، Yazd (Yz) and Guilan (GUS2) provinces and identified based on mitochondrial cytochrome oxidase sequences. The bioassay results showed that resistance ratios of ISR and Yz populations to chlorpyrifos were 176. 9- and 9. 78-fold compared to the GUS2 population، respectively. Determination of acetylcholinesterase activity with different substrates showed that resistant and susceptible strains have the highest and the lowest specific activity، respectively. In addition، AChE zymogram analyses were investigated in all populations. Lower acetylcholinesterase electrophoretic mobility in ISR population and high acetylcholinesterase band intensity of ISR population in presence of chlorpyrifos-oxon showed that resistance population has altered AchE. For determining the role of acetylcholinesterase insensivity in resistant mechanism، inhibitory effect of 11 inhibitors including OPs and carbamates on this enzyme were investigated in this study. IC50 of all inhibitors on acetylcholinesterase from ISR and Yz populations were higher than GUS2 population respectively. The results suggest that some modification occurred at the active site of the enzyme that reduces the AChE affinity to substrates and subsequently led to differences in electrophoretic mobility and insensivity of enzyme to OPs and carbamate inhibitors as competitive inhibitors.