Metabolic mechanisms of resistance to spirodiclofen and spiromesifen in Iranian populations of Panonychus ulmi

The European red mite, Panonychus ulmi (Koch), is one of the major pests of apple trees worldwide. Cyclic ketoenol compounds such as spimdiclofen and spiromesifen are frequently used to control phytophagous spider mites in agricultural crops, including P. ulmi on apple trees. Spider mites, however, can rapidly develop resistance against acaricides and, in this study, multiple P. ulmi populations from apple orchards in Iran were monitored for spimdiclofen and spiromesifen resistance. The Urmia and Shahin Dej population showed the highest spirodiclofen resistance ratio (more than 150-fold) compared to the susceptible Ahar population. Toxicity bioassays also revealed the presence of moderate cross-resistance between spiromesifen and spirodiclofen, but not towards the chitin synthase inhibitor etoxazole. As a first step towards elucidating spirodiclofen resistance mechanisms, the role of detoxification enzymes (cytochmme P450 monooxygenases, carboxyl/choline esterases and glutathione 5-transferases) was investigated by in vivo synergism and in vitro enzyme assays. PBO pretreatment synergized spimdiclofen toxicity in the populations of Urmia and Shahin Dej to a higher extent than in the susceptible Ahar population. Furthermore, enzyme activity measurements showed relatively higher activity of detoxifying enzymes in the resistant populations. In conclusion, increased detoxification is most likely underlying spimdiclofen resistance and results in limited cross-resistance to spimmesifen.