The present study is the first to investigate resistance levels of Tetranychus cinnabarinus (Boiduval) in mulberry in southwest China. The toxicity and resistance of seven common acaricides, including dichlorvos, phoxim, methomyl, pyridaben, propargite, abamectin and chlorfenapyr, were assayed for seven mulberry field strains of mites along with a laboratory susceptible strain (SS). The results indicated that most field strains exhibited low to moderate levels of resistance. Mengzi and Xiangyun had moderate resistance to phoxim (RR = 31.45 and 26.22, respectively). The Qiaojia, Luliang, Mengzi and Heqing strains showed moderate resistance to chlorfenapyr (RR = 14.98–22.52). Correlation analysis of resistance ratio in seven acaricides indicated that methomyl and abamectin were negatively correlated. The results provide information for acaricide rotations and resistance management of spider mite in mulberry plantations.

Understanding how endogenous and exogenous factors such as annual seasonal rhythm and photoperiodism affect the toxicity of pesticides can help design integrated pest management strategies. The two-spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae), a worldwide phytophagous pest species distributed in areas with different time zones, is a good model to explore how the photoperiodism and seasonality affect the pesticide toxicity. In this study, we conducted a laboratory experiment from March 2017 to November 2018 where spider mites were reared at three photoperiod regimes in environmentally controlled incubators. The toxicities of two acaricides, propargite and diafenthiruon, were measured on T. urticae every month. To determine potential mechanisms underlying the changes in the acaricide toxicity, we measured body size and total GSTs activity with the toxicity measurements in 2018. Our results showed that the toxicities of the two acaricides were not significantly different among the three photoperiod regimes. However, both had a consistent trend along the season which increased in the spring and summer and decreased in the winter in all photoperiod regimes for two consecutive years, suggesting seasonality be an endogenous factor affecting the susceptibility of the spider mites to pesticide. Pearson’s correlation analyses showed only the body size had a weak negative correlation with the acaricide toxicities, suggesting a minor contribution to the higher toxicity from the smaller size of T. urticae . Our study is the first to show that seasonality can have an endogenous effect on the pesticide toxicity, and the results can inform practical insights into the pest management strategies.