Seasonality but not photoperiodism affects pesticide toxicity to the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Understanding how endogenous and exogenous factors such as annual seasonal rhythm and photoperiodism affect pesticide toxicity is important to designing integrated pest management strategies. The two-spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae), a worldwide phytophagous pest species distributed across different zones with contrasting environmental conditions, is a good model for exploring how photoperiodism and seasonality affect their responses against pesticides. In this study, we conducted a laboratory experiment from March 2017 to November 2018 in which T. urticae populations were reared under three photoperiod regimes (10L:14D, 12L:12D, and 18L:6D) in environmentally controlled incubators, and assessed the toxicity of the two pesticides, diafenthiruon and propargite, on T. urticae every month. To determine potential mechanisms underlying the changes in pesticide toxicity, we measured body size and total GSTs activity of T. urticae along with the toxicity measurements in 2018. Our results showed that the photoperiod regime did not significantly affect the toxicity of each of the two pesticides to T. urticae. However, unlike photoperiodism, the toxicity of both pesticides was found to be season-dependent, being higher during spring and summer and lower during the winter. This suggests that seasonality could induce in those T. urticae populations, an endogenous mechanism that influenced their susceptibility to the tested pesticides. Pearson's correlation analyses showed that only the body size had a weak negative correlation with pesticide toxicity, thereby suggesting that pesticides could have higher toxicity on smaller T. urticae. Taken together, our study showed that seasonality can affect pesticide toxicity, thereby providing practical insights into the management strategies against T. urticae.