High-temperature acclimation during development modulates efficacy of emamectin benzoate against <i>Mythimna separata</i> (Lepidoptera: Noctuidae)

Global temperature increases require the reevaluation of pesticide efficacy under thermal stress conditions; however, most studies have focused on acute high-temperature exposure, neglecting the effects of natural diurnal temperature fluctuations and cumulative acclimation. This study investigated the effect of thermal acclimation during developmental on the sensitivity of Mythimna separata (Walker) (Lepidoptera: Noctuidae) to emamectin benzoate (EB). Larvae were reared under cyclic thermal regimes (25→34/38→25 °C), experiencing daily high-temperature exposures of 2, 4, or 6 hours from the egg stage. Continuous fluctuating high-temperature exposure did not have a detrimental effect on the life-history traits of M. separata but substantially increased its sensitivity to EB. Daily 6-hour exposure to either 34 °C or 38 °C significantly increased LC10 EB-induced mortality and reactive oxygen species (ROS) levels compared to constant 25 °C. Increasing thermal intensity progressively suppressed the levels of heat shock proteins (MsHsp70 and MsHsp90) and the enzymatic activities of superoxide dismutase and glutathione-S-transferase in the EB-exposed larvae. A pronounced temperature-dependent increase in cytochrome P450 monooxygenases (P450s) activity accompanied this suppression, suggesting an adaptive metabolic response that could accelerate the development of EB resistance under climate warming scenarios. Thus, thermal acclimation during insect development increases EB toxicity, potentially through ROS accumulation caused by inhibiting heat shock proteins and antioxidant enzymes. These findings demonstrate that fluctuating thermal conditions amplify EB toxicity and underscore the potential role of P450s in long-term resistance to global temperature increases. This study guides the development of climate-adaptive pesticide applications and region-specific integrated pest management strategies.