Rational Design of β-Ketonitrile Acaricides through Binding-Mode-Guided Isosteric Ring Replacement

β-Ketonitrile acaricides represent one of the most important acaricide classes targeting mite succinate dehydrogenase (SDH). The research of novel β-ketonitrile acaricidal derivatives is of great value for efficient management of phytophagous mites. Herein, a binding mode similarity-based isosteric ring replacement strategy was applied to the rational design of arylcarbonitrile and arylacrylonitrile target compounds containing SDHI fungicidal active ring moieties. The 3-trifluoromethyl-1H-methylpyrazole moiety containing optimal target compound A23 not only exhibited outstanding acaricidal activities against all developmental stages of Tetranychus cinnabarinus, with an LC₅₀ of 0.105 mg/L against adult populations, 0.087 mg/L against nymphs, and 0.849 mg/L against eggs, respectively, but also demonstrated a 3.2-fold decreased acute fish toxicity compared to cyenopyrafen. Mechanisms of action studies, including SDH inhibitory activity assay and computational binding mode analysis, revealed that the active metabolites of target compounds exerted inhibitory effects through conserved cation–π interaction and H-bonds within SDH. The optimal target compound showed promising improved acaricidal activity and an ecological safety profile, providing a potential candidate for the further development of β-ketonitrile acaricide and useful information for the rational design of other SDHI agrochemicals.