Life History Traits of Mythimana loreyi (Lepidoptera: Noctuidae) on Two Transformation Events (DBN9936 and Ruifeng 125)

Mythimna (=Leucania) loreyi (Lepidoptera: Noctuidae) is an important pest of summer maize (Zea mays) in China’s Huang–Huai–Hai region. The approved transformation events DBN9936 (Cry1Ab) and Ruifeng 125 (Cry1Ab/Cry2Aj) were primarily developed to target Ostrinia furnacalis, and their efficacy against other lepidoptera pests, such as M. loreyi, remains largely unknown. Using age-stage two-sex life tables under controlled conditions, we quantified lethal and sublethal effects on these transformation events on first- to third-instar M. loreyi larvae fed V6-V8 leaves, and further evaluated how host plant movement scenarios affected larval development. Within a 7-day exposure period, both transformation events exerted significant lethal effects on M. loreyi. Corrected mortalities for first- to third-instar larvae were 93.43%, 17.33%, and 8.61% for DBN9936, and 44.33%, 8.53%, and 3.34% for Ruifeng 125, respectively. Individuals that survived on Bt maize exhibited significant sublethal effects, including prolonged larval development (ΔT = 8.12~9.93 d), reduced pupation rates (decreased by 69.41~100.00%), lower pupal mass (reduced by 31.14~33.64%), and decreased fecundity (by 177.86~262.96 eggs/female). Life table analysis revealed that both transformation events suppressed M. loreyi population growth in an instar-dependent manner. First-instar larvae failed to survive on either Bt transformation event. For second-instar larvae, DBN9936 remained completely lethal, while Ruifeng 125 allowed minimal survival (R0 = 7.51, relative fitness = 0.03). For third-instar larvae, partial survival occurred on both Bt events (DBN9936: R0 = 21.68, relative fitness = 0.09; Ruifeng 125: R0 = 32.34, relative fitness = 0.14), with rm values (0.06–0.07), lower than those on conventional controls (0.14–0.16). Host plant movement scenarios also affected M. loreyi survival and mass gain in an age-dependent manner. Brief early exposure to Bt maize did not reduce survival, but significantly suppressed body mass, whereas continuous Bt exposure caused drastic mortality in young larvae (2.08–8.33% survival). Older larvae tolerated Bt maize without survival loss, yet still exhibited significantly reduced body mass (p < 0.05). These findings demonstrate that DBN9936 and Ruifeng 125 exert substantial significant lethal and sublethal effects on M. loreyi, but the ability of older larvae to survive on Bt maize, combined with potential larval movement between Bt and non-Bt plants may facilitate resistance evolution.