Agenda & Program

True armyworm, Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae), is a sporadic pest of wheat, Triticum aestivum L., in Ontario, but existing action thresholds may not reflect modern cultivar tolerance, growth-stage-specific compensation, or current production costs. This study aimed to quantify relationships among larval density, foliar defoliation, yield response, insecticide performance, and economic return. Replicated field cage experiments were conducted from 2022 to 2025 in winter and spring wheat (Triticum aestivum L.) across Ontario. Cages were artificially infested with 0–32 third instar larvae per 30 cm2 at boot, flowering, or milk cereal growth stages and were treated 24 h post-infestation with chlorantraniliprole (83 mL/ha), chlorantraniliprole (83 mL/ha) and pydiflumetofen + propiconazole (1000 mL/ha) or left untreated. Defoliation incidence and severity were assessed visually over time, flag leaf defoliation was evaluated using image-based analysis, and harvest measures were taken. Data were used to model yield loss and net return. Larval density increased foliar defoliation, and chlorantraniliprole-based treatments reduced defoliation; however, relationships among density, defoliation, and yield loss were weak and inconsistent. Linear, quadratic, and exponential regression models failed to identify a stable damage function or yield-loss breakpoint. Economic models incorporating grain value and treatment costs showed that insecticide applications did not consistently improve profitability relative to the untreated control across observed densities. These findings suggest current action thresholds for TAW may overestimate economic risk, and modern thresholds should integrate crop stage, modern cultivar tolerance, and economic return to optimize insecticide use, conserve natural enemies, and strengthen integrated pest management strategies.