Time‐Course of Attainment and Reversal of Thermal Acclimation in the Invasive Fall Armyworm

ABSTRACT Acclimation induces physiological and/or behavioural plasticity that promotes survival to environmental stress spatio‐temporally in insects. As such, the time‐course of acclimation and reversal may shape survival in insect pests under stress. The fall armyworm, Spodoptera frugiperda , elicits plastic responses that improve survival under temperature stress. However, its time‐course for attainment and reversal of temperature acclimation is unknown. We investigated whether acclimation temperature and duration could influence changes in critical thermal limits (CTLs), that is, critical thermal maxima (CT max ) and critical thermal minima (CT min ). If affirmative, how much time would be required for the reversal of the induced changes in CTLs (plasticity)? Third‐instar fall armyworm larvae were acclimated at high (33°C, 38°C) and low (23°C, 18°C) temperatures for 15 min to 48 h before recording CT max and CT min . To assess reversal, CT max or CT min was measured from 15 min to 120 h post‐acclimation. Fall armyworm rapidly acclimated to 38°C (15 min), than (33°C) (2 h). Durable heat tolerance gains followed prolonged acclimation (1–12 h) at 38°C. Acclimation gains were reversed faster at 33°C than at 38°C (24 vs. 72 h, respectively). Overall, prolonged acclimation slowed reversal of gains. In contrast, cold acclimation did not confer any measurable benefits, underscoring the pest's resilience to high‐temperature environments. These findings demonstrate that the fall armyworm's physiological high‐temperature plasticity explains its persistence under high‐temperature stress and that low temperature may limit the pest's persistence in cold environments. Acclimation time‐courses may dictate ecological resilience by shaping stress responses, persistence, and geographical range dynamics, informing early warning and strategies.