Time-course of attainment and reversal of acclimation temperature in the invasive fall armyworm

Phenotypic plasticity is near universal in insects, and buffers against these environmental stressors. However, given the costs and benefits of plasticity, little is known about the time-course of acclimation responses, despite its ecological relevance for insects living in constantly changing environments. We tested the time taken to induce and reverse acclimation responses in fall armyworm, Spodoptera frugiperda (J.E Smith). Using third instar larvae, we acclimated fall armyworm at 38 °C (for 1, 2, 12, 24 and 48 h), and were rested (reverse-acclimated) at 28 °C (control temperature). Heat tolerance (critical thermal maxima [CTmax]) of the reverse-acclimated insects were measured at 0, 24, 48, 72, 96 and 120 h post acclimation. These were compared to the negative and positive control treatments (3rd instars kept at 28 °C and, and individuals measured at 0 h post acclimation per each duration, respectively). Preliminary results showed that acclimation had significant influence on fall armyworm CTmax, with all acclimated individuals recording higher heat tolerance at time zero post acclimation than the negative control. Individuals acclimated for 48h had overall lower heat tolerance gains (CTmax = 48.7 °C) and lost it faster (72h) than those acclimated for shorter durations (96 - 120 h). On the other hand, individuals acclimated for 1h gave maximum heat tolerance benefits (CTmax = 49.5 °C and lost heat tolerance 120h post acclimation). Our results underscore the significance of short-term plastic responses for insects’ ecological fitness and that this phenomenon may explain the pest’s success across divergent geographies in invaded regions.