Thermal fitness costs and benefits of developmental acclimation in fall armyworm

Global increases in mean temperatures and changes in precipitation patterns due to climate change, coupled with anthropogenic pathways, have intensified biological invasions of pest insects. Continuous exposure to bouts of acute and chronic heat and fasting stresses (during e.g., droughts) might improve pest performance under recurring stresses, therefore enhancing/reducing fitness within- or across- life stages (i.e., ‘carry-over’ effects). Here, we examined developmental acclimation effects in the invasive fall army worm Spodoptera frugiperda — a highly invasive economic insect pest of cereal crops, particularly maize — using standardized heat tolerance metrics. Specifically, we assessed the effects of acute (3h) and chronic (3 days) heat treatments (at 32°C, 35°C, 38°C), as well as fasting (48h), on 3rd instar larvae, and tested fitness traits (critical thermal maxima [CT_max] and heat knockdown time [HKDT]) at a later life stage (4^th/5^th larval instar). Acclimation to heat stress and fasting had significant fitness costs (lower CT_max) across majority of treatments. However, both heat and fasting acclimation improved HKDT (except for 35 and 38°C [acute acclimation] and 35°C [chronic acclimation]). Our results suggest context-specific developmental acclimation costs and benefits in S. frugiperda. In particular, heat and fasting acclimation potentially have fitness costs and benefits for subsequent developmental stages facing high temperature stress. These results are important for estimating the effects of prior stressful events on future survival of invasive insect species and may be significant in predicting pest population dynamics under changing environmental conditions.