Carry-over effects of acute and chronic environmental stress as drivers of fitness in Spodoptera Frugiperda (J.E Smith)

Abstract

Global increases in mean temperatures and changes in precipitation patterns due to climate change, coupled with anthropogenic pathways, have intensified biological pest insects’ invasions. Continuous exposure to bouts of acute and chronic heat and fasting stresses (during e.g., droughts) might improve performance under recurring stresses, therefore enhancing/reducing fitness within- or across- life stages (i.e., within generation plasticity or carry-over effects). Here, I examined developmental acclimation effects in the invasive fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) - a highly invasive economic insect pest of cereal crops, particularly maize using standardized heat tolerance metrics. Specifically, I assessed (1) 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 [CTmax] and heat knockdown time [HKDT]) at a later life stage (4th/5th larval instar). Acclimation to heat stress and fasting had significant fitness costs (lower CTmax) 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]). The 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. Second, I tested whether FAW critical low temperature activity limits likely adaptively tracked the prevailing temporal ambient environments. Using field collected wild FAW populations, temporal changes (over two years/seasons) in larval low temperature tolerance traits were tested using standardised protocols vis critical thermal minima (CTmin), chill coma recovery time (CCRT) and supercooling points (SCP). Results varied but generally showed that FAW low temperature responses tracked environmental temperature. Winter collected FAW generally had higher cold tolerance (lower CTmin and CCRT). These results show rapid responses to changing temperature environments in FAW suggesting that both long-term (season) and short-term (conditions prevailing just before organismal collection [e.g., rapid hardening responses]) may shape FAW thermal traits and overall ecology. These results are important in informing FAW spatial adaptation and may help inform context tailored pest management strategies.