Discrepancy in Sterol Usage between Two Polyphagous Caterpillars, Mythimna separata and Spodoptera frugiperda

SIMPLE SUMMARY: Mythimna separata and Spodoptera frugiperda are two destructive pests worldwide. In this study, for the first time, we evaluated their sterol metabolic capacity. The results showed that Spodoptera frugiperda required less sterol for normal growth, which helped them survive better when cholesterol was unavailable. Both insects consistently showed high fitness when they fed on cholestanol. Cholestanone enabled most individuals of S. frugiperda to pupate but caused remarkable lethality to M. separata at their early developmental stage. Comparative studies indicated that S. frugiperda was more efficient in converting ketone into available stanol than M. separata. Therefore, they perform differently in terms of their sterol demand and metabolism although these two species are closely related. The divergences in sterol nutritional biology between the two closely related insect species reflect adaptive and evolutive changes in sterol metabolism, which may help us to better understand the potential of using phytosterol-manipulated plants to control pests. ABSTRACT: Insects are sterol auxotrophs and typically obtain sterols from food. However, the sterol demand and metabolic capacity vary greatly among species, even for closely related species. The low survival of many insects on atypical sterols, such as cholestanol and cholestanone, raises the possibility of using sterol-modified plants to control insect herbivore pests. In this study, we evaluated two devastating migratory crop pests, Mythimna separata and Spodoptera frugiperda, in response to atypical sterols and explored the reasons that caused the divergences in sterol nutritional biology between them. Contrary to M. separata, S. frugiperda had unexpectedly high survival on cholestanone, and nearly 80% of the individuals pupated. Comparative studies, including insect response to multiple diets and larval body sterol/steroids analysis, were performed to explain their differences in cholestanone usage. Our results showed that, in comparison to M. separata, the superiority of S. frugiperda on cholestanone can be attributed to its higher efficiency of converting ketone into available stanol and its lower demand for sterols, which resulted in a better survival when cholesterol was unavailable. This research will help us to better understand insect sterol nutritional biology and the potential of using atypical sterols to control herbivorous insect pests.