Fitness of Nutrition Regulation in a Caterpillar Pest Mythimna separata (Walker): Insights from the Geometric Framework.

Simple Summary: A critical environmental factor that affects the growth and development of insect herbivores is food quality, and insect herbivores require the correct blend and balance of different nutrients in order to meet their physiological demands. For the first time, we investigated the nutritional regulation of the oriental armyworm, Mythimna separata (walker), and the association between insect performance (i.e., developmental time, insect mass, and reproductive response) and nutrients. Caterpillars had different intake targets for two macronutrients, proteins and carbohydrates, at two larval stages, and food consumption and nutrient intake were closely correlated with the content of protein and carbohydrate, respectively. Interestingly, we also found that the trade-off between nutritional regulation and fecundity, a phenomenon often seen in this migrating species, may be attributed to food quality and, subsequently, physiological preparation. These results help us better understand the behavior of this economically important pest in the field. In nature, plants can contain variable nutrients depending upon the species, tissue, and developmental stage. Insect herbivores may regulate their nutrient intake behaviorally and physio- logically when encountering different foods. This study examined the nutritional regulation of the oriental armyworm, Mythimna separata, for the first time. In one experiment, we allowed the cater-pillars to choose between two nutritionally balanced but complementary diets. The caterpillars did not randomly consume the paired foods, but instead chose between the nutritionally balanced but complementary diets. This intake behavior was found to change with their developmental stages. Furthermore, the nutrient concentrations in food significantly impacted the insect's performance. In the other experiment, caterpillars were given one of eleven diets that reflected the different nutrient conditions in the field. The results showed that proteins were significantly associated with developmental time and fecundity. For example, by consuming protein-biased food, the caterpillars developed faster and produced more eggs. In contrast, carbohydrates were more strongly linked to lipid accumulation, and caterpillars accumulated more lipids when consuming the carbohydrate-biased food. Moreover, the caterpillars were also found to actively regulate their intake of proteins and carbohydrates based on food quality and to physiologically prepare for subsequent life stages. These findings enhance our understanding of how M. separata feeds and responds to different nutritional environments in the field, which could have implications for managing insect herbivores in agricultural settings. [ABSTRACT FROM AUTHOR]