The Effects of Diet on the Immune Responses of the Oriental Armyworm Mythimna separata

SIMPLE SUMMARY: It is known that food nutrients affect hosts immune responses in response to infection. However, the results found in the literature show that the effect of the quantity and quality of foods on immune responses differ across insects. In the current study, we revealed that high protein-to-carbohydrate ratio diets have different impacts on phenoloxidase-mediated melanization and antimicrobial peptide production, but no effects on cellular immune responses in the oriental armyworm, Mythimna separata Walker. ABSTRACT: Nutrients can greatly affect host immune defenses against infection. Possessing a simple immune system, insects have been widely used as models to address the relationships between nutrition and immunity. The effects of high versus low protein-to-carbohydrate ratio (P:C) diets on insect immune responses vary in different studies. To reveal the dietary manipulation of immune responses in the polyphagous agricultural pest oriental armyworm, we examined immune gene expression, phenoloxidase (PO) activity, and phagocytosis to investigate the immune traits of bacteria-challenged oriental armyworms, which were fed different P:C ratio diets. We found the oriental armyworms that were fed a 35:7 (P:C) diet showed higher phenoloxidase (PO) activity and stronger melanization, and those reared on a 28:14 (P:C) diet showed higher antimicrobial activity. However, different P:C diets had no apparent effect on the hemocyte number and phagocytosis. These results overall indicate that high P:C diets differently optimize humoral immune defense responses in oriental armyworms, i.e., PO-mediated melanization and antimicrobial peptide synthesis in response to bacteria challenge.