Host-specific larval lepidopteran mortality to pathogenic Serratia mediated by poor diet

Authors: Mason, Charles; PEIFFER, MICHELLE - Pennsylvania State University; FELTON, GARY - Pennsylvania State University; HOOVER, KELLI - Pennsylvania State University

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2022
Publication Date: 8/13/2022
Citation: Mason, C.J., Peiffer, M., Felton, G.W., Hoover, K. 2022. Host-specific larval lepidopteran mortality to pathogenic Serratia mediated by poor diet. Journal of Invertebrate Pathology. 194. Article 107818. https://doi.org/10.1016/j.jip.2022.107818.
DOI: https://doi.org/10.1016/j.jip.2022.107818

Interpretive Summary: Bacteria inhabiting an insect gut are commonly benign, but some can emerge as strong pathogens when the host is compromised by dietary stress. This study investigated the role of reduced diet quality in mediating Serratia pathogens in three larvae, fall armyworm (Spodoptera frugiperda), beet armyworm (Spodoptera exigua), and corn earworm (Helicoverpa zea) (Lepidoptera:Noctuidae). Reduced diet quality increased larval mortality in the presence of Serratia isolates, but only in beet armyworm and corn earwaorm. Lowering antimicrobial concentrations in nutrient-rich diet did not yield increased mortality, suggesting effects were due to nutritional diet components. These results indicate that opportunistic gut bacterial pathogens can behave as commensal members of the community, but emerge under conditions of host distress.

Technical Abstract: Insect guts often harbor an abundance of bacteria. Many of these members are commensal, but some may emerge as opportunistic pathogens when the host is under distress. In this study, we evaluated how diet quality can mediate a shift from commensal to pathogen, and if host species influences those interactions. We used the lepidopterans (Noctuidae) fall armyworm (Spodoptera frugipderda), beet armyworm (Spodoptera exigua), and corn earworm (Helicoverpa zea) as hosts and a Serratia strain initially isolated from healthy fall armyworm. Diet quality was altered by bulk reduction in nutritional content with cellulose. Our experiments revealed that diet mediated the mortality from Serratia for beet and corn earworm, where poor quality diet caused elevated mortality to the isolate. However, for fall armyworm, little mortality was observed in any of the diet combinations. These results were not simply explained by the reduction in dietary antimicrobials, as changing concentrations of diluted and concentrated diets did not strongly alter the trends. Diet quality and inoculation did not change the expression of two antimicrobial peptides in fall and beet armyworm, suggesting that other mechanisms that mediate mortality are involved. Our results have implications in how pathogens may persist as commensals in the digestive tract of insects. These findings also suggest that diet plays a very important role in the switch from commensal to pathogen. Finally, our data indicate that the host response to changing conditions is critical in determining if a pathogen may overtake its host, and imply that the three species have differing responses to enteric pathobionts.