Immunological responses and gut microbial shift of fall armyworm Spodoptera frugiperda challenged with entomopathogens.

Fall armyworm (FAW), Spodoptera frugiperda is a dangerous lepidopteran pest that threatens maize production globally. Biopesticides such as entomopathogenic fungi (EPF) and Bacillus thuringiensis present safer alternatives to chemical pesticides for FAW management. This study investigated the impact of Metarhizium anisopliae and B. thuringiensis on the mortality, cellular immune response, and microbiome of FAW under different temperature regimes (15 °C, 20 °C, 25 °C, 30 °C, and 35 °C). M. anisopliae isolates ICIPE 7 and ICIPE 78 caused higher FAW larval mortality than B. thuringiensis isolates Bt43 and HD263 across all temperatures. M. anisopliae infection caused a more notable reduction in total hemocyte count (THC) in FAW, in contrast with B. thuringiensis infection, compared to control FAW groups. Plasmatocytes and granulocytes dominated the differential hemocyte counts in both challenged and control FAW larvae. Microbiome analysis of entomopathogen-infected FAW showed marginal changes in the microbiome diversity, with larvae challenged with ICIPE 7 and Bt43 having higher bacterial species richness than control samples assessed on days 0 and 10 post-exposure. Hence, we detected that infection with entomopathogens compromises the immune system of FAW and that M. anisopliae isolates ICIPE 7 and ICIPE 78 would be better candidates than B. thuringiensis isolates Bt43 and HD263 for integration into an IPM program against FAW.