Histopathological and Ultrastructural Changes in Spodoptera frugiperda Larvae Following Infection with Metarhizium anisopliae

Abstract

Fall armyworm (Spodoptera frugiperda) is a highly invasive lepidopteran pest causing severe damage to cereal and vegetable crops, necessitating sustainable biological control approaches. The present study investigates histopathological and ultrastructural alterations in S. frugiperda larvae following infection with the entomopathogenic fungus Metarhizium anisopliae. Larvae were exposed to standardized conidial suspensions under laboratory conditions, and infected individuals were sampled at different time intervals for microscopic examination. Light microscopy of infected tissues revealed progressive degeneration of the midgut epithelium, disruption of muscle fibers, and disintegration of fat body tissues. Histological sections showed extensive hyphal penetration and colonization of internal organs during advanced infection stages. Ultrastructural observations using electron microscopy further confirmed severe cellular damage, including mitochondrial swelling, nuclear membrane disruption, and loss of membrane integrity in infected cells. Fungal proliferation within the hemocoel was associated with complete breakdown of tissue organization and eventual larval death. The study demonstrated that infection severity increased over time, with early stages characterized by localized gut invasion and later stages showing systemic colonization. These findings indicate that M. anisopliae exerts its pathogenicity through mechanical penetration and enzymatic degradation of host tissues, leading to irreversible physiological dysfunction. The observed histopathological and ultrastructural changes provide detailed insights into host–pathogen interactions and the mode of action of M. anisopliae against S. frugiperda. The results underscore the potential of this entomopathogenic fungus as an effective biological control agent for managing fall armyworm populations. Overall, the study contributes to a better understanding of fungal infection mechanisms and supports the development of mycoinsecticide-based integrated pest management strategies.