The Developmental Disorders of Fall Armyworm (Spodoptera frugiperda, Lepidoptera: Noctuidae) Caused by the Infection with Nosema sp. (Microsporidia: Nosematidae)

The fall armyworm (<i>Spodoptera frugiperda</i>), a globally invasive pest, poses substantial threats to corn in China. Microsporidia are a group of obligate intracellular parasitic fungi and are considered to have great potential in biological control. In this article, we investigated the pathology of <i>Nosema</i> sp. infection in <i>S. frugiperda</i> larvae at the organismal, cellular, and molecular levels. At the organism level, this microsporidian significantly prolonged the developmental duration of the host, reduced its body weight, caused molting failure, and led to a high mortality rate at 98.9%, 97.8%, and 64.0%, respectively, in 5 × 10⁵, 5 × 10⁴, 5 × 10³ spores/larva doses. Microsporidia infection caused severe damage to midgut cells, including the formation of vacuoles in the cytoplasm, mitochondria, and intercellular spaces, destruction of goblet cells, and partial encapsulation of spores by mitochondria. Transcriptomic profiling revealed significant alterations in gene expression profiles in <i>S. frugiperda</i> larvae following microsporidian infection. The expression levels of genes associated with the chitin synthesis pathway (<i>CHS1</i>, <i>G6PI</i>, <i>GFAT</i>, <i>GNPNA</i>, <i>PAGM</i>, <i>UAP</i>) were inhibited, which may contribute to the effects of <i>Nosema</i> sp. on the growth and development of <i>S. frugiperda</i>.