Oral Exposure to Cordyceps javanica Reduces Survival and Pupation of Spodoptera frugiperda and Alters Enzyme Activities and Immune-Gene Expression

Cordyceps javanica (Bally) Kepler, B. Shrestha & Spatafora 2017 is an entomopathogenic fungus capable of controlling a wide range of insect pests, demonstrating substantial potential for biological control. To date, the effects of C. javanica on Spodoptera frugiperda (J.E. Smith, 1797) and the underlying mechanisms by which S. frugiperda responds to C. javanica infection remain largely unclear. In light of this, the present study conducted bioassays on S. frugiperda following both feeding and immersion treatments with C. javanica, investigated the effects on survival rate, pupation rate, and female proportion of S. frugiperda, evaluated the changes in protective enzyme and detoxification enzyme activities in S. frugiperda post-treatment, and examined the expression patterns of immune-related genes in S. frugiperda after treatment. The results showed that the survival rate of S. frugiperda fed with C. javanica at a concentration of 1 × 106 conidia/mL was only 66%, and the pupation rate was 65.2%. In contrast, compared with the control group, the immersion treatment did not result in significant changes in the survival rate, pupation rate, or female ratio of S. frugiperda. In response to C. javanica infection, the activities of protective enzymes (POD, CAT, and SOD) and detoxification enzymes (CarE and GST) were either enhanced or inhibited. Additionally, the expression patterns of most immune-related genes were initially down-regulated and subsequently up-regulated, with the exception of lebocin 4 and PGRP-LB under the feeding treatment. Notably, the feeding treatment induced a more intense defense response in S. frugiperda. These findings indicate that treatment with C. javanica affects the survival and metamorphosis of S. frugiperda, and that S. frugiperda defends against C. javanica infection by activating immunity. This study provides a theoretical basis for the green prevention and control of S. frugiperda.