Metarhizium rileyi is a wide spread insect fungi with a good biocontrol potentiality to various pests, particularly noctuid insects. However, it is characterized by its slow growth, its sensitivity to abiotic stress, and the slow speed of kill to pests, which hinder its use compared with other entomopathogenic fungi. In this study, the responses of M. rileyi to eight types of lipids were observed; among the lipids, oleic acid and linoleic acid significantly promoted the growth and development of M. rileyi and enhanced its stress tolerances and virulence. An additional mechanistic study demonstrated that exogenous oleic acid and linoleic acid significantly improved the conidial germination, appressorium formation, cuticle degradation, and cuticle infection, which appear to be largely dependent on the up-regulation of gene expression in growth, development, protective, and cuticle-degrading enzymes. In conclusion, exogenous oleic acid and linoleic acid enhanced the stress tolerances and virulence of M. rileyi via protecting conidial germination and promoting cuticle infection. These results provide new insights for the biopesticide development of M. rileyi.

Spodoptera frugiperda (J.E.Smith) (Lepidoptera: Noctuidae) was first found in 2019 in Yunnan, China, and it was characterized as a corn strain; it was also found on rice strains there, and it damages rice in China, but little is known about the effect of host plant transfer on the intestinal microbiota and the activities of detoxification enzymes in the C-strain (corn strain) S. frugiperda. The intestinal microbiota and the protective enzyme activity of S. frugiperda that were transferred from rice plants were assessed, and the fourth generation of insects transferred from corn were studied; the gene types of S. frugiperda that were transferred from rice plants were tested using mitochondrial Tpi gene sequences. The results showed that the intestinal microbiota in the C-strain S. frugiperda were changed after the host transference, and the diversity and richness of the intestinal bacterial communities of the S. frugiperda feeding on rice were significantly reduced after the transfer of the host from corn. The predominant species of intestinal bacteria of the S. frugiperda on rice transferred from corn were Enterococcus and Enterobacter, with relative abundances of 28.7% and 66.68%; the predominant species of intestinal bacteria of the S. frugiperda that were transferred from rice and feeding on corn were Enterococcus (22.35%) and Erysipelatoclostridium (73.92%); and the predominant species of intestinal bacteria of S. frugiperda feeding on corn was Enterococcus, with a relative abundance of 61.26%. The CAT (catalase) activity of the S. frugiperda transferred from corn onto rice from corn was reduced, the POD (peroxidase) activity was significantly increased after the transfer from corn, and no significant variations were found for the SOD (superoxide dismutase), CarE (carboxylesterase), and GST (glutathione S-transferase) activities of S. frugiperda after the host plant transfer. The results showed that after feeding on rice, the activities of CAT and POD in the in S. frugiperda body changed in order to resist plant secondary metabolites from corn or rice, but there was no significant change in the detoxification enzymes in the body. In summary, switching the host plant between corn and rice induced variations in the intestinal microbiota in C-strain S. frugiperda owing to the strain difference between the C-strain and the R-strain (rice strain), and this was consistent with the results of the activities of detoxification enzymes. The results indicat that changes in intestinal microbiota and physiological enzymes may be important reasons for the adaptive capacity of C-strain S. frugiperda to rice.

Paralipsa gularis (Zeller) has become an increasingly destructive pest in both storage and field ecosystems, particularly affecting maize crops across China. As chemical control methods face limitations due to resistance development and environmental concerns, biological control presents a promising alternative. In this study, we isolated and identified a novel strain of Metarhizium sp. from naturally infected P. gularis larvae collected in Yunnan Province, China. Morphological characterization, along with ITS-rDNA and EF-1α-rDNA sequencing, confirmed the fungus as Metarhizium rileyi. The optimal growth medium for this strain was SMAY, and the optimal conditions were 25 °C under continuous light (L:D = 24:0). Laboratory bioassays showed that the strain exhibited high virulence against P. gularis larvae, with cumulative mortality reaching 82% following infestation with 5 × 10⁸ conidia/mL. Biochemical analyses revealed that fungal infection significantly inhibited the activity of the key antioxidant enzyme SOD in the host, while activities of POD, CAT, and detoxification enzymes (P450, CarE, AChE, and GSTs) were significantly increased. These results indicate that immune responses were triggered, and systemic colonization of the host was achieved. Overall, this native M. rileyi strain demonstrates strong potential as an effective biological control agent. Its ability to overcome insect defenses and induce high mortality supports its integration into pest management programs targeting P. gularis. This work advances the understanding of fungal–insect interactions and contributes to sustainable, environmentally safe strategies for managing a pest of economic importance in agricultural ecosystems.