Arbuscular Mycorrhizal Fungi Change the Growth and Metabolites of Perilla frutescens, with Subsequent Effects on the Development and Behavior of Spodoptera exigua

Arbuscular mycorrhizal fungi (AMF) influence plant-insect interactions, yet how they modulate volatile organic compounds (VOCs) to affect insect behavior remains unclear. This study integrates physiological assays, GC-MS volatile metabolomics, and insect bioassays to investigate how Funneliformis mosseae (Fm) and Rhizophagus intraradices (Ri) inoculation affects Perilla frutescens growth and its resistance to Spodoptera exigua. The results showed that Fm and Ri inoculation both promoted plant growth but differed in defense strategies. Ri inoculation increased the tannin content (16.98%) and polyphenol oxidase activity (40.52%), whereas Fm inoculation showed neutral effects. VOC profiling revealed that Ri inoculation upregulated leaf monoterpenes and phenylpropanoids, while Fm inoculation reduced sesquiterpenes (e.g., β-selinene). Insect bioassays showed that Ri inoculation inhibited the insect growth, whereas Fm inoculation promoted the insect growth and enhanced its feeding preference. These results revealed that AMF triggers a plant growth-defense trade-off, with sesquiterpenoid regulation critical for insect behavioral shifts. The findings provide a basis for sustainable pest management using AMF-plant symbiosis.