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.