D3381: Frass forward: Sorghum’s defense modulation by fall armyworm excreta

The fall armyworm (FAW; Spodoptera frugiperda) is a globally invasive pest posing severe threats to sorghum (Sorghum bicolor), a vital monocot crop cultivated worldwide. Plants respond to herbivore attack through complex defense mechanisms, many of which are chemically mediated. Caterpillar-derived secretions contain a mixture of elicitors and effectors that can activate or suppress plant defenses. In this study, we investigated the role of FAW frass in modulating sorghum defense responses. Our findings indicate that frass application significantly upregulated sorghum defenses and influenced subsequent FAW herbivory. Notably, frass-treated plants exhibited elevated levels of key phytohormones, including jasmonic acid and abscisic acid, both of which are central regulators of plant immune signaling. Additionally, we observed a transient increase in total flavonoid content, secondary metabolites previously associated with reduced FAW performance. Beyond chewing herbivores, FAW frass treatment also impacted non-chewing insects: proliferation of specialist sugarcane aphids was notably suppressed, whereas generalist greenbugs were less affected. These results suggest that frass mediates cross-talk between sorghum’s defenses against multiple feeding guilds, potentially enhancing its resistance spectrum. Our work highlights frass as a biologically active input in plant–insect interactions and suggests its potential application in sustainable pest management strategies through induced resistance.