Fall Armyworm Frass Induce Sorghum Defenses Against Insect Herbivores

The fall armyworm (FAW; Spodoptera frugiperda) is a global invasive agricultural pest. Sorghum (Sorghum bicolor), an important monocot crop cultivated worldwide, faces significant challenges from FAW, which has become a major threat to sorghum production. Plants have evolved a wide array of defense mechanisms to combat insect assault. Caterpillar secretions contain both elicitors and effectors, which can either amplify or suppress plant defenses, thereby influencing plant defense responses. In this study, we examined the role of FAW frass in modulating sorghum defenses. Our results suggest that frass application significantly induced sorghum defenses that impacted subsequent FAW herbivory. We also found that the exogenous frass application significantly elevated the phytohormone levels, specifically jasmonic acid and abscisic acid levels, potentially contributing to enhanced sorghum defense against FAW. Furthermore, FAW frass-treated plants exhibited transient increase in total flavonoids, a class of secondary metabolites, which was previously shown to have a detrimental impact on FAW growth and survival. FAW frass application on sorghum plants mitigated proliferation of specialist aphids (sugarcane aphids), though its effect on generalist aphids (greenbugs) was less pronounced. These findings highlight the role of FAW frass in mediating plant responses against both chewing and piercing-sucking insect pests, providing valuable insights into sorghum's defense mechanisms and its potential for pest management strategies.