Frass happens: Caterpillar frass manipulates plant defense responses

Caterpillar behaviors such as feeding, crawling, and oviposition are known to induce defenses in maize and other plant species. We examined plant defense responses to another important caterpillar behavior on plants, their defecation. Fall armyworm (FAW, Spodoptera frugiperda), a major threat to maize (Zea mays), is a voracious eater and deposit copious amounts of frass in the enclosed whorl tissue surrounding their feeding site where it remains for long periods of time. FAW frass is composed of molecules derived from the host plant, the insect itself and associated microbes and hence provides abundant cues that could alter plant defense responses. We observed that proteins from FAW frass initially induced wound responsive defense genes in maize; however, pathogenesis-related (pr) defense genes were induced as the time after application increased. Elicitation of pathogen defenses by frass proteins was correlated with simultaneous increase of salicylic acid and suppression of jasmonic acid accumulation in leaves, as well as increased herbivore performance and reduced fungal pathogen growth. These results indicate that the unavoidable deposition of frass in the whorl could ultimately benefit the herbivore by triggering plant defenses against pathogens.