Effects of Trichomes and Epicuticular Wax on Nutritional Physiology, Salivary Enzymes, and Gut Bacteria of Spodoptera frugiperda Feeding on Brassica juncea

Plant mechanical defenses constitute the first line of resistance against herbivorous insects, yet their independent effects on insect physiology and gut microbiota remain poorly understood. In this study, we investigated the individual effects of leaf trichomes and epicuticular wax of Brassica juncea (L.) Czern. on Spodoptera frugiperda (Smith) (Noctuidae: Prodeniini) using integrated nutritional, biochemical, and 16S rRNA amplicon sequencing approaches. Trichomes are known to reduce feeding efficiency and nutrient assimilation, whereas epicuticular wax influences insect attachment and feeding behavior. We hypothesized that these distinct modes of action impose differential metabolic and microbial constraints on herbivores. Larvae fed on leaves with intact trichomes but removed epicuticular wax showed an increased CI, but a decrease in the ECI compared with the control (with both wax and trichomes), suggesting metabolic demands. In contrast, leaves without trichomes showed higher ECD, indicating the dominant mechanical constraint shown by trichomes compared to epicuticular wax. Moreover, increased amylase activity and glucose oxidase activity were found in larvae fed on leaves lacking trichomes and wax, respectively. Trypsin‐inhibitory activity was reduced in larvae under trichome stress, indicating adaptive responses to dietary barriers. 16S rRNA amplicon sequencing showed differences in microbial composition in larvae fed with leaves differing in the presence or absence of trichomes and/or epicuticular wax. Wax‐bearing leaves supported higher microbial diversity, whereas the absence of both defenses resulted in a less diverse microbiome. Moreover, the presence of Enterococcus across treatments confirmed its role as a core microbiome member; however, trichome ingestion may disrupt gut integrity, leading to a decline in core taxa and a simultaneous increase in opportunistic genera such as Pseudomonas . Thus, our study shows that the plant mechanical defenses influence insect performance through coordinated changes in feeding efficiency, physiology, and gut microbiota.