Soil‐derived, gut‐dominant generalist bacteria shape the fitness of folivorous larvae

Abstract

Whether Lepidoptera harbor a conserved core gut microbiome has long remained contentious. Through large‐scale microbiome profiling of folivorous larvae, their host plants, and associated soils across three climatically distinct regions of China, we identify two soil‐derived generalist bacteria, Ralstonia insidiosa and Delftia sp., that colonize 97.92% of larval species examined, attaining mean relative abundances exceeding 47%, with the soil microbial reservoir as their principal source. Strikingly, these two taxa exhibit strong mutual exclusion within the larval gut yet govern host development through diametrically opposed metabolic strategies: R. insidiosa promotes larval weight gain, whereas Delftia sp. suppresses growth. This functional bifurcation, in which two widespread generalists exert opposite phenotypic effects, represents a previously undescribed phenomenon in insect‐microbe symbiosis. Our findings provide broad evidence that soil microbial reservoirs can shape aboveground herbivore fitness via horizontally acquired bacteria, offering mechanistic insights for microbiome‐based ecological management.