Glucosinolate - Guided Oviposition Enhances Immunity in the Specialist Herbivore, Plutella Xylostella

Insect oviposition success depends on selecting optimal host plants, guided by plant chemical cues critical for larval fitness. Yet, the specific metabolites shaping egg-laying choices remain unclear, as some enhance oviposition but inconsistently affect larval performance. Since larval success is wholly contingent on adult oviposition decisions, plant metabolites mediating both egg-laying behavior and larval fitness are pivotal to understand insect behavioral ecology and targeted pest control interventions. Using Brassica specialist, Plutella xylostella, we tested the impact of plant chemical defences on oviposition and larval fitness. We used eight varieties of Brassica plants to evaluate insect oviposition preference and subsequent larval fitness. Glucosinolates, key secondary metabolites of Brassica species influenced oviposition. Among these, 4-hydroxy-indol-3-ylmethylglucosinolate (4 H-I3M) was identified to impact oviposition, larval cellular immunity and survival against entomopathogens. Larvae reared on artificial diet containing 4 H-I3M also showed higher immunity and better survival against entomopathogens. Moreover, painting 4 H-I3M on plant cultivar lacking this compound or onto a paper disc, consistently induced oviposition behavior. This study demonstrates that (a) 4 H-I3M is both necessary and sufficient to induce oviposition (b) 4 H-I3M regulates larval cellular immunity and improves survival against entomopathogens. These results suggest that insects utilize plant compounds as proactive signals, guiding their choice of host plants to enhance larval immunity and ensure survival.