Abstract: Intensifying climate warming has increased the frequency of extreme hydrological events and insect outbreaks, creating compound stresses that threaten terrestrial plant growth and development. Waterlogging is a major abiotic stress but can also reshape plant resistance to herbivory by altering physiological and metabolic defense pathways. In this study, seedlings of Syzygium nervosum A. Cunn. ex DC. and S. cumini (L.) Skeels were subjected to four treatments: control, waterlogging, Targalla delatrix (Guenée, 1852) feeding, and combined waterlogging and feeding. Leaf area consumed by insects, signaling molecules (JA and SA), carbohydrate composition, defense enzyme activities (SOD, POD, PAL, PPO, and LOX), and secondary metabolites (total phenolics, tannins, and total flavonoids) were quantified to assess species-specific defense responses under single and combined stresses. Results showed that waterlogging generally reduced leaf area consumed by insects in both species, indicating that flooding stress enhanced resistance to insect herbivory. Under combined waterlogging and herbivory, S. nervosum maintained strong defensive homeostasis, characterized by significantly increased JA and SA levels, enhanced defense enzyme activities, and increased accumulation of phenolics, flavonoids, and tannins, while carbohydrate contents remained relatively stable, reflecting synergistic action between constitutive and inducible defense mechanisms. In S. cumini, SA responded mainly to insect feeding, whereas JA increased under combined stress. Defense enzyme activities and phenolic accumulation also increased, but cellulose content declined, suggesting reduced mechanical defense and greater dependence on inducible chemical defense. Overall, S. nervosum exhibited stronger integrated defense capacity under combined waterlogging and herbivory, supporting its suitability as a priority species for ecological restoration in reservoir drawdown zones and riparian habitats. The chemically mediated defense strategy of S. cumini provides a useful reference for breeding stress-resistant woody plants and designing insect-resistant and stress-tolerant vegetation assemblages in tropical regions.