Plant Chemical Defences against Insect Pests: Roles of Secondary Metabolites and Epicuticular Lipids-A Review

Plants resist insect herbivores through interconnected structural, chemical and inducible defence mechanisms that influence insect behaviour, development and survival. This review examines the contributions of plant specialised metabolites, particularly terpenoids and tannins, and epicuticular lipids to host-plant resistance and plant–insect interactions. Terpenoids can function as feeding deterrents, toxic compounds and volatile signals that influence herbivore behaviour and the recruitment of predators and parasitoids. Tannins may reduce food palatability, interfere with digestion and affect nutrient utilisation, although their effectiveness varies with chemical structure, concentration and insect physiology. Epicuticular lipids influence insect landing, attachment, locomotion, feeding and oviposition through their chemical composition and surface microstructure. However, individual compounds may function as either deterrents or stimulants, depending on the interacting plant and insect species. The review further considers insect counter-adaptations, plant defence signalling, environmental variation and interactions with beneficial organisms. Recent developments in metabolomics, lipid profiling and functional genomics provide opportunities to identify resistance-associated traits and investigate their underlying mechanisms. Nevertheless, the combined effects of specialised metabolites and epicuticular waxes remain insufficiently established through causal experiments and field validation. Future investigations should integrate chemical characterisation, insect behavioural assays, functional validation and multi-environment assessments. Incorporating experimentally validated defensive traits into crop improvement and integrated pest management may contribute to sustainable insect pest suppression while limiting reliance on broad-spectrum insecticides.