Chemical Components, Emission Dynamics, and External Immune Functions of Red Palm Weevil Larval Volatiles in Response to Changes in Developmental Stages and Pathogen Stress

Chemical defenses help insects resist pathogen infection. The volatile compositions, emission patterns, and external immune functions of the red palm weevil (RPW), a major invasive pest now established in numerous countries including the Mediterranean Basin, North Africa, Middle East, and parts of Latin America and the Caribbean, are largely unknown. In this study, we examined RPW larval volatiles, analyzing their emission patterns across developmental stages and under pathogen stress from feeding <i>Metarhizium anisopliae</i>. RPW larvae shared a number of volatile components across stages, but the emission dynamics were significantly different. These volatile chemicals were primarily alcohols, phenols and aromatic hydrocarbons, and styrene was the predominant volatile. Furthermore, pathogen stress induced distinct volatile profiles: phenylacetaldehyde unidirectionally decreased, whereas <i>n</i>-nonanol, 4-ethylguaiacol, 2-phenylethanol, hexanal, and benzophenone levels increased. Antimicrobial analysis showed that these upregulated compounds broadly inhibited fungal and bacterial growth. Therefore, our results illustrated the immune defense role of RPW larval volatiles and their potential bioactive compounds, including <i>n</i>-nonanol, 4-ethylguaiacol, 2-phenylethanol, hexanal, and benzophenone.