Stage-specific phototactic behavior and opsin-mediated light perception in the concealed-feeding palm pest Octodonta nipae.

BACKGROUND: Negative phototaxis is a widespread but underexplored behavior in concealed-feeding insects, and its adaptive value and sensory mechanisms remain poorly understood compared with light attraction. The invasive palm pest Octodonta nipae inhabits folded leaflets and shaded microhabitats, providing a suitable system to examine how light avoidance contributes to habitat adaptation and whether opsin-mediated perception regulates stage-specific phototaxis. RESULTS: Larvae of O. nipae exhibited strong negative phototaxis (>70% selected dark chambers), whereas adults displayed positive phototaxis (~70% selected illuminated chambers). Complete darkness promoted larval body-length growth, with larvae reaching 7.75 mm at 15 days, and shortened development by ~1.7 days under leaf-bound conditions compared with the standard 12 h light:12 dark cycle, while survival remained comparable. Five opsin genes were identified, including the UV-sensitive OnRh3 and long-wavelength-sensitive OnRh6, which showed pronounced adult-biased expression and strong transcriptional responses to light regimes. RNAi-mediated silencing of either gene reversed adult positive phototaxis, with approximately 80% of silenced adults selecting dark chambers, whereas ectopic overexpression in Drosophila melanogaster larvae shifted behavior from innate light avoidance (~70% selecting dark sectors) to increased light selection (>60% selecting illuminated sectors). CONCLUSION: These findings link larval negative phototaxis to developmental advantages in concealed habitats and identify OnRh3 and OnRh6 as key mediators of opsin-dependent phototactic behavior. They further indicate that opsin-mediated light perception contributes to stage-specific behavior in this invasive pest, providing insights for future light-based monitoring studies. © 2026 Society of Chemical Industry.