Spodoptera litura (Lepidoptera: Noctuidae) Adult Abundance and Larval Host Plant Use in the Ord River Irrigation Area, Western Australia

ABSTRACT Spodoptera litura (Fabricius) is a highly polyphagous lepidopteran pest; however, little is known about its seasonal population dynamics or larval host plant use in northern Australia. Improved understanding of the ecology of S. litura is increasingly important given the anticipated expansion of Bacillus thuringiensis (Bt) cotton production in the Ord River Irrigation Area (ORIA) in north‐eastern Western Australia and the species' status as a pest of cotton. This study investigated the seasonal abundance of adult S. litura in the ORIA from May 2023 to November 2024 and used stable isotope analysis of adult tissues to infer larval host plant use, distinguishing between C3 and C4 plant sources. Despite expectations that dry‐season cultivation of Bt cotton would not coincide with peak adult S. litura activity, substantial numbers of moths were collected within the ORIA during this period. Laboratory‐reared insects that had developed exclusively on maize (C4), cotton (C3 non‐legume) or pigeon pea (C3 legume) were used to establish baseline δ 13 C and δ 15 N values for each plant species. Analysis of δ 13 C values in the wing tissue of pheromone trap‐collected S. litura adults enabled inference of larval host plant use. Stable isotope analysis revealed that most moths likely originated from C3 plants across the season with smaller numbers developing on C4 plants despite the widespread cultivation of C4 crops, such as maize and sorghum, in the region. These findings suggest that uncultivated C3 plants, including weedy species, may play an important role in sustaining S. litura populations within the ORIA. This study provides the first assessment of seasonal S. litura host plant use in northern Australia and highlights the importance of understanding landscape‐level ecology when developing sustainable pest management strategies.