Strontium isotope and trajectory method elucidating overseas migration of Mythimna separata to Japan

The oriental armyworm, <i>Mythimna separata</i>, generally migrates from eastern to northeastern China in early summer, and some individuals are believed to migrate overseas to Japan depending on meteorological conditions. This potential migration was investigated with the immigrants' strontium radiogenic isotope ratio ⁸⁷Sr/⁸⁶Sr and backward flight trajectories from Japanese trapping sites. The results showed that the ⁸⁷Sr/⁸⁶Sr ratios of Chinese reared <i>M. separata</i> were significantly higher than those of reared insects of Japanese immigration areas. As some individuals trapped in western Japan had ⁸⁷Sr/⁸⁶Sr ratios higher than a statistical discriminating ratio, they likely originated in China. Trajectory analysis also indicated those individuals might have originated from the East Asian continent, such as the first-generation outbreak region in China and their migration waypoint regions. Our analysis, thus, suggests direct or multistep overseas migration of individual <i>M. separata</i> from the East Asian continent to Japan, giving insight into migration pathways and population dynamics.