Analysis of the occurrence of the fall armyworm, Spodoptera frugiperda, in the winter season on the southwestern islands of Japan using the insect's strontium radiogenic isotope ratio (87Sr/86Sr)

Spodoptera frugiperda, an invasive pest insect that targets maize and other crops, first arrived in Japan in the summer of 2019. This species occurs year-round in East Asian subtropical regions such as southern mainland China and the island of Taiwan, where the mean air temperature in the coldest month is above 10°C. Adults are similarly found throughout the year on the southwestern islands of Japan. Trap monitoring there showed continuous or intermittent S. frugiperda catches in the three winter seasons since 2019. However, it was difficult to distinguish between immigrants arriving from these neighbouring areas and local individuals occurring on each Japanese island. In this study, the possible natal origin of captured insects on five small islands (Yonagunijima, Taramajima, Okinawajima, Amamioshima and Tanegashima) was determined by investigating the strontium radiogenic isotope ratios (⁸⁷Sr/⁸⁶Sr) and comparing them with those of reference hosts and insects. Since trapping data and the ⁸⁷Sr/⁸⁶Sr values of trapped insects didn't support S. frugiperda's winter breeding on the northernmost island, Tanegashima, further analysis was limited to the four southern islands. The ⁸⁷Sr/⁸⁶Sr values of reference host plants and reared insects on the four islands ranged from 0.70929 to 0.71009, while those of catch insects ranged from 0.70885 to 0.71090. The ⁸⁷Sr/⁸⁶Sr values of the catch insects and the reference on the four islands did not differ significantly. In addition, the monthly averages of daily mean air temperature in January and February 2020–2022 were above 10°C, and the wind direction at the surface was mostly from the northeast or northwest. These pieces of evidence, together with winter host availability, suggested that S. frugiperda occurs year-round on the islands. In other words, the year-round occurrence area of S. frugiperda in East Asia extends to the Japanese southwestern islands below Amamioshima Island.