[H-TT16] Development and application of environmental traceability methods

[HTT16-07] The events of simultaneously occurring overseas and domestic immigrations of a migratory moth unveiled by strontium isotope analysis

*Naoya Hidaka¹, Akira Otuka¹, Yuuki Ikenoue², Ki-Cheol Shin³ (1.National Agriculture and Food Research Organization, 2.Kagoshima Prefectural Institute for Agricultural Development, 3.Research Institute for Humanity and Nature)

Keywords:invasive pest insect, Storontium isotope, Backward trajectory analysis

Some insects including agricultural pests migrate for long distances using air currents caused by low pressure systems and fronts. The fall armyworm, Spodoptera frugiperda, is a migratory insect pest native to the Americas. Its larvae are polyphagous and feed on many crops, mainly Poaceae species including maize, sugarcane, rice, sorghum, pasture grasses, and vegetables. Since 2016, this species has rapidly expanded its distribution from Africa to India, southern China, Taiwan, and South Korea, and arrived in Japan in July 2019. This rapid expansion was achieved primarily by its strong migratory characteristics and favorable summer monsoons in this region. Since this species cannot overwinter in the mainland Kyushu and northern regions, it immigrates mainly from overseas every spring and early summer. At the same time, since populations on southwestern islands in Okinawa and Kagoshima prefectures occur year-round, a spring generation may migrate northeastward to the mainland Japan. However, it is unknown whether such a domestic migration in the early season exists. Therefore, we performed an isotopic analysis using strontium radiogenic isotope ratios (⁸⁷Sr/⁸⁶Sr) to investigate the natal origin of trap catches, and backward trajectory analysis to estimate the migration route.
To identify the natal origin of trapped insects, reference ⁸⁷Sr/⁸⁶Sr values of host plants (maize, sugarcane, wheat, and Italian ryegrass) and reared insects fed on them in various prefectures were measured. In addition, from 2020 to 2022, wild adults were collected with pheromone traps at various sites in Japan. The ⁸⁷Sr/⁸⁶Sr values of the wild insects were measured and compared with the reference. As a result, an individual caught on 25 May 2021 in Nagakute city, Aichi prefecture (around 0.70900) indicated its ⁸⁷Sr/⁸⁶Sr value of 0.70714, and an insect caught on 8 June 2021 in Kameoka city, Kyoto prefecture (around 0.71000) indicated 0.70581, showing lower ⁸⁷Sr/⁸⁶Sr values than that of the references. Backward trajectory analysis found air currents from Kagoshima prefecture before the catches, suggesting that the individuals might have arrived from the region where low ⁸⁷Sr/⁸⁶Sr values are expected. Furthermore, in Minami-Satsuma city, Kagoshima prefecture, many individuals with ⁸⁷Sr/⁸⁶Sr values (>0.70900) higher than that of the reference of the prefecture’s mainland, around 0.70750, were found in early spring (March to April), while several catch individuals showed lower ⁸⁷Sr/⁸⁶Sr values (0.70600-0.70900). In addition, air currents from southern China, Taiwan as well as the Japanese southwestern islands were found for these catches. Since the reference ⁸⁷Sr/⁸⁶Sr values of islands of Okinawa prefecture and Amamioshima Island and the bioavailable Sr isoscape for southern China and Taiwan falls in the above higher range, the results suggested that the insects of the high values might be immigrants from these areas. Whereas the low-value individuals might have immigrated from the Tokara Islands and Tanegashima Island where the ⁸⁷Sr/⁸⁶Sr value of the Sr isoscape indicates low. These results suggested that not only the overseas arrivals but also the domestic dispersal of this species might have occurred as a series of the events.
This research was supported by the research program on development of innovative technology grants (JPJ007097) from the Project of the Bio-oriented Technology Research Advancement Institution.