Migratory Routes Of The Fall Armyworm In Eastern Asia

The fall armyworm(FAW),Spodoptera frugiperda(J.E.Smith)is a major migratory pest that has recently invaded China in the end of 2018.In 2019,27 provinces of China,Japanese islands and Korean peninsula as well,were invaded by FAW,causing serious damages to crop production.It is thus urgent to identify the migration routes,timing of the seasonal movements,and seasonal distribution of FAW in eastern Asia,in order to design strategies to monitor and control this pest.Here,the migration and development processes were modelled by a trajectory analytical approach combined with degree-day model,then selected the representative areas where FAW first invaded Zhejiang Province,Japan and South Korea,clarify the source areas and landing mechanism.Information on migration pathways and timings can be used to inform integrated pest management strategies for this emerging pest.The results show as follows:1.Migratory routes and occurrence divisions of FAW in ChinaBased on the meteorological data of the past five years(2014-2018),using a trajectory simulation approach and degree-day model,migratory flight and development processes of FAW moths from winter-breeding source areas on the Indochina Peninsula and in southern China were simulated.According to the modelling results,the occurrence area of FAW in China can be divided into four regions:(i)Southern part of the South China,i.e.the overwintering area of FAW,where FAW can complete 9-12 generations in one year.This part is located roughly south of the 10℃ isotherm in January,including Hainan Island,and the southern parts of Guangdong,Guangxi,Yunnan,Fujian and Taiwan provinces.Except the local overwintering population,there also will be lots of immigrants from Indochina countries in March and April.(ii)Most areas to the south of the Yangtze River,and Yunnan and southeastern Guizhou as well,where FAW can complete 6-8 generations.Here FAW moth can appear as early as March,but most immigrants will arrive in April and May.(iii)Huang-Huai-Hai area,Sichuan Basin and most Guizhou,where FAW can complete 3-5 generations in one year.Among these regions,Huang-Huai-Hai area covers a large area to the north of Yangtze River in eastern China,including the reaches of Huai,Huang and Hai rivers.FAW migrate from the south of the Yangtze River into the area between the Yangtze and Huang rivers in April-June,and the peak of immigration appears in May and June.To the north of the Huang River,FAW arrive in July and after,and only can complete 3-4 generations in one year.(iv)Northern China area where FAW can complete one or two generations includes eastern Gansu,the northern parts of Shaanxi,Shanxi and Hebei provinces,southern Inner Mongolia,and the Northeast China.In this part,FAW mostly arrive in July and after.2.Original area of FAW newly invaded in ZhejiangZhejiang is located in the transition zone of FAW,which migrated from south to north Bridge strip.On 8 May 2019,FAW was first detected in Jiande City of Zhejiang.Using a trajectory simulation approach,combined with flight behavior of FAW and meteorological data to analyze FAW in Zhejiang.According to the larval developmental period,the effective stage for the first time the FAW population moved into Jiande City may be from 26 to 30 April.Its source area is mainly distributed in southern China(western Guangdong and eastern Guangxi),and the suitable flight altitude of FAW was 1 250-1 750 m.After taking off,it could land in Jiande City after 2-3 nights’ flight.3.Risk analysis of FAW invading Japan and Korean PeninsulaTo evaluate the risk of FAW invasion in Japan and the Korean Peninsula,we modelled the rate of expansion and future potential migratory range of the insect by a trajectory analytical approach using the flight behaviour of FAW and meteorological data of the past five years(2014-2018).If FAW can fly for up to 36 continuous hours over water,then our results predict migration from southern and eastern China into Japan and Korea.Most likely,Japan would be invaded from Fujian and Zhejiang in 1 June-15 July,and Kyushu,Shikokuand southwestern Honshu could face the highest risk of FAW invasion.Korea would most likely be reached by FAW from northern Zhejiang,Jiangsu,Anhui and Shandong in 1 June-15 July and later.Our results indicated a very high risk that FAW would annually invade Japan and the Korean Peninsula and cause a possible significant decrease in agricultural productivity.4.Source area study on the immigration of FAW invading Japan and KoreaFAW was first detected in South Korea and Japan in June 2019.Here,the migration timing and path for FAW into these two countries were identified by a trajectory simulation approach,combined with flight behavior of FAW and meteorological data.The result showed that FAW in both South Korea and Japan came from eastern China by crossing sea migration in 14-35 hours.Specifically,there were three series of FAW migrations into Japan and South Korea in June 2019.(ⅰ)FAW moths arrived Jeju Island during May 22 to 24,and most likely were from Zhejiang and Anhui after 1-2 nights’ flight.(ⅱ)The FAW moths were migrated from Jiangxi,Zhejiang,Guangdong and Fujian,and them arrived south of South Korea and the coastal areas of southwestern Japan between June 4 to 10;(ⅲ)FAW migrated from Taiwan to Enna,Japan on June 21 to 24.During these migration processes,southwesterly low-level jet(LLJ)with a speed ≥12 m/s was observed in the northwestern periphery of the western Pacific Subtropical High,and it spanned from eastern China to Japan and/or southern Korea.A large amount of downdraft airflow also appeared in immigrating areas of FAW,where were close to the 13.8℃ isotherm.Therefore,we inferred FAW migrants were carried by LLJ and landed in Japan and South Korea when encountering low temperatures and downdraft.