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Simple Summary: Identifying source areas and migratory pathways is very important for monitoring and providing an early warning of migratory pests. The newly invasive pest Spodoptera frugiperda (Smith), which is a serious threat to China's food production, migrates along two migratory pathways in China: the eastern pathway and the western pathway. The Sichuan Province, located in Southwest China, is an important node along the western pathway of S. frugiperda. However, the source areas and migratory pathway of S. frugiperda in Sichuan are not clear. As a result, trajectory simulations were used to study the source areas and migratory pathways of S. frugiperda in Sichuan. The results showed that the source area of S. frugiperda in Sichuan was widely distributed. The source areas were located not only along the western national migration pathway but also along the eastern pathway. S. frugiperda migrated to Sichuan from the source areas via 6 potential pathways, 1 pathway into southwest Sichuan and 5 pathways into the Sichuan basin. This study can provide new information on the migratory pathways of S. frugiperda in China and can help with the monitoring and early warning of the presence of S. frugiperda in Sichuan and throughout all of China. The Sichuan Province, located in Southwest China, is one of China's main maize-producing areas, and is also an important node along the north-south migratory pathways that pests follow within China. After its invasion, the fall armyworm (FAW), Spodoptera frugiperda (Smith), was found in 70.81% of all counties in Sichuan. However, FAW source areas and their migratory pathways into Sichuan remain unclear. This study simulated FAW sources and their migratory pathways into Sichuan during 2020 and 2021 using the trajectory simulation platform HYSPLIT with flight behavior parameters. Additionally, the seasonal horizontal wind field was also analyzed with the meteorological graphics processing software GrADS. The results showed that sporadic FAW migration into Sichuan began in April. By May, FAWs were found in much of the Sichuan Basin and moved further north and west in June. Except for year-round breeding areas, FAW sources varied monthly and expanded northward and eastward. The source areas were concentrated in Yunnan, Guizhou, Chongqing, and Myanmar on the western pathway of national migration and also in Vietnam, Guangxi, and Hunan of the eastern pathway. At various times, parts of Sichuan have also served as sources for other parts of Sichuan. FAWs migrated to Sichuan from the source areas via 6 potential pathways, 1 pathway into southwest Sichuan and 5 pathways into the Sichuan basin. The southwestern airflow from the Bay of Bengal, the southeastern airflow controlled by the western Pacific subtropical high, and the local topographically influenced airflow could provide the airflow needed for FAW migration. This work provides new information that can assist the monitoring and warning of the presence of FAW and support integrated management strategies for this pest in Sichuan and throughout China. [ABSTRACT FROM AUTHOR]