The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), is an important agricultural pest with a strong migratory ability.  While the species is native to the Americas, it has recently invaded China, ravaging crops in many provinces (autonomous regions, municipalities).  Its long-distance migration, which is critical to understand for pest management programs, has been well studied in its native region.  In other regions however, the moth’s migration patterns have not yet been characterized.  Here, the migratory behavior of FAW populations in China was studied on an isolated island located in the center of the Bohai Strait from spring to autumn 2019, the year in which FAW first reached this region, by using searchlight trapping, stable carbon isotopes and trajectory simulation.  The main results were summarized as follows: (i) The number of FAW moths caught by searchlight trapping provided direct evidence that the species migrated across the Bohai Sea.  (ii) Species identification was confirmed by both morphology and molecular methods, and only the “corn-strain” that preferentially infests maize and sorghum was found in the collections.  (iii) Stable carbon isotope measurements showed that up to 95.07% of captured moths displayed the C4 isotope signature, thus providing additional evidence that this species is a migrant as there are no major C4 plants at the trapping site.  (iv) Backward trajectory analysis indicated that the pest now threatens to expand its range into the agriculturally important region of Northeast China, and thus authorities in this region should be vigilant to the threat posed by this species.  Together, these findings add to our knowledge concerning the occurrence of FAW in northern China, and will help us to develop sustainable and effective monitoring, forecasting, and pest management strategies.