Morphological Trait Analysis Showed the Existence of a Migratory Ecotype in the Fall Armyworm, Spodoptera frugiperda

<i>Spodoptera frugiperda</i> (fall armyworm, FAW) has rapidly spread across Asia and Africa in recent years, with its seasonal long-distance migration ability serving as the biological basis driving its region-wide outbreaks. Although the migratory biology of FAW has been extensively studied, it remains unclear whether there is stable differentiation between migratory and non-migratory individuals. In this study, we revealed the significant differences in morphological parameters between migratory populations and laboratory-reared populations. The migratory populations exhibited a greater body length and width and forewing size, as well as a lower body weight, compared to the laboratory colony. After three generations of indoor rearing, the migrants' morphology and flight capacity converged to the laboratory phenotype, indicating the existence of a migratory ecotype in FAW. Through further investigation, a method for identifying the migratory ecotype of FAW was proposed based on the corrected wing loading (WL) and forewing aspect ratio (FA), which was successfully applied to distinguish individuals of the migratory ecotype in field populations. Our results confirm that FAWs exhibit stable differentiation into a migratory ecotype, and using WL and FA provides a robust, field-deployable tool for regional FAW monitoring, early warning systems, and targeted FAW control.