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

## TL;DR

This study shows that fall armyworms have a migratory ecotype with distinct physical traits that help them fly, and these traits fade in lab conditions.

## Contribution

The study identifies a migratory ecotype in fall armyworms and proposes a field-deployable method to distinguish it using morphological traits.

## Key findings

- Migratory fall armyworms have morphological traits like larger body size and lower weight that support flight.
- These traits diminish after three generations of lab rearing, indicating a migratory ecotype.
- A method using corrected wing loading and forewing aspect ratio can identify migratory individuals in the field.

## Abstract

Determining whether species differentiation exists between migratory and non-migratory ecotypes of Spodoptera frugiperda (fall armyworm, FAW) is crucial for elucidating its migratory mechanisms and for developing effective monitoring and early-warning systems. This study systematically compared morphological traits between migratory populations and laboratory-reared strains and analyzed the trends in the morphological traits and flight capability of the migratory populations after three generations of indoor rearing. The results indicated that the migratory population of FAW exhibited more favorable morphological traits for flight compared to the laboratory colony, and these traits decayed rapidly under indoor rearing conditions over three generations. Furthermore, based on model analysis of the morphological parameters of the migratory population, a method for identifying migratory ecotype FAW was established and applied to distinguish these individuals in field populations. These findings confirm that FAWs have adopted a strategy of developing a migratory ecotype to enhance habitat adaptability, providing a theoretical basis for the development of monitoring and forecasting technologies for migratory populations.

Spodoptera frugiperda (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.