A 2026 Outbreak of Locusta migratoria (Orthoptera: Acrididae) in Korea: Evidence of Partial Gregarization from Sex-Specific Morphometrics

Abstract

The migratory locust, Locusta migratoria, exhibits density-dependent phase polyphenism. In June 2026, a large outbreak occurred on reclaimed agricultural land in Goheung, Republic of Korea. We quantified adult morphometric phase status using Korean general-reference and laboratory-crowded populations. Six body dimensions were measured in 173 adults comprising 47 general-reference, 46 laboratory-crowded, and 80 Goheung individuals. The ratios of hind femur length to head width (F/C) and forewing length to hind femur length (E/F) showed strong discrimination between the reference groups and were used to establish sex-specific classification boundaries. Of the 80 Goheung adults, 23 (28.8%) were classified as gregarious-like, 47 (58.8%) as transitional, and 10 (12.5%) as solitarious-like. In contrast, all 28 fifth-instar nymphs examined showed conspicuous coloration associated with gregarization. These findings indicate partial morphometric gregarization in the adult population and show that gregarization-associated nymphal coloration did not correspond to uniformly gregarious adult morphology in this outbreak. This study provides the first sex-specific morphometric assessment of a Korean outbreak population of L. migratoria and a preliminary framework for monitoring adult phase composition using locally calibrated criteria.