Shift in Biotype Composition of the Culex pipiens Complex Following the Monsoon Season in Seoul, South Korea

Background: Climate change and rapid urbanization are altering the ecological niche of the Culex pipiens complex, a primary mosquito-borne disease vector in Seoul, South Korea.Heavy rainfall during the East Asian summer monsoon can affect surface-and underground-dwelling biotypes differently, but how biotype composition differs before and after the monsoon season has only rarely been examined.Objectives: This study compared the relative abundance of Culex pipiens pallens (Cx.p. pallens) and Culex pipiens molestus (Cx.p. molestus) in Seoul between the first half (April~July, including the monsoon season) and the second half (August~November) of the study period.Methods: A total of 2,097 specimens collected from 25 administrative districts between April and November 2025 were analyzed: 828 from the first half and 1,269 from the second half of the study period.Biotyping was performed using acetylcholinesterase 2 (ACE2) gene-based PCR.Results: Analysis of the first half revealed that Cx. p. pallens (45.4%) and Cx.p. molestus (40.2%) showed comparable proportions.In the second half, Cx. p. molestus increased to 66.3% while Cx.p. pallens declined to 24.4%.Unidentified and hybrid specimens accounted for 14.1% and 0.2% in the first half, and 8.9% and 0.4% in the second half, respectively.This biotype shift was statistically significant after false discovery rate correction in nine of the 25 districts, but its magnitude varied across Seoul. Conclusions:In the second half, which falls after the monsoon season, Cx. p. molestus became the predominant biotype.This pattern is consistent with, but does not establish, the hypothesis that monsoonrelated disruption of surface breeding sites favors underground-dwelling populations.These findings suggest that biotype-aware, seasonally targeted mosquito control strategies may be warranted in urban environments.