"Not Rice Paddies, but Irrigation Ditches"... Malaria Mosquitoes Near Ganghwa DMZ Show Habitat Shift and Multiple Insecticide Resistance [Reading Science]

Seoul National University Team Conducts Three-Year Tracking Study:
Anopheles sinensis Larvae Concentrated in Weedy Irrigation Ditches
Simultaneous Spread of kdr and ace-1 Resistance Genes Detected,
Including Discovery of New Resistance Mutations

As Plasmodium vivax malaria cases are once again increasing in areas near the Demilitarized Zone (DMZ) in Korea, scientific evidence has emerged showing that the larval habitats of malaria vector mosquitoes are changing compared to the past, and that insecticide resistance is spreading in a complex manner.

The research team led by Professor Yeo Sunjoo of the Department of Biomedical Sciences at Seoul National University announced on January 16 that, after conducting a long-term survey of Anopheles mosquitoes-the main malaria vectors-in Ganghwa County near the DMZ in Incheon from 2022 to 2024, they found that larvae of the primary vector species, Anopheles sinensis, are now predominantly inhabiting irrigation ditches surrounding rice paddies rather than inside the paddies themselves. The team also confirmed that resistance genes to both pyrethroid and organophosphate insecticides are simultaneously spreading.

Map showing the malaria incidence rate in Korea and the main collection sites of Anopheles mosquitoes investigated in this study. (A) Map of the research area in Ganghwa County, Incheon. Three major breeding sites of the malaria vector mosquito Anopheles were marked. Red dots indicate the malaria incidence rate in Korea as of 2024 (per 100,000 population), and red circles represent the collection areas. (B) Satellite images showing the natural habitats of malaria mosquitoes identified in Ganghwa County from 2022 to 2024. Red dots indicate the survey sites: Wolgot-myeon (WG, 2022), Naega-myeon (NG, 2023), Gukhwa-ri (KH, 2024), and Seonhaeng-ri (SH, 2024). Provided by the research team

This research was published in the international journal Scientific Reports on November 25, 2025.

From Rice Paddies to Irrigation Ditches: Structural Changes in Malaria Vector Mosquito Larval Habitats

Over the three-year period from 2022 to 2024, the research team collected larvae and adult mosquitoes monthly at major sites in Naega-myeon, Gukhwa-ri, and Seonhaeng-ri in Ganghwa County, and conducted comparative analyses of their habitat environments.

As a result, the 2024 survey found that larvae of Anopheles sinensis were rarely detected inside rice paddies, which had previously been considered their primary habitat. Instead, they were mainly found in irrigation ditches with weeds and shade. These irrigation ditches are located within a 100-meter radius of livestock farms and residences, providing a cool environment. The findings suggest that changes in agricultural water management are having a significant impact on the formation of larval habitats.

In contrast, Anopheles sineroides was primarily found in shallow, muddy, and weedy areas adjacent to forests, while Anopheles lindesayi was mainly found in sections of streams where clear water flows and then stagnates. This shows that the habitats of different Anopheles species are clearly differentiated.

The research team explained, "Since there has been limited systematic investigation of malaria vector mosquito larval habitats in Korea since 2007, the actual field environments have already undergone significant changes."

Spread of 'Dual Resistance' to Insecticides: Identification of New Resistance Genotypes

An even greater concern from a control perspective is the spread of insecticide resistance. The research team analyzed the kdr gene (knockdown resistance) and ace-1 gene (acetylcholinesterase-1), which are associated with resistance to pyrethroid and organophosphate insecticides, the primary chemicals used for vector control in Korea.

The results showed that most Anopheles sinensis individuals carried the ace-1 resistance allele, and the kdr gene was also found in various combinations of mutations. Notably, some individuals exhibited mutations in insecticide target genes with different mechanisms of action simultaneously, indicating that multiple insecticide resistance has already developed in malaria vector mosquito populations near the Ganghwa County DMZ.

Additionally, during the analysis of the kdr gene, the team identified a new resistance genotype that had not been previously reported in domestic studies, demonstrating that resistance evolution is currently ongoing under insecticide selection pressure.

The genetic analysis results are also noteworthy. The distribution of ace-1 genotypes in Anopheles sinensis deviated from Hardy-Weinberg equilibrium, suggesting that gene frequencies within the population are not being maintained under random mating conditions and that genetic structure may be changing due to insecticide selection pressure.

The research team stated, "The particularly high proportion of heterozygous individuals could serve as important baseline data for long-term tracking and analysis of the evolution of insecticide resistance."

Distribution of Anopheles larvae collected in Ganghwa County in 2024. Surveys were conducted in Naegamyeon, Gukhwa-ri, and Seonhaeng-ri. Yellow stars indicate livestock farms, and white circles represent larval breeding sites. Larvae of Anopheles sineroides, Anopheles lindesayi, and Anopheles sinensis were found in irrigation ditches near livestock farms. Provided by the research team

Providing 'Reference Data' for Malaria Control

The research team emphasized that this achievement is significant as a rare field-based study that simultaneously reflects the latest changes in malaria vector mosquito habitats and the spread of insecticide resistance in Korea. By empirically demonstrating that larval habitats, previously thought to be centered inside rice paddies, are shifting to irrigation ditches, the findings can serve as scientific evidence for redesigning future malaria control policies and field management strategies.

Furthermore, the detailed analysis of the distribution of kdr and ace-1 genes is highly valuable as reference data for long-term monitoring and evaluation of the speed and direction of insecticide resistance evolution in malaria vector mosquito populations in Korea. The team noted that as such genetic reference data accumulate, it could also contribute to more precise adjustment of insecticide selection and usage strategies.

The researchers further emphasized that these results could serve as a starting point for developing tailored control strategies focused on irrigation ditches, scientifically improving insecticide use policies, and establishing an integrated malaria vector mosquito management system that incorporates ecological and genetic information. Beyond short-term control outcomes, the study clearly highlights the need to shift toward an integrated malaria management strategy that considers both ecological changes in vector mosquitoes and the evolution of resistance.

This research was supported by the Ministry of Health and Welfare and the Korea Health Industry Development Institute (KHIDI) through the Global Vaccine Business Support Program, as well as by the National Research Foundation's Bio-Medical Technology Development Program and the undergraduate research program of the Seoul National University College of Medicine.