Public health center workers fumigate for mosquitoes near Hwaseong Haenggung Palace in Paldal District, Suwon, Gyeonggi, on Aug. 25. Mosquito numbers across the country collapsed during the summer's extreme heat and have climbed back since the weather cooled.YONHAP
Mosquitoes went missing from Korea during the worst of this summer's heat. They are back, and September is now the worst month for bites.
Traps at 55 sites across the city caught 1,978 mosquitoes on Friday, the highest single-day count in more than a month, according to the Seoul Metropolitan Government's Digital Mosquito Monitoring System. The last time the number ran higher was Aug. 2, at 2,064.
Counts fell sharply from the start of August, when the extreme heat set in, and have been climbing again since. The Korea Disease Control and Prevention Agency's mosquito index for seven regions came to 100.9 for Aug. 24 to 29, nearly double the 53 of the week before.
The pattern is not a one-year fluke. Over the past five years, Seoul's traps have averaged 67,535 mosquitoes in September against 62,114 in August, roughly 9 percent more.
Heat is the reason, and there can be too much of it. Mosquitoes are most active between 25 and 30 degrees Celsius (77 and 86 degrees Fahrenheit). Push the thermometer well above that and their lifespan roughly halves and they move less.
This summer delivered exactly that. Extreme heat ran at severe-warning levels for weeks, and drought dried up the standing water mosquitoes breed in, so their habitat collapsed. Then heavy rain arrived in mid-August at the scale of an autumn monsoon, mornings and evenings turned cool, and the numbers turned with them.
“Mosquitoes are cold-blooded and struggle to survive above 35 degrees Celsius, so when temperatures came down in September the conditions became good for them,” said Kim Dong-geon, a professor at Sahmyook University who heads its Institute of Environmental Ecology. “Forest mosquito eggs that could not hatch while the water was drying up have now hatched as well, which has pushed numbers higher still.”
A family shelters under a canopy with a mosquito net tent at Busan Citizens Park in Busan and cools off with a fan and a hand fan on July 10.YONHAP
Autumn mosquitoes have a reputation for being nastier than summer ones. Autumn is the egg-laying season, and females need a blood meal to produce eggs, so they feed harder. The itch and the swelling come from the body's immune reaction to proteins in mosquito saliva, and how strongly any one person reacts varies.
The longer-term problem is that climate change is stretching the biting season into late autumn. As the weather cools, mosquitoes look for warmth and head indoors, which is where most bites now happen.
Sealing the drainage holes at the bottom of window frames, along with any other route inside, does more than anything else to keep them out. Dark clothing attracts them, so wear light colors outdoors and cover up with long sleeves and long trousers where possible. Repellent loses strength as it mixes with sweat, so reapply every three to four hours.
This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.
