'Don't open the windows!' Mosquitoes surge as autumn arrives — and disease risks rise with them

"Barely saw them all summer — and now they're everywhere."

September has arrived with a welcome break from a record-shattering heat wave, bringing cooler temperatures and the feel of early autumn. But the season has also brought an unwelcome guest.

Mosquitoes are back. While many Koreans spent August losing sleep to the oppressive heat, a growing number now find themselves kept awake by the familiar high-pitched whine buzzing past their ears the moment the lights go out.

Data show that as the heat broke, domestic mosquito activity nearly doubled almost overnight.

The bigger concern is that this year's mosquito surge is only just beginning. Climate change is transforming autumn into an ideal breeding environment for the insects.

Milder winters are extending their active season further into the year, widening the window during which mosquitoes can transmit life-threatening infectious diseases.

If winters continue to warm as climate projections suggest, experts warn that mosquito-borne illness could become a year-round problem — interrupted only during the most extreme summer heat.

According to the Seoul Metropolitan Government, the city's mosquito activity index stood at 44.3 on Tuesday — up roughly 10 points from 33.4 recorded just two days earlier on Sunday. The reading pushed the mosquito alert to Level 2 "Attention (High)," at which point residents are advised to use window screens and avoid opening windows late at night for ventilation.

The index had climbed even higher the previous Wednesday and Thursday, surpassing 50 and reaching Level 3 "Caution." At that level, authorities recommend refraining from opening doors and windows for ventilation altogether.

Having largely disappeared during the peak summer heat, mosquitoes are surging back now that temperatures have eased. Real-time surveillance data from the Korea Disease Control and Prevention Agency's AI-based monitoring system make the shift even more striking.

During the week of Aug. 17–23, the average total mosquito index stood at 53 individuals. The following week it jumped to 100.9 — a near-doubling of almost 48 individuals in just seven days. The index remained elevated at 104.7 during the week of Aug. 30 to Sept. 5.

Mosquitoes have long been considered a quintessential summer insect that thrives in hot weather. But a clear trend is emerging: their activity now peaks in spring and autumn rather than summer. Summers have grown so hot that even mosquitoes struggle to cope, while spring and autumn temperatures have become increasingly mild and hospitable.

According to the Korea Meteorological Administration, the national average temperature this summer was 25.4 degrees Celsius — 1.7 degrees above the historical norm and the third highest on record since a nationwide weather observation network was established in 1973. The past three years have ranked first, second and third on that all-time list, marking three consecutive summers of record-breaking heat.

Mosquitoes grow and reproduce more actively in warm conditions, but hotter is not always better for them. They are generally most active at temperatures in the upper 20s Celsius, and extreme heat can suppress both their activity and reproduction — though the threshold varies by species and environment.

That does not mean fewer mosquitoes overall — it means their range is expanding. According to a spring climate report the Korea Meteorological Administration released in June, the national average temperature this spring was 13.3 degrees Celsius, a full 1.4 degrees above the historical norm of 11.9 degrees and the second highest on record since 1973.

The trend has grown more pronounced of late. Seven of the past 10 years rank among the 10 warmest springs ever recorded. Last year's autumn told a similar story: the national average temperature was 16.1 degrees Celsius, 2 degrees above the norm and the second warmest autumn on record.

The result is a lengthening window of mosquito-friendly weather — arriving earlier in spring and lingering later into autumn with each passing year.

The greatest concern goes well beyond lost sleep. The longer mosquitoes remain active, the more time they have to transmit pathogens to humans. The WHO has noted that changes in temperature and rainfall are expanding both the seasons and the geographic areas in which disease-carrying vectors can operate.

The threat is already materializing. According to data the KDCA released Wednesday, South Korea confirmed its first domestic Japanese encephalitis patient this year — a woman in her 60s who visited a medical facility on Aug. 15 with symptoms including high fever, chills and vomiting. Last year's first case was confirmed on Oct. 14, meaning this year's case arrived about five weeks earlier.

This is not an isolated development. Over the past decade, as the national average temperature rose by about 1.4 degrees Celsius, the date on which the Japanese encephalitis advisory is issued moved roughly 16 days earlier. The active season for disease vectors has been expanding steadily, stretching from spring through late autumn.

The coming weeks are particularly dangerous. The KDCA says most domestic Japanese encephalitis cases occur between August and November, with about 80 percent concentrated in September and October. An extended mosquito season like this year's raises the risk of other mosquito-borne diseases as well, including dengue fever, malaria and Japanese encephalitis.

According to the Korea Institute for Health and Social Affairs, every 1-degree Celsius rise in temperature is associated with a 4.27 percent increase in the average incidence of major infectious diseases, including scrub typhus and malaria. Beyond human health, livestock diseases such as lumpy skin disease are also emerging as a growing concern.

As climate change amplifies infectious disease risks, the KDCA plans to expand its regional vector surveillance network from 16 sites to more than 30 by 2029 and build a "smart surveillance system" using AI-based mosquito monitoring equipment.

The WHO has also warned that climate change is altering the very epidemiology of infectious diseases, stressing that countries must have "the means to monitor and track risks, the capacity to respond to outbreaks, and access to countermeasures such as vaccines and treatments."