A cicada emerges from its shell along a park walkway near an apartment complex in Gyodong, Gangneung, Gangwon Province, on the night of Aug. 2, as a tropical night kept temperatures from falling. [Yonhap]
A cicada emerges from its shell along a park walkway near an apartment complex in Gyodong, Gangneung, Gangwon Province, on the night of Aug. 2, as a tropical night kept temperatures from falling. [Yonhap]

Kim Ye-rin, a 30-year-old office worker living in Eunpyeong-gu, Seoul, has not had a full night's sleep in the past week. The culprit: cicadas screaming around the clock. "I live on the sixth floor of an apartment building and I could still hear them," she said. "They were so loud even in the early morning hours that I couldn't open the windows no matter how hot it got."

This summer, a growing number of city residents are losing sleep to cicadas that keep calling well past midnight. The insects grow more active as temperatures rise, and with heat waves and tropical nights overlapping, they have extended their hours deep into the night. The horse cicada — a species that thrives in hot conditions — has been particularly loud in urban areas.

Several environmental factors, including temperature and light, influence when cicadas call. Each species has its own preferred temperature range, which determines when it starts and stops singing. "Species like the small brown cicada, which prefer cooler periods, mainly call from early morning through the late morning," said Kim Tae-woo, a researcher at the National Institute of Biological Resources. "Species that prefer hotter conditions, such as the horse cicada and the evening cicada, can keep calling from midday through the evening."

Cicadas call to reproduce. Males produce sound to attract females, and once one starts, nearby males are stimulated to join in competitively. When many call at once, the noise in urban areas can become overwhelming.

Cities are also full of light sources that stay on through the night. Bright surroundings extend cicadas' active hours. "In the countryside, once the sun sets there are few light sources and it gets dark," said Park Hyeon-cheol, a professor in the Department of Life and Environmental Chemistry at Pusan National University. "In cities, the light is extremely bright — it creates conditions where cicadas can keep courting even at night." Cicadas normally stop courting when it gets dark because they cannot find mates, but the abundance of artificial light from streetlamps and building lights in urban areas allows them to remain active after dark.

Cicada calls typically register between 70 and 90 decibels. Among urban species, the horse cicada is considered the primary source of noise. Unlike the small brown cicada, which produces a rhythmic "maem-maem" sound, the horse cicada emits a harsh, unbroken shriek perceived as far more disruptive in city environments. Measured at close range, its call can reach 80 to 100 decibels — well above the residential noise standards of 65 decibels during the day and 60 decibels at night.

Residents cool off by dipping their feet in the water beneath the southern end of Mapo Bridge in Yeouido, Seoul, on the morning of Aug. 3, as extreme heat exceeding 40 degrees Celsius gripped the country for several consecutive days. [Lee Sang-sub]
Residents cool off by dipping their feet in the water beneath the southern end of Mapo Bridge in Yeouido, Seoul, on the morning of Aug. 3, as extreme heat exceeding 40 degrees Celsius gripped the country for several consecutive days. [Lee Sang-sub]

The horse cicada's outsized noise comes down to its size and the nature of its call. "Because the horse cicada has a larger body, it can produce a louder sound," Kim said. "Other cicadas call with a rhythmic rise and fall, but the horse cicada calls in a flat, unrelenting burst — which is why it registers as noise to human ears."

The horse cicada also prefers hotter conditions than other species. A 2018 study by researchers Kim Yun-jae and Ki Gyeong-seok analyzed the start and end times of calls by small brown cicadas and horse cicadas and found that temperature and sunrise time both influenced when horse cicadas began calling, with temperature having the greatest effect.

According to Kim, horse cicadas begin calling when temperatures reach around 27 degrees Celsius and grow more active as it gets hotter. This year has brought conditions particularly favorable for the species. As of Sunday, the Korea Meteorological Administration recorded 16.7 heat wave days so far this year — the fourth highest on record. The average daily high of 30.1 degrees Celsius also ranked fourth highest on record.

When tropical nights persist and temperatures do not drop sufficiently — as has been the case this year — horse cicadas can keep calling until very late. A 2021 study by the citizen lifestyle research team at the Seoul Metropolitan Government's Institute of Public Health and Environment found that cicada noise levels inside apartment complexes were 8 to 10 percent higher during tropical nights than on other nights. Horse cicadas in particular were observed calling 3 to 4 hours longer in areas with strong nighttime lighting.

"When tropical nights continue, conditions are like daytime, so they just keep calling," Kim said. "They also tend to call late into the night near street trees lit by bright lamps." This year has been especially conducive to nighttime cicada activity. As of Sunday, the number of tropical nights this year stood at 14.7 — the highest on record for the same period and 4.5 days more than last year.

Warming temperatures push cicadas' first calls earlier

The changing behavior of horse cicadas is also tied to climate change. The species was once found mainly in southern regions such as Jeju, but rising temperatures and the urban heat island effect have made it an increasingly common sight in warm city centers. The Ministry of Environment designated the horse cicada as a climate change indicator species precisely because of its sensitivity to temperature shifts.

Climate change is also affecting when cicadas begin their seasonal calls. According to data published by the Korea Meteorological Administration in 2021, the average date of the first cicada call observed during the 1991–2020 climate normal period was July 10 — three days earlier than July 13, the average for the 1981–2010 normal period. The administration noted that the timing of the first cicada call each year has tracked closely with the earlier onset of the climatological summer.

This year, the first cicada calls were recorded earlier than average in major cities across the country. According to the administration's seasonal observation data, the first call was heard two days ahead of the average in Seoul, 10 days early in Busan and 15 days early in Daejeon.

As temperatures continue to rise, cicadas' active season is likely to grow even longer. "In tropical regions and Southeast Asia, cicadas call heavily even in the middle of the night," Kim said. "If temperatures keep rising in Korea, cicadas here could become active for as long as they are in hotter regions."


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