The summer heat wave seems to have largely silenced a seasonal annoyance--the mosquito--in the nation’s hot spots.

The city of Tajimi, Gifu Prefecture, for example, has had four days this year when the temperature topped 40 degrees.

Mizuki Yamada, 20, a college student who grew up and lives in Tajimi, sensed something unusual other than the sweltering weather.

“I have not been bitten by mosquitoes and I haven’t heard many cicadas singing,” Yamada said.

A dearth of the bloodsuckers was also noticed in Kumagaya, Saitama Prefecture, which baked under 41.1-degree heat on July 23, the highest temperature ever recorded in Japan.

“It has been hot since the early stages of summer, and there have probably been fewer mosquitoes,” said a municipal government official in charge of countermeasures for destructive insects.

The city’s department of environment enhancement provides subsidies to neighborhood associations to spray insecticide into roadside ditches to prevent mosquito larvae from growing.

The number of applications for the subsidy was 11 by the end of July, compared with 12 for the same period in 2017.

But the official, who is responsible for the mosquito-killing subsidy, said, “If the number of inquiries for the subsidy is added, it seems to be a decrease from last year.”

Yoshikazu Shirai, head of the Institute of Pest Control Technology, which is located in Yachiyo, Chiba Prefecture, said, “In theory, it is safe to say there are fewer mosquitoes this year.”

Shirai, who studies how to prevent the emergence of mosquitoes, focuses not only on temperatures but also precipitation. Mosquito larvae cannot grow without puddles created by rain, he said.

“If the temperature is high, puddles are more likely to evaporate, and without rain, puddles cannot be formed,” Shirai said. “Due to these two reasons, the source of mosquitoes was reduced, and it is likely that some areas experienced fewer mosquitoes.”

The amount of rain that fell on Kumagaya throughout July was 83 millimeters, less than half of the level in the same month last year. The city’s average temperature in July was up by 1.1 degrees from last year, and the daytime high was 35 degrees or higher on 18 days within the month, according to the Japan Meteorological Agency.

The July rainfall in Tajimi was 160 millimeters, down by more than 100 mm from the previous year, and temperatures topped 35 degrees on 18 days during the month, according to the agency.

“Research found that 35 degrees is the threshold for mosquitoes to be able to fly,” said Toshihiko Sunahara, an assistant professor of epidemiology and entomology at the Institute of Tropical Medicine of Nagasaki University. “According to the research, mosquitoes can fly for the longest time in temperatures of 21 degrees.”

The research was conducted on the Aedes aegypti mosquito species in the United States half a century ago.

That species, also known as the “yellow fever mosquito,” inhabits Southeast Asia and Africa and can transmit the viruses of dengue fever and Zika fever.

Aedes aegypti is closely related to Aedes albopictus, which inhabits Japan. The heat-resistant abilities of the two species are believed to be similar.

“It is apt to conclude that Aedes albopictus cannot be active in temperatures over 35 degrees,” Sunahara said. “That is why people in hot areas of the Kanto and Chubu regions are less likely to get bitten by mosquitoes in the daytime.”

But Sunahara warns residents to stay alert because around 125 types of mosquitoes live in Japan, some of which can transmit pathogenic viruses.

Aedes albopictus, which is in frequent contact with humans, is believed responsible for an outbreak of dengue fever in Tokyo in 2014.

“Mosquitoes are more likely to take a break in shaded areas when the temperature is high and become active in the morning and early evening,” he said. “I would advise people to wear long-sleeved clothing and apply insect-repellent to their bodies if they work outside near thickets in the morning or early evening.”