KYOTO - Every spring, as visitors flock to Kyoto's Uji City to see landmarks such as Byodo-in Temple and sites associated with The Tale of Genji, another seasonal attraction appears in overwhelming numbers: swarms of tobikera, or caddisflies.
The insects emerge in large numbers around the Golden Week holiday period, covering trees, buildings, and public spaces throughout the city. While harmless to humans, their sheer numbers have made them a source of frustration for tourists and residents alike. Yet experts say the insects are also a vital part of the local ecosystem and an indicator of the Uji River's clean water.
Walking through central Uji, it is difficult to miss the insects. The undersides of leaves are often blanketed with caddisflies, with few leaves left uncovered. The insects resemble moths and spend most of their lives underwater as larvae before emerging as adults in spring and early summer. Their adult lifespan lasts only about a week.
Although caddisflies neither bite nor sting, many visitors react with surprise when encountering the swarms. Some foreign tourists described the insects as itchy or simply too numerous, while local children complained that the insects cling to clothing and bicycles.
Businesses in the city's popular tea district have adopted measures to reduce the impact. One shop owner said visitors often wonder what the insects are when they first see them. To limit the problem, the business applies special coatings to windows during the peak season to prevent insects from gathering, a measure that comes with additional costs but is considered necessary.
For local residents, the insects are an annual nuisance. Laundry can be difficult to hang outside, and cyclists frequently find caddisflies sticking to their clothes or landing on them while riding.
According to Associate Professor Kobayashi of Kyoto University's Disaster Prevention Research Institute, caddisflies inhabit only fast-flowing rivers and require exceptionally clean water. Around 20 species live in the Uji River system.
The insects were not always present in such large numbers. Kobayashi explained that major changes to the river began after the completion of Amagase Dam in 1964. The dam, built following the devastating 1953 Yodo River flood caused by a typhoon, remains an essential water source for the Kyoto and Osaka regions.
However, the dam also traps sediment flowing downstream. As a result, the riverbed environment gradually changed. Fine sand and sediment were washed away, leaving larger stones exposed across the riverbed.
These large rocks proved ideal for caddisfly larvae. The larvae construct tent-like nests attached firmly to the stones, where they spend most of their lives underwater. Larger stones provide a stable foundation that is less likely to be moved by river currents, improving the insects' chances of survival.
While some fish species that lay eggs on sandy riverbeds declined, caddisfly populations increased severalfold from the 1960s onward.
Researchers say the insects are far more than a nuisance. Adult caddisflies serve as a major food source for swallows, other birds, bats, spiders, and numerous insects. Their abundance helps support a diverse river ecosystem.
"They play an extremely important role in the ecosystem," Kobayashi said.
Recognizing both the ecological importance of the insects and the inconvenience they cause, Uji City has pursued measures aimed at coexistence rather than eradication. Since around 2013, the city has introduced insect-control devices and other initiatives to reduce numbers in heavily affected areas.
The Kinki Regional Development Bureau and other organizations have held meetings on caddisfly countermeasures since 2014. In 2020, river improvement works began primarily to restore the river environment, though officials hope the projects may also help suppress caddisfly populations as a secondary benefit.
City officials emphasize that the insects' presence reflects the health of the river. "The fact that caddisflies live here can be seen as evidence that the river environment is good," one official said. "We are pursuing coexistence with caddisflies while considering ways to control their numbers."
Some residents have already embraced that approach. One local said people eventually become accustomed to the insects, even when they fly into faces or shoes while cycling. Another suggested promoting a mascot character to help improve public understanding of the species.
Efforts to change perceptions are already underway. In 2021, university students in neighboring Shiga Prefecture launched a project to promote awareness of the ecological importance of caddisflies. The group developed protective covers for ice cream cones to keep insects away during outdoor events and created educational manga explaining the insects' role in the environment.
Whether caddisflies will continue to be viewed as troublesome pests or become a symbol of a healthy river ecosystem remains an open question. For now, their annual arrival remains one of Uji's most distinctive signs of the changing seasons.
Source: ABCTVnews
