Larvae collection in Polvijärvi aims to reduce mosquito problem in Imatra

In Polvijärvi, larva collected from the Rauanjoki River are expected to help alleviate the mosquito problem faced by residents of Imatra. Larvae of the large caddisfly, known as the northern caddisfly, are being collected from the riverbed using nets. Brick traps have been set up to which the larvae are hoped to attach.

Next summer, the larva collection effort will continue in Lieksa, with bricks already placed in Ruunaa’s Siikakoski. Kari-Matti Vuori leads this unique project aimed at restoring moss and predator larvae to the riverbed of Imatra. Vuori works as a development manager at the Finnish Environment Institute’s Nature Solutions unit.

Currently, the impact on the mosquito population in South Karelia is minimal, as only 22 northern caddisfly larvae were collected from Rauanjoki. These will be transferred to the stream in Imatra. Vuori states that hundreds or even thousands of insects are needed for a significant effect.

Historically, disturbances to the Vuoksi River, such as damming and the concreting of banks in the 1950s, destroyed the riverbed vegetation, which in turn eliminated predators such as caddisfly larvae that relied on moss for attachment. Meanwhile, the larvae of mosquitoes thrived, leading locals to report overwhelming numbers.

Vuori points out that the absence of northern caddisfly larvae impacts water quality as well. This insect, recognized for its strong yet flexible silk, has even been studied by NASA. The larvae behave like spiders, trapping prey in nets and communicating with chirps when threatened. The northern caddisfly is rare in southern Finland but still found in healthy populations in certain areas, prompting efforts to relocate larvae from North Karelia for repopulation. Overall, this initiative represents a novel attempt to enhance river ecosystems.