Riihimäki reduces Spanish slug population with community efforts in Finland

Riihimäki has significantly reduced the population of Spanish slugs in the city, thanks to collaboration with residents. This summer, nearly 4,950 kilograms of slugs were collected, which is almost double the amount from the previous year, according to invasive species coordinator Anne Kulhoranta. In recent years, the city has gathered between 2,500 and 8,200 kilograms of slugs.

Efforts to eliminate the non-native species included the “Etanapassi” campaign and city-organized volunteer events. Additionally, residents collected slugs from 28 designated collection points throughout Riihimäki. The Spanish slug is an omnivorous invasive species that has adapted well to Finland’s cool climate, with local specimens typically exhibiting various shades of orange and brown. Adult slugs range from 7 to 14 centimeters in length and weigh about 10 grams.

Since 2020, efforts have been focused on deploying pesticides, organizing volunteer events, and managing park areas to control the slug population. Parks manager Satu Nummela noted a marked decrease in slugs over the years. Community involvement has been crucial in this success; without it, the reduction wouldn’t be possible. Specific areas like Vahteristo, Metsäkorpi, and Pohjankorpi have thus far managed to avoid significant slug invasions.

Despite the efforts, the Spanish slug remains a persistent challenge due to its adaptability and voracious appetite, including occasional cannibalism among individuals. Each summer, a single slug can produce up to 300 eggs, which can withstand winter conditions. The most effective strategy is to gather slugs early in the season, as this ensures that hundreds fewer will emerge by autumn.