Black-headed slugs overrun gardens in Lappeenranta

A newly spreading invasive slug is damaging gardens and limiting how residents use their yards in Lappeenranta, where people have collected thousands this year, Yle reports.

The slugs have spread across the Ruoholahti neighbourhood. Maisa Hautala, a gardener with the city’s maintenance department, said she receives reports every day. In some places, she has counted 20 to 50 slugs per square metre.

Resident Outi Pyysing-Anttila first spotted them in her garden in 2024. She hoped winter frosts would kill them, but they returned in greater numbers the following year. She says she has collected thousands from her yard, yet more remain. Her children no longer feel comfortable playing outside, and the family left its vegetable garden unplanted this summer after slugs damaged potatoes the year before.

Another resident, Katja Mykkänen, said she had collected about 500 slugs from her garden and nearby city land. After she reported them, the city treated the area with slug poison the next day.

The black-headed slug is a new species in Finland. The first sightings were recorded in 2018, and its numbers have since grown rapidly. It appears to tolerate cold well and can overwinter both as an adult and as an egg. In large numbers, it can damage gardens and the natural environment.

The slug’s mucus may contain listeria bacteria that can be harmful to people, so it should only be handled with protective gloves. The slugs should be collected and killed promptly. Plants, soil and garden waste should not be moved from an infested area.

Lappeenranta has treated affected areas this autumn, but city staff cannot collect the slugs from municipal land. Hautala has asked residents to help collect them there as well.