Black-headed slugs are spreading through gardens in Finland, with residents in Lappeenranta reporting thousands of the invasive molluscs on their properties. A Yle report published on 28 October describes how infestations in the Ruoholampi neighbourhood have disrupted gardening and children’s outdoor play.
Thousands of slugs in a single neighbourhood
Outi Pyysing-Anttila told Yle that she first noticed the slugs in summer 2024. They returned in greater numbers the following year, despite her hopes that winter would kill them. She has collected thousands and left her vegetable patch unplanted this summer after damage to potatoes the previous year.
“The children no longer dare to be in our garden. It’s a shame.”
City maintenance gardener Maisa Hautala said she receives reports daily and has seen concentrations of 20–50 slugs per square metre. Lappeenranta has treated affected areas this autumn, but municipal staff cannot collect all the slugs on city land. Hautala has asked residents to help.
Black-headed slugs were first recorded in Finland in 2018
The species, Krynickillus melanocephalus, is known in Finnish as mustapääetana. Its arrival predates the latest infestations: a 2020 scientific paper documenting its spread to Finland records sightings in a Sipoo garden in 2018 and a Helsinki arboretum in 2019.
Native to the Caucasus and the eastern Black Sea region, it inhabits damp woodland, sheltering beneath leaves, stones and fallen timber. The paper reported abundant populations at both of Finland’s first known sites.
Adults reach around four to five centimetres. Their black head, tentacles and neck contrast with a paler body, according to Finland’s official invasive species portal. The species appears cold-tolerant and probably overwinters both as eggs and adults.
Plants and soil help the species spread
An official guidance bulletin identifies imported plants, soil and gardening materials as routes of spread. It also lists lettuce, courgettes, cabbage and strawberries among the foods the slugs consume. Early removal is more effective before populations become widely established.
Finland already faces problems with another invasive mollusc, the Spanish slug, previously covered by NordiskPost in its reporting on municipal collection reward schemes.
Control measures include hygiene and limits on moving soil
The black-headed slug is on Finland’s national list of harmful invasive species. Official guidance says its mucus may contain harmful bacteria, including Listeria and E. coli. It recommends protective gloves, handwashing and tools reserved for collection.
The guidance calls for collected slugs to be killed quickly without unnecessary suffering and, in built-up areas, disposed of securely with mixed waste. It also warns against moving soil, plants, compost or leaf piles from infested sites, which can carry slugs and eggs into new areas.
Research on the species’ effects on native biodiversity remains limited. Its dense populations are a concern, but the extent of ecological damage in its northern range is still poorly understood.
