Slug Information

A slug freezer is available and can be found in the outbuilding behind Peltorivi 16.

Spanish Slugs and Black-Headed Slugs Are Harmful Invasive Species

The removal of harmful invasive species is the responsibility of the property owner. Slugs move and can travel several tens of meters per day, so we hope everyone will remove them from their yards to prevent further spread.

The Spanish slug has been observed in Fiskars for approximately five years. It has already spread along the northern side of the river from the museum to the Stone House and has begun advancing up Shoemaker’s Hill and toward the threshing barn. The map (link to map) shows the distribution area in autumn 2023. The species has not yet been observed on the other side of the river.

The black-headed slug was first identified in Fiskars in autumn 2023. It has been seen around the K-Market area in Shoemaker’s Hill, along the footpath leading uphill, and all the way to Milk Path. There is still limited information available about the species, but Vieraslajit.fi recommends treating it the same way as the Spanish slug because both species reproduce very rapidly and displace native fauna.

In addition, they:

• cause damage in home gardens and cultivated areas
• may carry bacteria harmful to humans and pets, such as listeria and E. coli
• may act as intermediate hosts for so-called heartworm parasites harmful to dogs and related animals
• may lower property values when present in large numbers

  

  The image above shows a striped leopard slug, which is not considered harmful, and below it a solid-colored Spanish slug. Photos: Anna Kamara.

IDENTIFICATION

SPANISH SLUG

The Spanish slug is 7–14 cm long. It is a single color ranging from orange-red-brown to almost black. Light-colored individuals have also been observed.

It should not be confused with the leopard slug, which does not need to be removed. The leopard slug is usually black and gray striped and has a ridge along its back. In the leopard slug, the breathing hole is located at the rear end of the mantle, while in the Spanish slug it is located closer to the head. In the image, the Spanish slug is on the right and the leopard slug on the left.

BLACK-HEADED SLUG

The black-headed slug is smaller and faster than the Spanish slug, approximately 4–5 cm long. It can be identified by its black head and neck, which clearly contrast with the rest of the body. The body color ranges from dirty pale tones to brown and bluish gray. Older individuals are darker than younger ones.

The rear part of the mantle, beginning behind the head, is lighter than the rest of the body and displays a fingerprint-like pattern visible in good light. The breathing hole is located on the right side near the back of the mantle. The underside is uniformly pale, and the mucus is clear and colorless.

More identification help can be found at Vieraslajit.fi.

PREVENTIVE MEASURES

Slugs thrive in moist environments. Their preferred habitats include unmanaged tall meadows, piles of leaves, boards and brushwood, and dense bushes.

SPRING CLEANING AND REMOVAL OF HABITATS

• Rake leaves and cut down perennials low in spring.
• Remove piles of leaves and brushwood from the property or compost them in compost bins or enclosed compost systems.

The Martha Organization provides instructions for leaf composting even in garbage bags. A simple compost frame can be made from approximately one-meter-high wire fencing. The compost should be about one meter in diameter. Place boards around the compost as slug traps. Slugs seek food in compost and shelter under the moist boards during the day. Check the traps daily.

HANDLING GARDEN WASTE IN INFESTED AREAS

If Spanish slugs or black-headed slugs have been observed on the property, leaves and other garden waste should be packed in bags and taken in sealed bags to burnable mixed waste in Karis.

Rosk’n Roll accepts invasive species waste from private individuals free of charge for incineration when transported by car, van, or trailer.

Do NOT move soil, leaf piles, compost, or plants to other areas. This helps prevent the spread of slugs and eggs to new locations and municipal compost sites.

MOW THE GRASS

Keep meadows and lawns short at least during early summer or as long as slugs are present so they can be collected more easily.

WATER ONLY WHEN NECESSARY

Water directly at the roots in the morning or daytime so the area can dry out overnight. Avoid using sprinklers.

CHECK NEW PLANTS

Remove the pot and shake the soil over, for example, a garbage bag. Inspect the root ball carefully. Spanish slug eggs are white, round, and approximately 4 mm in size. They occur in clusters of 20–40 eggs and are therefore easy to spot in the soil. Remove and destroy the eggs in the same way as adult slugs. Replant the plant afterward.

Before ordering soil or fill material, ensure that it is verified clean and free of invasive species. Bagged soil is recommended.

COLLECTION AND KILLING

A single Spanish slug can lay up to 400 eggs per year. The species begins laying eggs at a size of approximately 5–6 cm. Therefore, it is important to begin collecting slugs already in May. When slugs are removed before laying eggs, no new generation develops.

Picking slugs is an effective control method when done regularly. Collect slugs daily as long as they are present. They are active early in the morning, in the evenings, and during wet weather. They often appear repeatedly in the same moist areas. Collect them, for example, in an empty milk carton.

To avoid unnecessary suffering, kill the slugs before or immediately after collection. Remember to wear gardening gloves because slugs may spread harmful bacteria. This is especially important in yards with pets or children.

KILLING METHODS

• Cut the slug with scissors, a knife, or a shovel about 0.5 cm in front of the mantle, or crush the head with a stone.
• Place slugs in a bucket or sealed container and pour plenty of boiling water over them.
• Place slugs in a sealed container and freeze them for two days.

SLUG FREEZER

The slug freezer is located in the outbuilding behind Peltorivi 16. You may bring collected slugs there in a tightly sealed container.

Dead slugs must be disposed of with mixed waste in a tightly sealed container or bag.

SLUG WASTE BIN

A slug waste bin is available during the growing season at the recycling point on Peltorivi opposite the knife factory. Dead slugs can be brought there in tightly sealed bags.

Do NOT leave dead slugs in your garden. Spanish slugs are cannibalistic and feed on one another.

WE DO NOT RECOMMEND FERRAMOL

We have investigated the side effects of Ferramol (Tukes, VieKas-LIFE, and The Finnish Association for Nature Conservation) and do not recommend its use in the ironworks area. Ferramol kills all slugs, not only the target species. In the worst case, it may also affect endangered native species. Herb-rich deciduous forests are important habitats for native and rare mollusks. If Ferramol enters waterways, it may have devastating effects on fish.

Please REPORT SLUG OBSERVATIONS to: siruetanat.fiskars@gmail.com. You can also receive help with identification there if needed.

Special attention should be paid to infestation sites and routes of spread, such as ditch edges and drainage channels, where the species can spread easily. It is important to prevent spread via plants, garden waste, and soil transport. Soil, leaf piles, compost, or plants must not be transported from infested areas to other locations. This helps prevent slugs and their eggs from spreading to new areas.

Let’s take care of our local environment together!

• Remove invasive species and prevent their spread from your own property.
• In this way we protect local plants and pollinators.
• Never dump garden waste in nature. Nature will thank you!

With thanks for your cooperation,
The Ironworks LUMO Project within Fiskars byaförening

Sources and Further Information

• Vieraslajit.fi
• City of Helsinki guide on Spanish slugs
• Martta guide to leaf composting
• Snigelakuten – life cycle of the Spanish slug

Map: Tuulia Minkkinen

Spanish slug eggs are white, round, and approximately 4 mm in size. They occur in clusters of 20–40 eggs. Image: YLE article 3 August 2019.

The black-headed slug is recognized by its black head and tentacles, and by the distinctive fingerprint-like markings on its mantle. Photos: www.vieraslajit.fi