Spanish slug – Arion vulgaris

Invasive alien species included in the national list (National list of alien species)

Spanish slug has been regulated as a nationally invasive alien species. It may not be released into the environment or imported into Finland from another EU country or from outside the EU, held, bred, transported, disposed of, passed on, sold, placed on market, or otherwise handed over.

According to the current research, when present in large quantities, Spanish slugs can cause damage to other species and biodiversity. In addition, because of the risk of diseases associated with them, they can also pose a risk to human health.

The owner or holder of a property must take reasonable measures to eradicate or limit the spread of an invasive alien species on the property if the occurrence or spread of the species can cause significant damage to biodiversity or a risk to health or safety. Thus, the owner and the holder of the property are obliged to combat the Spanish slug.

The Spanish slug occurs in masses and has adapted to the cool climate in Finland. It can move up to 50 metres in a night and usually eats its own weight (10 g) a day. As an omnivorous species it can cause great damage in home gardens and fields. It feeds on leaves, flowers and bulbs of ornamental and useful plants. Spanish slugs also eat animal carcasses, including dead individuals of their own species, which is why they have been given the vernacular name "killer slug".

Climate change is increasing the size of populations and thus, also the damage of the species in the northern cultivation areas, and further, the risks of plant production. Also, studies carried out in other parts of Europe have found that Spanish slugs can interbreed with other snails, thus causing ecological damage and harm to native species as well. In addition, mucus secreted by the Spanish slugs may contain bacteria (including listeria), which can be harmful to both humans and domestic animals. The Swedish Veterinary Institute (SVA) has found that the Spanish slug can act as an intermediate host for the so-called heartworm (Angiostrongylus vasorum), which is harmful to canines.

In most cases, it is possible to combat the spread of the slug with rather small measures. In the worst cases, however, even large masses of soil need to be treated to destroy the slug population.

The control of the Spanish slug is most effective in the spring before the overwintered individuals have laid eggs. A slugl lays about 20 to 30 eggs at a time and continues to lay eggs throughout the summer and autumn during the wet seasons. Therefore, th slugs in reproductive age should be destroyed immediately in the spring and always before laying eggs.

In home gardens, Spanish slugs can be collected with, for example, barbecue pliers, which protect against the addhesive, sticky mucus. Because of the risk of infection (listeria, E.coli), disposable gloves should be worn when handling snails. Collecting slugs can be made more efficient by placing, for example, boards or jute bags on the ground to collect moisture and thus to attract and gather slugs.

Mollusks have a developed nervous system and probably the ability to feel pain. Since the Animal Welfare Act states that animals must not be subjected to unnecessary suffering, Spanish slugs must be killed as quickly as possible. Effective and recommended methods of killing include cutting the head of a slug with scissors longitudinally between the sensory horns, causing the nerve ring of the head to break, or crushing the head of a slug with sufficient force e.g. with a stone on a hard surface so that the impact is as effective as possible. Also, it is recommended to stun a slug in mild (5%) and kill in strong alcoholic solution (e.g. denatured spirit, ethanol content above 80%). Slugs can also be killed, like crabs, by dropping them one by one into boiling water, so that the water should boil vigorously all the time. Alternatively, you can pour plenty (at least double amount) of boiling water over the snails. Also, 10% spirits vinegar solution or freezing has been used to kill slugs, but they are not recommended for use. Salt or other low-intensity toxins must not be used. Also, slugs should not be buried alive, nor cut across, as then the front end can survive. Dead slugs should be packed well and delivered for incineration with mixed waste.

For control of slugs, iron phosphate preparations and biological nematode preparations are available for use in arable and horticultural crops in fields, in the garden and in the greenhouse. Some products are also allowed for consumer use.

 In large affected areas, the soil infested by slugs is peeled at a depth of 10 cm. The exfoliated soil is poured into an excavated pit or piled up in heaps on the spot. The pit and soil piles are covered with a thick layer of soil. A half a meter thick layer of soil is probably sufficient. Compaction increases the effectiveness of the soil layer. A thick and compact layer of soil prevents the access of developing slugs to the surface of the soil. Also, products containing iron phosphate should be used in the area.

The mucus secreted by the Spanish slugs protects it from predators. The natural enemies of the species include e.g. ground beetles, nematodes, hedgehogs, badgers, some birds (thrushes, mallards, ducks).

When encountering Spanish slugs in yards, gardens and public areas, it is recommended to remove them or organize events for volunteers to combat them.

If Spanish slugs are found in the area, the lawn and all vegetation in the yard should be kept low and neat. In this way, the slugs do not find protection against dehydration. Also, it is good to remove and destroy the heaps of leaves and brushwood so the slugs don't have a place to overwinter. Another method of controlling slugs is to sprinkle lime on the top of the compost, then the lime absorbs moisture from the slugs. The adding lime should be restricted to the control of the Spanish slug only, because lime interferes with the function of the micro-organisms in composts. Also, open composts should be avoided.

In the control of the Spanish slug, special attention should be paid to its deposits and pathways through which they spread easily into the environment. It is important to prevent the spread of the species with seedlings, garden waste and soil transfers. No soil, piles of leaves, compost or plants may be transported from the areas where the slugs occurs. This will prevent them and their eggs from entering new areas.

A professional operator has the duty to prevent the spread of Spanish slugs outside the area under his control, for example through humus soil, soil or plants.

Because the species is regulated as a nationally invasive alien species, its import, breeding, sale and other possession as well as release into the environment is prohibited.