The common assumption that all slugs are plant pests couldn’t be further from the truth, since it is only a relatively small number that primarily feed on plants.

So, here in the British Isles, less than 20% of the 40 or so resident slug species cause problems to gardeners and growers, whilst the remaining species have important roles within the various natural habitats and help to sustain a balanced ecosystem.

Whether it’s providing a vital food source for the creatures that exist further up nature’s food chains, such as ground dwelling beetles, centipedes, small mammals, frogs, toads, blackbirds and hedgehogs, or by eating and recycling dead organic matter back into the soil. 

Knowing which are the few plant-damaging slug species would enable gardeners to decide how they protect their garden plants without harming the remaining beneficial slugs which will be recycling and cleaning the garden ecosystem of decaying organic matter.

The Problem Slug Species

1.     The Black Garden Slug (Arion hortensis)

Description:

·      A small black slug with a distinctive orange underside

·      Around 2.5cm in length when fully grown

·      Produces yellow slime

Habitat & Damage:

·      Lives above and under ground

·      Nocturnal feeder

·      Eats small seedlings, roots and tubers

·      The common cause of holes in potatoes where it feeds

2.     The Grey Field Slug (Deroceras reticulatum)

Description:

·      A small tan or grey coloured slug with mottled darker markings

·      Around 3.5cm in length when fully grown

Habitat & Damage:

·      Lives above ground

·      Nocturnal feeder

·      Eats plants and seeds

3.     Sowerby’s Keeled Slug (Tandonia sowerbyi)

Description:

·      A medium sized brown slug

·      Has a keel shape running along its back which is a lighter colour than its body

·      Around 7.5cm in length when fully grown

Habitat & Damage:

·      Lives mainly underground but can be found feeding above ground

·      Nocturnal feeder

·      Eats bulbs, plants, roots and tubers

4.     Budapest Keeled Slug (Tandonia budapestensis)

Description:

·      A medium sized brown-grey slug, mottled with dark brown spots

·      Has a keel shape running along its back which is a darker colour than its body

·      Has a dark stripe underneath which runs the length of its body

·      Around 6.0cm in length when fully grown

Habitat & Damage:

·      Lives underground and above ground

·      Nocturnal feeder

·      Eats bulbs, plants, roots and tubers

5.     The Greenhouse Slug (Ambigolimax valentianus)

Description:

·      A medium sized pale brown slug with a watery appearance

·      Has 2 or 3 dark lines at the head end, and two dark lines down the length of its body

·      Around 7.0 cm in length when fully grown

Habitat & Damage:

·      Often found in greenhouses and in vegetable beds, under pots and bark

·      Nocturnal feeder

·      Eats plants, with a particular preference to marigolds

Slug Species to Protect

Although the majority of slug species in the British Isles are not herbivorous plant feeders, or only cause insignificant levels of damage, they should be accepted within a garden environment as an important part of its biodiversity.

Three of these species are exceptionally good for the garden and should be encouraged and protected if possible. These are:

6.     The Spanish Slug (Arion vulgaris)

Description:

·      A large slug, usually brown in colour with a bronze sheen, although darker and lighter colours are common

·      Around 15cm in length when fully grown

·      Usually found in large groups, sometimes many hundreds

·      Exceptionally slimy

Habitat & Damage:

·      Lives above ground

·      Nocturnal feeder but commonly seen mornings and evenings

·      Tolerates dry conditions

·      Eats a vast range of plants and flowers to destruction

·      Omnivorous, often found eating dead animals and dog faeces

7.     The Leopard Slug (Limax maximus)

Description:

·      A large slug, usually dark grey in colour but could also be light grey

·      Distinctive lines of dark grey or black spots down the whole length of its body

·      Around 15cm in length when fully grown

·      Known for its unusual mating behaviour, entwined and suspended on a mucus string

Habitat & Damage:

·      Lives above ground

·      Nocturnal

·      Feeds on dead organic matter and also predates other slug species

8.     The Black Slug (Arion ater)

Description:

·      A large slug, usually dark brown or black in colour with distinct grooves down its back. Often similar in appearance to the Red Slug (Arion rufus)

·      Around 15cm in length when fully grown

·      Forms a domed bell shape when at rest

·      Usually found individually

·      Not very slimy

·      Not to be mistaken for the Spanish slug which is a serious plant pest

Habitat & Damage:

·      Lives above ground

·      Nocturnal feeder of dead and decaying organic matter

·      Very occasionally it will eat living plant tissue

·      Becoming less common in gardens.

·      Can be found around and within compost heaps where it will be breaking down organic matter as part of the natural composting process

9.     The Red Slug (Arion rufus)

Description:

·      A large slug, usually reddish brown in colour with distinct grooves down its back. Often similar in appearance to the Black Slug (Arion ater)

·      Around 15cm in length when fully grown

·      Forms a domed bell shape when at rest

·      Usually found individually

·      Not very slimy

·      Not to be mistaken for the Spanish slug which is a serious plant pest

Habitat & Damage:

·      Lives above ground

·      Nocturnal feeder of dead and decaying organic matter

·      Very occasionally it will eat living plant tissue

·      Becoming less common in gardens.

·      Can be found around and within compost heaps where it will be breaking down organic matter as part of the natural composting process.