Slugs, like everything in an ecosystem have a role. They are composters so break down vegetation, and they are also a source of food for other wildlife. There are approximately 30 species of slug in the UK and there are four main pest species that are native to the UK. These four pest species cause approximately £8 million in damage to vegetable crops each year. On average a UK garden is home to over 20,000 slugs and it is estimated that an acre of farmland can support over 250,000 slugs.
Slugs move in rhythmic waves by contracting muscles on the underside of the foot. At the same time a layer of mucus is produced that helps to smooth the slugs path across the ground. This mucus is also used as the slugs navigation system, as slugs will find their way back to their tunnels and feeding sites by following their mucus trail.
A slug has two retractable pairs of tentacles. The upper pair of tentacles are called the optical tentacles and are the eyes of a slug. The optical tentacles have light sensitive eyespots on the end and can be re-grown if lost. These are also used for smell. The lower pair are two smaller tentacles and are used for feeling and tasting.
The mouth parts are below the tentacles and the slugs eat using a radula which is a tongue-like organ that is covered with approximately 27,000 tiny tooth-like protrusions that are called denticles.
Slugs have a large respiratory pore called the pneumostome. This leads to a single lung and is generally found on the right hand side.
The mantle is an area behind the head of the slug and is made of thicker flesh than the head. If a slug is frightened or not active the slug will retract its head into the mantle for protection. The mantle also forms the respiratory cavity. In some slug species there is a small piece of shell in the mantle, this is because slugs have evolved from snails.
The keel is a ridge that runs the length of the back of some species of slug.
For more, see the SlugWatch slug anatomy diagram
Most slugs live underground and the ones that can be seen feeding above ground are only approximately 5% of the total slug population.
Cool weather, rain and fog is the best weather for slugs because since they don’t have an outer shell to protect them, they may dry out in dry and warm weather. When the weather is warm and dry slugs will try and find a cool, dark and damp area to hide in. However if there is a long dry spell then slugs will encase themselves in a papery cocoon-like structure and attach themselves to a wall or a tree and wait it out.
Slugs will grind up their food and will eat things such as:
- Tender leaves
- Soft fruits
- Decaying matter
Most species of slugs are herbivores however some are carnivores and the diet of a slug varies depending on what species it is. For more information about a certain species’ diet see the Identification guide.
Slugs have a variety of predators such as:
- Birds (e.g. Thrushes)
- Ground Beetles
When a slug is attacked by a predator it will contract its body to make it a smaller target. The mucus that covers a slug’s body doesn’t taste very nice and is slippery, so this is why you will often see birds wiping slugs on the grass before they eat them.
There are four species of slug that are classed as pests; they are the netted or grey field slug, the garden slug, the keeled slug and the large black slug.
The Netted or Grey Field slug (Deroceras reticulatum) is normally a light brown colour with a chain of darker veins and blotches, it can grow to between 3cm and 5cm in length when extended. It is a pest on agricultural land as it feeds mostly on seeds and plants above ground, and is a major threat to cereal crops.
The Common Garden slug (Arion distinctus/ Arion horntensis) is brown in colour and can grow to 3cm when mature. This species is a major pest of potatoes and attacks both leaf and root crop.
The Common Keeled slug (Tandonia budapestensis) is black or grey in colour and can grow up to 6cm when mature. This species of slug will feed on newly drilled seeds underground and will badly affect crops such as potatoes. As it spends most of its time underground it is very hard to control.
The Large Red slug and Black slug (Arion ater) is either orange-red or black in colour and can reach a size of 12cm in length. This species of slug will eat seedlings on agricultural land in spring, but it is much less damaging than the other pest species.