Garden slug & snail species: which are useful & which should be kept away?
Few of the species of snails in gardens feast on lettuce and other green plants. Many snails are even useful and should be encouraged in gardens.
Although we are often disgusted by the slimy little critters, many land snail species are useful and important to our ecosystem. Nevertheless, especially in wet years, some species of snails develop into a real plague of snails in the garden. This article looks at some of the snail species that are common in our country and points out which snails are more useful to us.
- Types of snails in the garden: How many are there?
- Useful species of snails in the garden
- What species of snails are a threat to garden plants?
Types of snails in the garden: How many are there?
Snails, whose scientific name is Gastropoda, form a separate class within the mollusks (Mollusca). How many species of snails there are in total, you can not say exactly, but their number is enormous. Estimates put the number of species at least 85,000. But the most interesting for us are the snails that can be found in gardens. These belong to the land snails (Stylommatophora), of which there are about 25,000 species worldwide.
Useful species of snails in the garden
For what are snails useful? Not all snail species prefer fresh, green plant material. Some species feed primarily on dead plants and carrion and are thus important destructors, or decomposers, in the material cycle. In some cases, useful snails in the garden even eat other snails or their clutches, so they actively contribute to a reduction of unwanted snail species. The useful snail species found in our gardens come mainly from two families: the snails (Helicidae) and the slugs (Limacidae).
Almost all snails with a shell, which are found in our gardens, come from this family. It is very rich in species and widespread in Europe. The various species of these shell snails feed mainly on dead plant material, so they usually do not because much damage.
Roman snail (Helix pomatia)
Roman snails are one of the snail species native to our region. However, the number of vine slugs in the garden has decreased sharply in recent years, which is due to continuous destruction of the habitat of vine slugs and effective chemical control. That is why the Roman snail is nowadays a protected species in Germany. The shell of Roman snails reaches a size of 3 to 5 cm and is in the vast majority of cases right-handed. Since Roman snails usually survive the winter by going into frost rigidity in their covered shell, Roman snails can reach an age of up to eight years in the wild. The habitat of Roman snails should have a calcareous, rather moist soil and ideally be shady and warm. They are often found in sparse forests or bushes. The natural enemies of slugs are diverse. For example, ants, mites, birds of prey, and small mammals are especially targeting the young snails with a still soft shell. For this reason, only about 5 out of 100 snails reach an age of more than two years.
Tip: Have you ever found a Roman snail on your salad and wondered if Roman snails are really useful? If vine slugs cannot find enough dead plant material, which can be the case in very clean and tidy gardens, for example, they sometimes have no choice but to feast on fresh plant parts as well. So feel free to leave a few dirty spots so that the snail does not have to compete with you for food.
Garden banded snail (Cepaea hortensis)
Garden banded snails are much smaller than Roman snails, with a shell diameter of about 2.5 cm. The case is characterised by the light colouring with the dark bands. The mouth of the shell is usually white, which serves as a distinguishing feature from the otherwise very similar grove banded snails. Garden banded snails can adapt to a wide variety of habitats, but are often seen in woodlands, shrubberies, hedgerows, or raised beds. They are often found somewhat higher up on trees or hedges. Since they feed primarily on algae rather than herbaceous plant parts, they generally do no harm in our gardens.
Often all snails without a house are lumped together and basically portrayed as harmful and unwanted. However, there are definitely species of slugs that are useful – for example, most species of snails. A total of about 200 species of snails are currently known. They feed mainly on dead plant material, algae, fungi and sometimes the eggs of other snails. Horticulturally relevant are the genera Lehmannia, Limacus, Limax, Malacolimax. The tiger snail (Limax maximus) is also not afraid to attack and exterminate the rampant Spanish slugs, making it one of the few natural enemies of Spanish slugs. The black snail (Limax cinereoniger) is commonly found on composts, where it eats plant debris and fungi. The greenhouse slug (Lehmannia valentiana) is the only known species of slug that causes damage – but only in greenhouses.
What species of snails are a threat to garden plants?
With increased occurrence, the Genetted Slug (Deroceras reticulatum) and the Spanish Way Slug (Arion vulgaris) pose a threat to garden plants.
What is always important to remember is that not all plants are affected equally. Many are uninteresting to slugs or can even help repel slugs on the bed.
Common to the different species within the family of slugs is that the shell of the slug is largely regressed. Only the so-called mantle, which covers the front part of the body of adult slugs, remains. The breathing hole of slugs is on the right side, in front of the center of the mantle. Another characteristic that distinguishes them from many other snail species is that path snails can curl up. In the garden is primarily relevant genus Arion.
Spanish slug (Arion vulgaris)
For a long time, Spanish slugs, preferably referred to as capuchin snails, were used as a prime example of harmful invasive species. It has been suggested that this species of nudibranch was introduced from Spain in the 1960s and has since spread massively, displacing our native snail species. However, recent studies show that the Spanish slug does not occur in Spain at all and its actual origin cannot be clearly proven. Its strong spread within the last decades is undeniable, so that it is probably the most common snail species in Germany by now.
Adult animals are relatively large snails with a length of 7 to 14 cm. Their exact identification is difficult, because they are very variable in colouration – from brown to orange to gray-green – and thus very similar to other slugs. Caution: they are therefore often confused with the red (Arion rufus), brown (Arion fuscus) or black (Arion ater) slug. These species often because comparatively little or no damage to the bed, but are nevertheless – and usually superfluously – fought with all their might. The red slug is even considered endangered and should not be eliminated if possible.
To avoid confusion, the capuchin snail should be accurately identified. The identification works better than with adults on the juveniles, which are usually much lighter, of an almost bright orange-yellow and have two coffee-brown longitudinal stripes. Spanish way slugs are omnivorous but prefer fresh plant material and particularly target student flowers (Tagetes), Baldrian (Valeriana officinalis), and pumpkin plants (Cucurbita). Spanish slugs are also cannibals and eat the clutches of other snail species, giving them a competitive advantage.
Garden slug (Arion hortensis) and Common slug (Arion distinctus).
These two species of snails are very closely related and difficult to distinguish, so in the past they were often simply grouped under the name garden path snail. The adults grow to a maximum length of 5 cm, but often remain smaller. The back of the animals is usually blackish to dark blue with a tinge of brownish. Separated by a lighter, yellowish stripe, there is a darker longitudinal stripe on both sides, the so-called bandage. The sole is usually yellowish in colour and the body mucus is also yellowish in colour. The two species of garden snails feed on herbaceous, chlorophyll-rich – that is, green, fresh – plant material. They often live in the soil, where they eat plant parts already below the crumb. In all stages – from egg to adult – the garden snails can overwinter, which is why you can also find all stages in your garden at any time of the year. As a rule, garden path snail and common path snail live about nine months.
The mantle of field slugs usually covers at least one-third of the body. The relatively small breathing hole sits in the back half of the mantle. Their tail end is keeled, i.e. pointed, like the keel of a ship. Despite being the largest family of slugs, one species in particular is perceived as harmful in our gardens – the Genetted Slug.
Genetted field slug (Deroceras reticulatum)
The Genetted Field Snail grows to a length of 3.5 to 6 cm and is the most common native snail species in our country. Their colour varies from creamy whitish to slate gray to reddish brown. Characteristic is the darker spot pattern, which sometimes results in a real net, and the clearly furrowed skin. Genetted field slugs are particularly dependent on moisture and are sensitive to light, which is why they are nocturnal. Genetted field slugs are omnivorous, but prefer fresh plant material. This species of slugs can also feast on seeds and young seedlings, which often leads to non-emergence of the crop. Genetted field slugs lay their eggs into the fall. The young snails then hatch in the spring and, under optimal conditions, create two to three generations per year.
Tip: The reason the Genetted Slug has such a high damage potential is that, unlike other slugs, it is capable of developing several generations per year.
In mild winters, even the adult snails survive and become active on particularly warm days. These surviving slugs can do a lot of damage early in the year.