They're slimy, brown and hungry: Spanish slugs will sometimes destroy entire vegetable gardens in a single night, even if the plants are grown in raised beds. The animals are therefore feared by gardeners, but for zoologists they are exciting research objects.
If the carefully grown lettuce disappears overnight and glistening ribbons of slime cover the beds, it was probably at work: the Spanish slug. "It's a real super snail," says Michael Schrödl from the Munich State Zoological Collection (ZSM). The brownish-reddish animals are excellent climbers. "Raised beds are no problem for them." And even a vessel hanging at a dizzy height doesn't stop them: "They rappel down their own slime threads."
They are lured by lettuce and vegetables, which they can smell from dozens of meters away. They also remember the locations of delicious meals. Simply throwing a Spanish slug into the neighbor's garden is often of little use: "If there isn't something delicious there, it will come back," says snail expert Schrödl. "If you want to eliminate this snail by throwing it, you should practice long-distance throwing beforehand."
According to Schrödl, there are around 300 species of snails in Germany, around two dozen of which are slugs. "The taxonomy is still quite unclear." In the case of snails with shells, their coloring and peculiarities often provide good classification features, while in the case of slugs with variable shapes and colours, it is more difficult to clearly classify them as a species based on their external appearance. Genetic classifications are intended to remedy the situation - but so far there are not enough data sets for this, as Schrödl explains.
The problem is also evident in the Spanish slug, technical name Arion vulgaris, also known as the large slug. It leaves its slime trail in many places - but only very sporadically in Spain of all places, as researchers have found out in recent years. Contrary to what was long assumed, it was probably not brought in by fruit and vegetable imports from the Iberian Peninsula after the Second World War - so the name is misleading. Rather, she has probably been living for a very long time, at least in southwest Germany, as Schrödl says.
It is still not clear where the species originally came from, explains Heike Reise from the Senckenberg Museum of Natural History in Görlitz. The region of origin is probably somewhere between south-west Europe - say south-west France - and the south-western tip of central Europe, perhaps south-west Switzerland or east France. But one thing is clear: Since the 1960s, it has been appearing more and more north and east, often in high densities. The species causes damage and problems in Norway and Sweden as well as in Poland.
"There are new records in the spread to Northern and Eastern Europe almost every year," says Schrödl. The transport of lettuce, vegetables and flowers, for example, played a part in the spread, explains Reise. It often first appears in places such as nurseries, cemeteries or garden waste storage and spreads from there.
Relatives such as the black slug (Arion ater) and the red slug (Arion rufus) did not succeed in such a triumphal procession. Why? "The Spanish slug is a real roughneck," says Schrödl. Dry lawns and gravel paths may be a problem for other native slugs, but not for Arion vulgaris. It multiplies faster, eats more and, if necessary, sits in the blazing sun to eat without being harmed. In addition, genetic analyzes show that it mixes strongly with other species - and in this way it may acquire new favorable characteristics for the respective environment, as Reise says.
In addition, the snail does not taste good. Schrödl tried all sorts of options with his colleagues, but even with a lot of garlic or pickled in broth, the animals remained one thing above all: "bitter". Only Indian runner ducks are really crazy about Spanish slugs. Hedgehogs, blackbirds, certain ground beetles, shrews, toads - all classic snail hunters only eat young animals of the species. A serious enemy of adult specimens is therefore only the gardener.
It often has to deal with the robust mollusks: the Spanish slug is almost the only one to be found in urban allotments and gardens - and often in alarmingly large numbers. "The snail simply chooses the most comfortable option," says Schrödl. It's warmer in cities than in the countryside, and it's easier to survive the winter. In addition, allotment gardeners in particular ensure the best living conditions: They water their greenery every evening and thus ensure a comfortable moisture level. Animals that eat young snails, such as hedgehogs and toads, are rarely found in plots without any wild growth. "Arion vulgaris is a beneficiary of the wasteland in the Gardens."
Another consideration is that many vegetables have had their natural defenses bred away from being eaten by pests to make them more palatable. It just makes them more attractive not only to humans, but also to pests, as Reise explains. "Bitter substances, for example, have an important function, they deter many animals, including snails." Schrödl is also convinced: "This outbreeding definitely plays a role."
In general, however, Spanish slugs are not particularly choosy, Reise explains. "They also like to try new things." This can be fatal for the flora of a region newly colonized by the species. According to Reise, many small seedlings were eaten away during a resettlement project for rare plant species in Upper Lusatia. In Sweden, the composition of herbs in the forest is changing. "When certain plant species are severely depleted, it affects the entire ecosystem."
The problems are exacerbated by the fact that Arion vulgaris can occur in much greater densities than native species. "According to reports, 50 animals per square meter are possible," says Reise.
There is still a lot of research to be done on the impact on the animal world, says Reise. It is clear that the snails eat small nestlings from ground breeders, other defenseless or seriously ill animals and weakened conspecifics. They also displace related species. In the years after the arrival of Arion vulgaris, the red slug in the Görlitz area initially mixed up heavily with the latter and eventually disappeared completely. Things look a little better for the black slugs that live in forests - but here, too, there is a risk of complete displacement. Reise emphasizes that throwing garden waste or snails collected in the garden into the forest is really bad.
Eventually, a new balance will be established in the habitats conquered by Arion vulgaris. It will not disappear again any time soon, especially in the course of climate change. "We can't get rid of this snail, no chance," said Reise.
Gardeners often take advantage of the appetite to decimate animal populations. For example, slugs find the smell of fermenting substances, which can indicate food, tasty - which is what their preference for beer is based on, as Schrödl explains. Death in a beer trap is probably the most pleasant way to kill a snail - but this method of combating it is not recommended. The seductively scented "Trinkhalle" attracts snails from all over the area - but only a small number of them drown, the rest start eating in large numbers.
There's absolutely nothing stopping Schrödl from sprinkling salt on the animals. "It's like putting salt in an open wound." In addition, salt does not kill safely and contaminates the garden soil. Slug pellets are also not recommended. "There's always a risk that the wrong person will eat it." Two active ingredients are approved: metaldehyde, which is dangerous for hedgehogs and other animals, but also for small children, and iron-III-phosphate, which is also used in organic farming. However, metaldehyde preparations only work well in dry conditions and, like those with iron III phosphate, do not have a particularly attractive taste for slugs.
Rather, it is recommended to only water in the morning, to border beds with border strips of sand or snail fences and to regularly turn potential egg-laying sites, such as boards lying on the ground, into the sun to dry out. Or, for people who can, "A quick cut in the front third kills the snails instantly." Collecting it is also an option - but what to do with the slimy creatures? "Under no circumstances should you tip into nature," emphasizes the Munich expert Schrödl. Urban dog parks, on the other hand, are a good place: "The droppings are eaten by the slugs."
In general, despite all the worries about the lettuce, it should not be forgotten that slugs are the vultures of the gardens: They remove excrement and carcasses, get the composting going and thus contribute to the health of the small ecosystem, as Schrödl says. "Arion vulgaris may be a nuisance in the garden, but it's also super useful."