That’s nice. If you have just placed strawberries and young lettuce plants in your vegetable garden box on your balcony three floors behind, the snails suddenly appear to have eaten them. Where do they come from? And, even more intriguing: how did they know that there was something to be gained there?
There are according the online ‘land snail species finder’ from Naturalis Biodiversity Center 95 species of terrestrial snails in the Netherlands. In addition, there are, judging by a search map of the Dutch Malacological Society25 species of slugs.
It is mainly the slugs that have a bad reputation when it comes to eating garden and balcony plants. They mainly focus on the “softer plant parts and young seedlings”, according to the website of the Belgian organization Natuurpunt. That snails also often eat green plants “is a bit of a myth,” says evolution biologist and Naturalis researcher Menno Schilthuizen. “Most species live on dead leaves, algae and fungi and only very occasionally on green plants, and then really only the young leaves or cultivated crops made edible by man, from which all antibodies have been bred.”
Sticky mucus layer
So also the garden snail eats, Cepaea nemoralis, especially algae and lichens – including on trees or balconies. Because the garden snail is “a real tree snail that easily climbs twenty to thirty meters high trees, and can therefore also do that with buildings,” says Schilthuizen. Also the shagri snail Cornu aspersum is a climber, and can then end up on balconies. “Other snail species, such as the shrub snail Arianta arbustorum, are much more ground-bound and you rarely find them higher than fifty centimeters above the ground, even in the forest.” Not all species of slugs can climb either. Good climbers include the red and black road snail and various species of keel snails. Thanks to the sticky slime layer on their lower body, they do not fall down.
Snails can smell with their antennae on their heads. In an article from 2017in the magazine Invertebrate Neuroscience, the Hungarian biologist Tibor Kiss underlines the importance of the snail’s sense of smell when searching for food. It does seem, he emphasizes, that snails can only trace food with their sense of smell if they have eaten the plant, algae or lichen in question before. In addition, the distance over which they can smell food is most likely small. A 1999 experiment in which a snail could detect an ethanol cloth or a piece of apple by scent was carried out over a distance of six centimeters – now not exactly comparable to the distance from the ground floor to a balcony.
Avoid warm soil
What can play a role is the search for a cool, moist place, especially on hot and dry days. In the article ‘Snails in the sun’which will be launched in 2019 Ecology & Evolution German researchers write that snails climb into artificial or non-artificial objects to avoid the warm ground. Although they are sometimes more exposed to the sun during such a climb, the air temperature a little above the ground is so much lower that it is actually worthwhile, the biologists conclude. And then once they are on a cool balcony, they may accidentally come across food there.
Finally: not every snail has to climb. Some individuals hitch a ride with potting soil or balcony plants, and snail eggs can even be spread through bird droppings.