Uncovering the fascinating life of the world’s most resilient garden pest
If you have ever gardened, you have met them. Not only have you encountered their slimy tentacled bodies, but you have very likely also cursed them. Chances are you have secretly thrown them in your neighbour’s gardens, or in the hedge. If your living, like theirs, depends on your garden plants, you have probably spent some of your chilly spring nights religiously picking them before they did any damage. The least lucky among them have faced the brutality of your rage and ended up in a bucket, under your sole, or poisoned. Perhaps, however, you proudly feed them to ducks and hedgehogs, thus relieving some pain from your dirty conscience. Regardless of how cruel you have been to them, I am certain that you have also admired the fascinating smoothness and changeability of their shape, as well as their insurmountable skill at obliterating plant tissue.
As strange as it may sound, when it comes to food, slugs are amongst our worst competitors. According to the Royal Horticultural Society, they…
…are persistent and widespread pests which can cause havoc in the garden, eating holes in leaves, stems, flowers, tubers and bulbs. They can cause damage throughout the year on a wide range of plants(*).
To me, both as a vegetable grower and as a scientist, the most amusing aspect of our centuries-long interaction with slugs is our apparent inability to deal with them. According to the vast majority of gardeners, farmers and scientists around the globe, there isn’t an economic or effective, let alone sustainable or regenerative way to control slugs. But what makes them so devastating and uncontrollable? How have they developed such resilience?
In this article, I will try to answer these and more questions, by discussing:
Where slugs are found and why;
How they move;
How long a slug lives;
What slugs eat;
Why we are studying their brains.
If you have read any of my previous posts, you won’t be surprised to discover that this is a non-technical, yet fully-referenced article.
1. For the love of moisture: where slugs live and why
Although most of the terrestrial slug species have not come at odds with humans, several have become major horticultural pests. Slugs are gastropods (a family of molluscs) and belong to the genera Hygromia, Limax, Discus and Arion. The most recognisable slug in Europe, Arion vulgaris or Spanish slug, has become widespread across the continent and is considered one of the 100 most invasive species². However, despite their success as garden pests, the life of slugs is not as easy as you may think.
First of all, slugs don’t have a shell to protect their soft bodies, as opposed to snails, from which they have evolved. Besides increasing their need for protection from predators, the absence of a bite-proof case makes them highly dependent on a moist environment. Indeed, slugs can easily fall victim of dehydration due to evaporative water loss across their epidermis and lungs, and through the deposition of their slime trail³. Within 2 hours, an active slug can lose up to 40% of its initial body weight⁴. For this reason, slugs are more common in wet climates: the percentage of slug species in the land mollusc population of Sicily is 6%, in contrast with 24% in the UK⁵. Slugs also favour heavier soils, as they have developed the ability to survive over summer in soil cracks under clods⁶. However, not having a shell also has evolutionary advantages. The obvious one is the lower need for calcium that allows slugs to survive in acidic conditions (such as English moorland)⁷.
2. Not as slow as you think: how a slug moves
Not having a cumbersome addition to their slender bodies makes slugs more agile and faster than their shelled counterparts⁸.
These terrestrial crawlers must secrete a ribbon of mucus when moving. This is known as pedal (from the Latin, foot) mucus and acts as an adhesive under small strains, while flowing like a viscous liquid for large enough ones⁹. Thus, slugs have developed the ability to create regions of flow in the mucus by locally shearing it while the rest of the mucus is glued to the solid surface they are moving on. These regions (or shear waves) propagate along the length of the foot, allowing the slug to crawl forwards¹⁰.
Along its slimy trail, the large banana slug (gen. Ariolimax) has an average speed of 13 inches per hour¹¹. Even though such speeds would make tortoises look like the flash, it has been pointed out that slugs can transport things faster than broadband can transfer data¹² (this obviously says more about the many paradoxes of internet communication than it does about slugs).
Survival is a matter of balance
To compensate for their low speeds and exposed bodies, slugs protect themselves from predators with a mix of chemical strategies¹³, while they can easily avoid humans due to their night-dwelling habits¹⁴. Slugs are in fact active also on wet days, when their bodies don’t risk desiccation and most birds are occupied with finding a shelter¹⁵. During dry spells, they hide in the ground, where they can surround themselves with a viscous mucus that prevents them from drying out¹⁶. Freezing conditions are as dangerous for them as dry ones, so they prefer temperatures of 10–25°C, although they can be active wherever the air temperature is higher than 5°C¹⁷.
3. From foreplay to motherhood: how long does a slug live?
Slugs are hermaphrodites, which means that any given individual can act either as a female or as a male. This is isn’t good news for slug haters, obviously, as it implies that all slugs can lay eggs — up to 300 each. Slugs sexual encounter can last up to several hours and are mostly foreplay; at the end, the lug penis does its jobs and inseminates its partner. Some individuals can store the opposite sex sperm on them until the conditions are suitable for hatching offspring¹⁸. At that point, eggs made of a translucent sticky substance containing water, calcium and proteins¹⁹ are laid in batches of 10–50, in moist but not waterlogged soil. Most slugs die right after deposing their eggs²⁰. However, one of the crucial reasons for their adaptability is their eggs’ high tolerance to substantial water loss²¹.
Like many of the plants they eat, slugs live on average one year²², but since they breed whenever the temperature conditions allow for it, several generations present in a given environment.
4. From salad to beer: what a slug eats
The slug is a generalist herbivore with distinct preferences for particular plant foods²³.
Not only so, but slugs are also pretty sophisticated when it comes to eating: they are particularly attracted to novel food items (this tendency is called neophilia²⁴) and their preferences vary in response to what they have eaten recently. Neophilia in slugs is not associated to taste, instead, they select food that contains nutrients which were deficient in earlier “meals”²⁵. This virtuous inclination for a balanced and varied diet makes slugs’ eating behaviour hard to predict, and many a plant fall victim to their appetite.
There are, however, some plants slugs systematically avoid, such as those containing tannins and phenolic compounds: onions and garlic (Allium spp.), beech trees (Fagus spp.), most berry plants (Rubus spp.) and aromatic herbs (Origanum, Rosmarinus, Lavandula, etc.), grasses (Poa spp.), tomato and potato leaves (Solanum spp.), Ranunculus, Geranium, Potentilla and Verbascum²⁶. On the slugs menu, the most appreciated greens are ground elder (Aegopodium podagraria), bean leaves (Phaseolus spp.), potato tubers, strawberry fruits (Fragaria spp.), beets (Beta spp.), rapeseed (Brassica napus.), cabbages and other brassicaceae including kale, Brussel sprouts, pak choi and other oriental greens as well as many common horticultural weeds²⁷. A study to assess the palatability of 78 wildflowers to common slugs found out that overall, they prefer man-sown species to naturally occurring ones and favour annuals to perennials²⁸.
Slugs, together with earthworms, beetles and millions of bacteria, fungi and other microorganisms, are highly beneficial members of the soil food web²⁹. They feed on fungi and dead as well as live plant matter, thus accelerating decay and recycling minerals into the soil ecosystem. It has been observed that slugs favour a diet of live plants whenever the conditions are dry, in order to take advantage of their higher water content³⁰.
You might be surprised to discover that some slugs are also carnivorous. The Ghost Slug (Selenochlamys) and the Worm-eating Slug (Testacella) feed mostly on earthworms, whereas giant slugs such as Limax maximus eat almost anything they can find, including the dead and live bodies of other slugs. They have been known to chase other slugs at a top speed of 15 centimetres (6 inches) per minute³¹.
Depending on their eating habits, different slug species are equipped with different types of teeth. Yes, that’s right: slugs have teeth (better known as radulae)! On average a slug has 27'000 radulae, organised in a ribbon-like structure which it uses as a circular saw³².
As many gardeners know, slugs are attracted to beer at least as much as to any other food source, if not more. This peculiar weakness is used to trap and drown them³³ and has been studied over the last decade. Polish scientists have gone as far as assessing the smell preference to several beer brands under field and laboratory conditions, in order to determine which compounds make beer so attractive to slugs. They measured the amount of CO2 emitted in the by gastropods as they smelled the beer, thus effectively judging their response by the amount of excited panting. Unfortunately, although an early series of experiments seemed to point in the direction of decanoic acid, new results published in 2018 were not conclusive³⁴. What we know is that anything fermented or fermenting is hugely attractive to slugs, be eat a solution of yeast and sugar or a pint of cheap beer.
Fig: The teeth of some slug species: Candidula intersecta (herbivore), Arion vulgaris (herbivore), Lauria cylindracea (surface grazer), Oxychilus cellarius (carnivorous)³²
5. Inside the slug brain
Slugs can evaluate evidence and make decisions³⁵. They use primarily their sense of smell to choose what to eat, find out whom to mate with, and how to avoid death. In a series of experiments conducted in the 1970s, slugs brains showed the ability to learn a sequence of liked and disliked foods³⁶. Moreover, a slug’s brain can be kept alive for several hours in a test tube and has neurons as big as 200–300 microns, which means they are big enough to be seen without a microscope³⁷. For these reasons, these surprising molluscs have been used as a model organism to discover the mechanisms of learning and memory storage in mammalian brains. Prof. Gelperin, who has dedicated his life to studying their brains, says
Before you step on a slug, or sprinkle the poison, pause and consider the creature’s marvellous complexity and place in the scheme of things.³⁸
The scientific articles and all the other references corresponding to the numbered footnotes can be opened by clicking on the link below. They are in a google doc format. I suggest that you open them in a separate tab (by pressing CTRL+left-click) so that you might refer back to them while you read the article, without the need to scroll back and forth.