Slugs are common garden inhabitants, often seen sliming across damp surfaces, particularly after rain. Many people have heard or wondered about the dramatic effect of pouring salt on these creatures. This widespread curiosity stems from a quick, visible reaction that raises questions about the biology behind it.
The Visible Reaction
When salt is applied to a slug, the immediate visible effect is the slug appearing to shrivel or “melt” away. Foamy, bubbly mucus typically exudes from its body. The slug’s body rapidly shrinks and contorts, transforming from its soft, elongated shape into a much smaller, shriveled mass. This rapid physical change happens within moments of contact.
The Science Behind the Reaction
The reaction when salt touches a slug is a direct consequence of osmosis. Slugs possess a highly permeable skin, allowing for easy passage of water. Their bodies also contain a high percentage of water, often 80-90%. When salt, or sodium chloride, is applied to a slug, it creates a highly concentrated solution on the outside of its body.
This external concentration gradient triggers osmosis, which is the movement of water molecules across a semi-permeable membrane from an area of higher water concentration to an area of lower concentration, aiming to equalize solute concentrations. In this scenario, the salt on the slug’s exterior creates an environment with a lower water potential compared to the water-rich interior of the slug’s cells. Consequently, water rapidly exits the slug’s body through its permeable skin to dilute the external salt solution. This rapid water loss leads to severe dehydration, causing the slug to shrivel and appear to dissolve. The foamy substance observed is the slug’s mucus, largely water, expelled as its body undergoes osmotic shock.
Considering the Slug and Other Approaches
The rapid dehydration caused by salt is widely considered a painful and inhumane method. Slugs are living organisms with nervous systems, and such extreme desiccation causes significant distress and a slow demise. While the exact level of pain slugs experience is not fully known, their frantic attempts to escape suggest considerable discomfort.
For those looking to manage slug populations in a garden more considerately, several alternative methods exist:
Encouraging natural predators like birds, frogs, toads, and hedgehogs can help keep slug numbers in check.
Setting up beer traps, where slugs are attracted to and drown in shallow containers of beer, is a common approach.
Physical barriers, such as copper tape or a band of diatomaceous earth around plants, can deter slugs from reaching vulnerable foliage.
Hand-picking slugs during their active hours, typically at night, and relocating them or disposing of them quickly is also an option.
Practices like watering in the morning to allow surfaces to dry by evening and maintaining a tidy garden to reduce damp hiding spots can naturally discourage slugs.
Some gardeners also use specific plants like garlic, thyme, or coffee grounds as repellents.