Skip to content
Grow In Backyard
Slugs & Snails

Slugs vs Snails: The Difference, and Why It Matters

A slug is essentially a snail that gave up its shell. What that one change did to where they live, what they eat, and how hard each is to keep off plants.

By Zakaria 4 min read Save to Pinterest — opens Pinterest in a new tab
A garden snail clinging to the edge of a citrus leaf, its coiled shell facing the camera
Photo: Grendelkhan, CC BY-SA 4.0, via Wikimedia Commons
On this page

A slug is, in evolutionary terms, a snail that gave up its shell. They are both gastropod molluscs, close relatives, and shell loss has happened independently in several snail lineages. Every practical difference between them traces back to that one thing.

Side by side

What the shell changes
SnailSlug
ShellExternal, coiled, can withdraw into itNone visible; usually a small internal plate
Dry weatherSeals the opening and waits it outMust go underground or into damp cover
ColdOverwinters sealed in the shellOverwinters underground, often as eggs
Where it feedsClimbs readily — leaves, stems, low fruitSoil level and below — roots, tubers, seedlings
Getting into tight spacesLimited by shell widthSqueezes through almost anything
EggsClusters in soilClusters in soil, often deeper and more numerous
Calcium needsHigh — the shell demands itLow

Why slugs cost you more

Gardeners tend to blame snails because snail damage is visible — a stripped hosta with a snail sitting on it makes the case itself.

Slugs are the bigger problem in most gardens for three reasons:

  1. They feed below ground. Keeled slugs eat potato tubers, bulbs and roots you never see them reach.
  2. They take seedlings whole, at ground level, overnight — the single most demoralising loss in a vegetable bed.
  3. They stay active longer. Cool, damp conditions that suit a slug perfectly send snails into shelter, so the slug feeding window is much wider.

The calcium point

This one has a practical consequence. A snail must build and repair a calcium carbonate shell, so snails concentrate where calcium is available — chalky and limestone soils, old mortar, gravel paths.

Slugs have no such constraint, which is why acidic, peaty and heavily organic soils often have plenty of slugs and hardly any snails. If your garden seems slug-dominated, that is usually the reason, not luck.

Which is why crushed eggshells do not work

Eggshells are recommended as a barrier constantly. In practice, slugs and snails cross them without difficulty — and for a snail, a scattering of crushed shell is a calcium supply rather than an obstacle. Trials consistently find no useful barrier effect.

Do slugs really have shells?

Mostly yes, hidden. Behind the head, under the saddle-shaped mantle, most garden slugs carry a small flat internal shell plate — the vestige of the external shell their ancestors had. Slice open the mantle of a common garden slug and there it is.

Three arrangements exist:

  • Internal shell plate — most familiar garden slugs
  • Small external shell, too small to retreat into — the semi-slugs
  • No shell at all — a minority

The ones worth keeping

Not every slug in a garden is eating your plants.

  • Leopard slug (large, grey, spotted) — eats other slugs, plus fungi and decaying matter. A genuinely useful animal to have.
  • Many woodland and keeled species feed mainly on decaying plant material and fungi, doing the same recycling job as a worm.
  • Roman and other large snails do far more scavenging than leaf-eating.

This is the argument against the scorched-earth approach: slug pellets do not distinguish, and they also poison the hedgehogs, birds, frogs and ground beetles that eat poisoned slugs.

Controlling either

The good news in all of this is that you do not need to tell them apart to deal with them. The same measures work:

  • Go out at night with a torch after rain and collect by hand. Unglamorous and by far the most effective single method.
  • Beer traps, sunk to the rim, emptied every couple of days.
  • Copper tape around pots — works reasonably on pots, poorly as a garden-scale barrier.
  • Nematodes (Phasmarhabditis), watered on — genuinely effective against slugs specifically, and they reach the ones underground.
  • Encourage predators: frogs, toads, hedgehogs, ground beetles, thrushes and slow-worms. A pond and some undisturbed cover do more over a season than any barrier.
  • Water in the morning, not the evening, so the surface is dry when they are active.
  • Wool pellets and sharp grit — modest effect, better than eggshells.

And if you use pellets, choose ferric phosphate over metaldehyde, which is now banned outdoors in several countries precisely because of what it did to everything further up the chain.

Common questions

What is the difference between a slug and a snail?

The shell. Both are gastropods and they are closely related — a slug is essentially a snail lineage that reduced or lost its external shell, and this has happened independently many times. Everything else follows from that one difference: snails can seal themselves into the shell to survive dry weather and so range further into exposed ground, while slugs have to stay near moisture and spend dry spells underground, which is why they are so much better at getting into dense, damp planting.

Are slugs and snails the same thing?

Not the same, but far closer than most people assume — they are both gastropod molluscs, and slugs are not a separate group so much as snails that lost the shell. The relationship is close enough that most garden slugs and snails eat the same food, leave the same slime trails, are eaten by the same predators, and are controlled by exactly the same methods.

Do slugs have shells?

Many do, just not one you can see. Most garden slugs carry a small internal shell plate, called a mantle shield, buried under the skin behind the head, which is a remnant of the external shell their ancestors had. A few groups, such as semi-slugs, have a small external shell far too small to withdraw into. Only a minority of slugs have no shell remnant at all.

Which does more damage in a garden, slugs or snails?

Slugs, in most gardens, and it is not close. Slugs are usually more numerous, they feed at soil level and underground on roots, tubers and seedlings where snails cannot easily reach, and they stay active in cool damp conditions that send snails into shelter. Snails do more visible damage to leaves at height — hostas, lettuce and low fruit — because climbing is easier for them.

Do slugs and snails bite or carry disease?

They do not bite, and they are harmless to handle in temperate regions apart from ordinary hygiene. The one genuine concern is rat lungworm, a parasite carried by slugs and snails in parts of the tropics, the southern US, Hawaii and Australia, which is a real reason to wash produce well and wash your hands after handling them or their slime — and a reason never to let a child or a pet eat one on a dare.

Are all slugs and snails bad for the garden?

No. A good share of slug species eat mainly decaying plant matter, fungi and even other slugs rather than living plants — the leopard slug, a large spotted species, is a predator of other slugs and worth leaving alone. Roman and other larger snails do more scavenging than people credit. Blanket killing therefore removes some of your own pest control along with the culprits.

Backyard Weekly

Know what to plant, week by week

Join the weekly email for seasonal jobs, fixes for what is going wrong, and gear worth buying.

Not ready to subscribe? The planting calendar is free and needs no email.

By subscribing you agree to our privacy policy. Grow In Backyard will never sell your address.

Keep reading