Invertebrates
Slug
Gastropoda · also called land slug
A slug is a snail that threw away its house. Freed of the shell, it can pour itself into a crack in a wall or under a plant pot, but it pays for that with a body that dries out fast, which is why slugs come out mainly at night and in the wet. And despite their reputation, most of them are not after your lettuces.
Overview
Slug is a body form rather than a single group — a land snail with the shell shrunk or gone, which has evolved many times over. A slug glides on a muscular foot on a ribbon of mucus, senses the world with two pairs of tentacles (eyes on the upper pair), breathes through a single hole on the right side, and rasps its food with a toothed radula. Most species eat dead and rotting matter, not living plants.
Slime and courtship
A slug makes two mucus types, a thin one to glide on and a sticky one to climb and hang. Slugs are hermaphrodites, and the leopard slug’s courtship is a spectacle — two slugs entwined and hanging in mid-air from a rope of mucus while they mate.
Friend more than foe
Of the many slug species in a garden, only a few, chiefly the grey field slug, do real damage to plants, while the rest recycle dead material and the big leopard slug even hunts the pest species. Hedgehogs, thrushes, frogs, toads, slow-worms, and ground beetles all eat slugs.
Status
Slugs as a group are Not Evaluated. Most are abundant, several are major invasive species — the Spanish slug is spreading across northern Europe — and a few narrow-range natives, like the Kerry slug, are protected.
Slug: common questions
What is the difference between a slug and a snail?
A snail carries a coiled external shell large enough to withdraw its whole body into, while a slug does not. Beyond that they are the same kind of animal, land gastropod molluscs, with the same foot, mucus, tentacles, and radula. Slug is simply the name for the shell-less or nearly shell-less body plan, and it is not one branch of the family tree but a form that many separate snail lineages have arrived at independently, because losing the shell frees an animal to squeeze into tight, damp hiding places and saves the calcium and energy a shell costs. Many slugs keep a small, flat remnant shell hidden under the shield on the back, and a few, the semi-slugs, have a shell too small to hide in.
Do slugs really eat my plants, or is that unfair?
It is partly unfair. Of the dozens of slug species found in a typical garden, only a handful cause meaningful damage to living plants, and the rest feed mainly on decaying leaves, fungi, and dead matter, doing the useful job of recycling. The grey field slug is the main culprit for seedling and salad-crop damage in temperate gardens and farms, along with a few keeled and netted slugs, while the large black and leopard slugs that people most often notice are mostly harmless recyclers, and the leopard slug will even hunt and eat the pest species. So a garden with lots of slugs is not necessarily a garden losing lots of plants.
How can I protect plants from slugs without poison?
Several approaches used together work well. Encourage predators by providing cover for hedgehogs, frogs, toads, slow-worms, and ground beetles, and by leaving some wild corners. Water in the morning rather than the evening so the soil surface is drier at night when slugs move. Use physical barriers around vulnerable plants, such as copper rings, sharp grit, wool pellets, or crushed eggshell, and raise seedlings to a robust size under cover before planting out. Remove daytime hiding places near beds, such as loose boards, pot rims, and dense low weeds. Hand-collect after dark or after rain, especially in spring. Nematode biological controls, watered into the soil, target slugs specifically and spare other wildlife.
Are any slugs dangerous?
Not directly to people, but there is one health link worth knowing. Slugs and snails can carry the larvae of the rat lungworm, a parasite that in rare cases causes a form of meningitis in humans, mostly in parts of Southeast Asia, the Pacific, and increasingly the southern United States, and infection comes from eating a slug or snail, raw or in unwashed salad leaves, not from touching one. The sensible precautions are to wash garden produce well, wash hands after gardening or handling slugs, and never eat a wild slug or snail on a dare. The slime itself is harmless, though it is very hard to wash off skin and clothing.