Why get to know slugs?

I’ve been studying snails and especially slugs since my school days, and one of their great virtues is their diversity in towns and cities: they’re one of the few groups of wildlife where you can see a bigger variety in a garden than in the countryside. I’ve known gardens with over half the British species – imagine a garden with over 100 breeding bird species.

Surprising mollusc facts

Slugs and snails have some surprising features - the largest species live for four-five years, twice as long as mice, voles or garden birds. Some, like the green cellar slug, have distinctive marbling patterns so that, with practice, you can recognise individuals, and follow their movements and behaviour from one year to the next. Acrobatic leopard slugs mate in mid-air dangling on a rope of slime. Some are rather beautiful - cellar slugs are yellow and green marbled, with blue tentacles, and lay strings of lemon shaped eggs.

Rapid changes

When, as a student, I helped write the standard guide to slugs back in 1983, we included 30 species. The latest guide, published in 2014, has 44. I can’t think of any other group of plants or animals which has changed so much in 30 years.

Despite their reputation for being slow and sedentary, molluscs have proven remarkably successful colonists and about half our species have arrived in Britain since the Romans. And some have spread right across the country in just a few years. The slug most likely to make holes in your potatoes is the Budapest slug first seen in Britain in the 1930s. As its name suggests, it’s originally from eastern Europe, but is now ubiquitous. One of the weirdest and least obtrusive species, the worm slug, spread from the Caucasus since the 1920s, and wasn’t seen in Britain till 1972. Within a decade had been seen on islands off Ireland, on loch shores in Scotland, and halfway up Snowdon. It spends much of its time underground, and is believed to feed on soil fungi and small invertebrates. It turned up in the garden of the Wildlife Trust BCN’s offices in Cambourne this year.

Good for gardens

Most slugs and snails much prefer to eat dead and dying plant material than to attack healthy plants. They are probably the most effective ‘detritivores’ in many gardens, because of the way they feed. Whereas earthworms, millipedes and woodlice pull or bite chunks off their food, molluscs rasp away at the surface with their unique tongue or radula, which has several thousand microscopic teeth on its surface. Each tooth with snag the surface of a dead leaf, and tear off a tiny fragment. The finely-chopped leaf then passes through the slug's digestive system, gets thoroughly coated with digestive bacteria, and is excreted as a vegetable pulp, which is probably broken down more rapidly by fungi and bacteria, releasing humus and nutrients back into the soil.

Good for other wildlife

Many birds will feed on slugs and snails, a thrushes specialise, using a stone (the “thrush’s anvil”) to break large and strong snail shells. Frogs, toads, lizards, shrews and hedgehogs will all eat molluscs, and glow-worms feed almost exclusively on small snails. Molluscs can also help disperse the seeds of plants, as they pass through their bodies intact, and are deposited with their own helping of finely-prepared compost.

Avoiding problems

A handful of slug and snail species will attack the plants people are trying to grow, preferring young plants with fresh foliage, but even they may target plants that are already sickly or being attacked by fungi. If you live in an area where molluscs are particularly prevalent, you can minimise damage in a flower garden by growing seedlings larger before planting out, and by sowing seeds in small groups rather than rows. I experimented with honesty seeds a few years ago: over 90% of those in dense rows were attacked by slugs, but only about 5% of seedlings sown in small groups among other plants were damaged.

Not all plants are equally vulnerable: hostas are famously attractive to molluscs (though even among these some varieties are less likely to being munched). Shrubs fare better than herbaceous plants, and plants which are rough, hairy, prickly or strongly aromatic tend to be ignored by slugs and snails. Some plants, such as hardy crane’s-bills and lavenders are well known to be unappealing to slugs, but I’ve pulled together a list of over 70 different plants which are recommended. Some are still vulnerable as seedlings, but are well defended when mature. Note that a few of the plants, such as wolf’s-bane and foxglove are highly poisonous to mammals, too, including people.

In a vegetable garden, you may decide you need to control molluscs. An effective biological control is now available using minute nematodes (eelworms) only 1-2mm long, which enter the bodies of slugs and snails and kill them in a week or two. Rather gruesome, but far better for other wildlife than slug pellets.

Plants less likely to be eaten by molluscs

The following have been recommended by gardeners as slug and snail resistant. Individual experience varies, depending on which species of molluscs are around, and what alternatives they have to eat – most prefer dead leaves, and will resort to healthy plants as a last resort.