Q What conditions encourage slugs and snails?
A Slugs and snails are molluscs and need a moist environment to survive. Slugs thrive in damp soil or plant debris, retreating deep into the soil when the weather is dry. Snails maintain their moisture level by going into their shells. Both are active mainly at night, especially in spring and autumn when the weather is warm and wet.
Caption: Which? Gardening magazine trials found that organic slug pellets are as effective as metaldehyde ones
Q Why are slugs and snails so common?
A There are thought to be around 200 slugs in every cubic meter of garden soil. Slugs lay around 40 eggs and snails around 60 eggs up to six times a year. They hatch as soon as the temperature is over 5C. Because they’re so slimy they’re relatively unpopular with predators. However, some are eaten by birds, hedgehogs, moles, shrews, frogs and predatory beetles.
Caption: You may find slug or snail eggs in compost or soil
Q What do slugs and snails like to feed on?
A Slugs and snails feed on a wide variety of plants – although they prefer succulent plants, such as hostas – and adapt to many environments. Seedlings and young plants are vulnerable as they are so small, whereas larger plants will continue to grow if they lose a few leaves.
Underground, they bore into potatoes or similar plants, but they attack plants above ground, too. They are about 6cm long, black, brown or grey.
Q How many kinds of slug are there?
A There are many kinds of slug, but only a few are serious pests.
The grey field slug (Deroceras reticulatum) is the most common and destructive. It’s about 40mm long and light grey to fawn in colour. It feeds above ground even in cool weather.
Keeled slugs (Milax gagates, Tandonia budapestensis, Tandonia sowerbyi) have a ridge down their backs and are mainly soil-dwelling. Uney.
The chestnut slug (Deroceras panormitanum) is a faster-moving version of the grey field slug. It prefers gardens and greenhouses.
Garden slugs (Arion hortensis, Arion distinctus) are very common. Although only about 3cm long, they do much damage both above and below ground.
The large black slug (Arion ater) is very big, up to 20cm long, and may be black or orangy-brown. It's often noticed after rain in the summer. It’s less destructive than smaller slugs, though is by no means harmless.
Caption: Keeled slugs tunnel into potato tubers underground
Q Are slugs beneficial?
A Most slugs live underground, and although some will eat roots and leaves, many others feed on plant debris and so contribute to keeping soil healthy.
Q Are all snails garden pests?
A Most snails aren’t significant pests in the garden. The banded ones do little harm, but the garden snail (Helix aspersa) and strawberry snail (Trichia striolata) are more common and damaging. The garden snail is the large, common kind with a grey-brown shell. The strawberry snail is small, about 13mm long, grey to brown with a flattened shell. Snails are less numerous than slugs, but are seen more often. They climb higher, as their shells protect them from drying out.
Caption: Snails often hide in plants
Q How is slug or snail damage different from other troubles?
A To check if slugs or snails really are to blame, look for slime trails and under pots, stones and foliage for the pests themselves. If you’re still not sure, a night-time foray with a torch may help. Below-ground damage to seedlings, roots and tubers could be the work of soil pests, such as chafer grubs or wireworms. Usually, these pests will be found nearby; simply scrape away the surface layer of soil. However, you may have to look in the soil around neighbouring plants.
Potato tubers and bulbs are usually attacked after watering or rain in summer, or in moist autumn conditions. Attacks at other times could be by cutworms (a soil-dwelling caterpillar pest). If you see millipedes, springtails or woodlice, they are just taking advantage of the food supply and are not the cause of the damage. Nibbled produce, such as celery stems and lettuce hearts, could be the work of caterpillars, but these leave dark droppings (frass). If slugs are to blame, they will usually be close to the damage.
Q Does soil type make a difference to slugs and snails?
A Snails prefer a calcium-rich chalky soil to form their shells. Slugs are at their most prolific on soils that retain moisture, contain a lot of organic matter and have many gaps below ground where they can hide: clay and heavily manured soils are ideal. Sandy soils are much less suitable, as they dry out more quickly and there are few hiding spaces below ground.
Q What can I use to deter and get rid of slugs?
A Gardeners traditionally use things such as crushed eggshells, sharp sand and soot to repel slugs, but these have limited success. There are also a wide range of barrier methods available, from granules that are supposed to dry them up, to copper rings. In our trials (Mar 15) we found that none of them work well.
When Which? Gardening magazine tried chemical controls (Apr 11) we had good results. Slug pellets contain either metaldehyde (eg Bio Slug Mini Pellets) or ferric phosphate (eg Growing Success Advanced Slug Killer). For plants in the ground, we found the biological control Nemaslug is effective against slugs, but not snails (May 08).
Caption: Scatter slug pellets sparingly
Q What chemical controls can I use against slugs and snails?
A There are two chemical options: metaldehyde and ferric phosphate. Metaldehyde, the active ingredient in most slug pellets, causes slugs to become paralysed. Once immobilised, the slugs dehydrate, although in very wet conditions they may recover. Ferric phosphate-based pellets stop slugs and snails feeding. They are currently approved for use by organic gardeners. In our tests we found these to be as effective as metaldehyde-based slug pellets.
Q Do slug pellets pose a risk to wildlife and pets?
A In high doses metaldehyde is toxic to birds and mammals, including humans. However, there is no conclusive evidence that metaldehyde-based pellets are harmful when used according to instructions. To minimise any risk, the pellets are dyed blue (to deter birds) and many contain a bitter-tasting animal repellent.
Ferric phosphate pellets are not thought to be damaging to mammals and birds. However, some research suggests that ferric phosphate pellets may not be as environmentally friendly as first thought. Some ferric phosphate pellets also contain a 'chelating agent', called EDTA, to make the iron more soluble. This may increase the amount of iron in ground water, which can be toxic to earthworms in high doses.
If you use either kind of pellets, concentrate your efforts on vulnerable plants; don’t attempt to eliminate slugs from the whole garden. Scatter the pellets thinly, as directed on the packet, as doses have been calculated to offer the best protection. Don't leave them in heaps. Regularly collect up the poisoned slugs and snails and consign them to the dustbin. Always store pellets in their original container, and safely out of reach of children and pets.
Q Are there any biological slug controls for slugs?
A Phasmarhabditis hermaphrodita is a biological control, sold by mail order only. It's watered into the soil, so is only effective against slugs. It contains millions of naturally occurring nematodes that penetrate the slug’s mantle, carrying a bacterium that infects and kills the slug. It’s best used in spring and autumn when the soil is warm and moist.
Q How do I keep slugs off potatoes?
A Potato damage can be controlled to some extent by applying chemical or biological control during late July or August. Some potato varieties are much less affected than others. Avoid 'Marfona', 'Estima' and 'Maris Piper' if your soil is slug-infested. Instead try 'Pentland Dell', 'Lady Rosetta' or 'Romano', which are less susceptible. Alternatively, use an early-maturing kind, such as 'Heather', 'Kestrel' or 'Wilja', and lift these before the end of August, while the soil is still dry. In a wet summer, lift as soon as the trouble becomes apparent.
Caption: Keeled slugs tunnel into potato flesh
Q Can slugs and snails be discouraged?
A Keep weeds down and clear away dead plants and rubbish. Often the number of slugs and snails varies around the garden; try growing vulnerable plants in areas where you see none. Fresh or partly rotted organic matter is a banquet for slugs, so use well-rotted manure or compost instead, and avoid mulching vulnerable plants such as lettuce and potatoes
Suppliers of biological controls for slugs
Defenders 01233 813121 defenders.co.uk
Green Gardener 01493 750061 greengardener.co.uk
Ladybird Plant Care 0845 0945 499 ladybirdplantcare.co.uk/slug.html
The Organic Gardening Catalogue 01932 253666 organiccatalogue.com