What are slugs?

Slugs are gastropods; single-shelled, soft-bodied animals in the mollusc group of animals. Slugs can use their rasping tongues to make holes in leaves, stems, buds, flowers, roots, corms, bulbs and tubers of many plants. There are many control options available for slugs and snail but despite this they remain a persistent pest. The RHS is carrying out a scheme of research on gastropod control methods to improve the advice we can give to home gardeners.

Most slugs feed at night, and the slime trails, if present, can alert you to the level of activity. Damage is usually most severe during warm humid periods.

Slugs can make a meal of a wide range of vegetables and ornamental plants, especially seedlings and other soft growth. Hostas, delphiniums, dahlias, gerberas, sweet peas and tulips are regularly attacked by slugs, and it can be difficult to grow these plants if you have a big slug problem. In the vegetable garden peas, beans, lettuce, celery and potato tubers are often damaged.

Many larger slugs primarily feed on decomposing organic matter such as dead leaves dung and even dead slugs. In the compost heap they can be a valuable part of the composting process.

Symptoms

You may see the following symptoms:

• Slugs sometimes leave behind slime trails, which can be seen as a silvery deposit on hard surface, leaves and stems
• Slugs can make irregular holes in plant tissue with their rasping mouth parts. They can kill young seedlings by completely eating them
• Long-keeled slugs of the Milacidae family live underground and tunnel into potato tubers and bulbs. In some gardens these slugs can damage a large proportion of the tubers of maincrop potatoes. Damage is similar to that of wireworms

Control

Slugs are so abundant in gardens that some damage has to be tolerated. They cannot be eradicated so targeting control measures to protect particularly vulnerable plants, such as seedlings and soft young shoots on herbaceous plants will give the best results.

Non-chemical control

Biological control

A biological control ('Nemaslug') specific to molluscs, with no adverse effect on other types of animal, is available in the form of a microscopic nematode or eelworm that is watered into the soil. The nematodes (Phasmarhabditis hermaphrodita) enter slugs' bodies and infect them with bacteria that cause a fatal disease.

A moist warm soil (temperatures of 5-20ºC (41-68ºF)) is required, therefore control is most effective during spring to early autumn. Best results are achieved by applying in the evening to moist but well-drained soils; control may be less successful in heavy soils, such as clay. The nematode is available from refrigerated cabinets in some garden centres or by mail order from suppliers of biological controls (download pdf below).

Other non-chemical controls

There are many preventive measures that have been used by gardeners to minimise slug damage. Most of these do not have any scientific evidence to prove that they are effective. These measures include:

• Transplanting sturdy plantlets grown on in pots, rather than young vulnerable seedlings. Transplants can be given some protection with cloches
• Torchlight searches on mild evenings, especially when the weather is damp; hand-picking slugs into a container. They can then be taken to a field, hedgerow or patch of waste ground well away from gardens, or killed in the freezer before being added to the compost heap or put in the bin
• Some birds, frogs, toads, hedgehogs, slow-worms and ground beetles eat slugs and these predators should be encouraged in gardens
• Raking over soil and removing fallen leaves during winter can allow birds to eat slug eggs that have been exposed
• Traps, such as scooped out half orange, grapefruit or melon skins, can be laid cut side down, or jars part-filled with beer and sunk into the soil near vulnerable plants. Check and empty these regularly, preferably every morning. Proprietary traps are also available from garden centres and mail order suppliers
• Barriers, thought to repel slugs, include rough or sharp textured mulches and substances thought to be distasteful or strong smelling. Copper-base barriers have been shown to repel slugs in some studies. A recent RHS study in a garden-realistic scenario however, found no reduction in slug damage from barriers made of copper tape, bark mulch, eggshells, sharp grit or wool pellets

Most of these non-chemical control options have very little scientific research into them, but the RHS is hoping to address this knowledge gap and is carrying out a range of scientific studies.

Potatoes and slugs

The slugs that damage potatoes spend much of their time in the soil where they do not come into contact with slug pellets. The nematode treatment (see above) can be effective. Damage usually begins during August and becomes progressively worse the longer the crop is left in the ground. Early potatoes usually escape damage; maincrop potatoes should be lifted as soon as the tubers have matured if the soil is known to be slug infested. Heavy applications of farmyard manure and other composts can encourage slugs, and so inorganic fertilizers should be used where slugs are a problem.

Potatoes vary in their susceptibility to slugs. ‘Maris Piper’, ‘Cara’, ‘Arran Banner’, ‘Kirsty’,  ‘Maris Bard’, ‘Maris Peer’, ‘Kondor’, ‘Pentland Crown’ and ‘Rocket’ are frequently damaged, whereas ‘Romano’, ‘Pentland Dell’, ‘Pentland Squire’, ‘Wilja’, ‘Charlotte’, ‘Golden Wonder’, ‘Kestrel’, ‘Estima’, ‘Stemster’, ‘Sante’ and ‘Pentland Ivory’ are less susceptible.  Damaged potatoes are more vulnerable to storage rots and the crop should be sorted into sound and damaged tubers, with the latter being stored separately for early consumption.

Pesticides

Following the manufactures instructions scatter slug pellets thinly around vulnerable plants, such as seedlings, vegetables and young shoots on herbaceous plants. It is important store pellets safely and scatter them thinly as they can harm other wildlife, pets and young children if eaten in quantity.

As of March 2021 there is only one type of slug pellet approved for use in gardens, those based the active ingredient ferric phosphate. This type of slug pellet is approved for use by organic growers. Products include Growing Success Advanced Slug Killer, Solabiol Garden Slug Killer, Vitax Slug Rid, Doff Super Slug Killer, Sluggo Slug & Snail Killer and SlugClear Ultra3. To protect children and pets pellets must be used and stored carefully,  as directed on the packaging. 

Metaldehyde ban 
In September 2020 the government announced the withdrawal of metaldehyde slug control, with product sales ending in March 2021 and remaining stocks to be used up or disposed of by 31^st March 2022. The withdrawal was planned following advice from the UK expert committee on pesticides and the Health and Safety Executive (HSE) who consider that metaldehyde poses an unacceptable risk to birds and mammals. An initial withdrawal in 2019 was overturned due to problems with incorrect implementation.

Product names for slug pellets for home garden use containing metaldehyde that will be affected by this withdrawal include: Ultimate Slug and Snail Killer, Deadfast Slug Killer, Doff Slug Killer Blue Mini Pellets and Westland Eraza Slug and Snail Killer. Please be aware that these and other product names may be relaunched using ferric phosphate as the main ingredient instead, and check any products you have stored to make sure they are used up or disposed of safely by the end of March 2022. 

Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Most plants, once established, will tolerate some slug damage and control measures can be discontinued. The RHS is researching sustainable control options for slugs and snails.

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf)

Biology

Slugs and snails, along with other animals including conches and limpets, are gastropods. A gastropod is a single shelled, soft-bodied animal in the mollusc group of animals. The name gastropod comes from the Greek words gaster, meaning stomach, and poda, meaning feet. All gastropods have a muscular foot that they use to move around. Slugs have evolved to have a reduced shell that often cannot be seen from the outside.

There are 44 species of slug in the UK, only some of which cause damage to plants. Many species feed on dead plant matter so some slugs of the slugs in the garden may actually be beneficial. Slugs vary in size from the grey field slug (Deroceras reticulatum), which is no more than 5cm (about 2in) long, to the large black slug (Arion ater), which can be 12cm (about 5in) when fully extended. Some slugs vary in colour; Arion ater can be black, orange-brown or buff coloured.
 
Most slugs feed in or on the soil surface, but keeled slugs (Milax species) live and feed mostly in the root zone.

Slugs remain active throughout the year, unlike snails, which are dormant during autumn and winter. Warmer weather, combined with damp conditions greatly increases their activity. Slugs are most active after dark or in wet weather.

Reproduction occurs mainly in autumn and spring, when clusters of spherical, yellowish-white eggs can be found under logs, stones and pots.

RHS slug research

Slugs and snails come out near the top of the RHS Top Ten most enquired about pests every year. Despite the array of control measures available they are still considered persistent pests. To tackle this the RHS launched scheme of research in 2015. Current and recent work includes investigating:

• Which slug species occur in gardens, and which of those are actually behaving as pests, are neutral or beneficial garden wildlife? This study uses a citizen science approach.
• How is Britain’s slug fauna changing? New species are arriving and others are declining; the RHS is asking for records of the Cellar Slugs as a case study.
• How can gardeners use ‘Integrated Pest Management’ techniques to get better control of damaging gastropods? Can existing control measures be combined to give better control? This study focussed on combining the most well evidenced control measures and its results and evidence based advice will be published later in 2020.
• Why does the nematode biological control (Phasmarhabditis hermaphrodita) work for some gardeners but not all? Working with collaborators at Liverpool John Moores University we are testing survival of P. hermaphrodita in different soil types and under different conditions.
• Can we use slug and environment friendly methods to protect our plants? Unfortunately the five slug barriers tested in this experiment did not give any protection! The search is not over though, and testing sustainable control measures is a priority for RHS research.

Some plants less likely to be eaten by slugs

Some herbaceous plants are less likely to be eaten by slugs and snails, these are listed below

Acanthus mollis (bear's breeches)
Achillea filipendulina
Agapanthus hybrids and cultivars
Alchemilla mollis (lady's mantle)
Anemone × hybrida (Japanese anemone), A. hupehensis (Japanese anemone)
Antirrhinum majus (snapdragon)
Aquilegia species
Armeria species
Aster amellus, A.× frikartii, A. novae-angliae (Michaelmas daisies)
Astilbe × arendsii
Astrantia major
Bergenia (elephant's ears)
Centaurea dealbata, C. montana
Corydalis lutea
Cynara cardunculus (globe artichoke)
Dicentra spectabilis (bleeding heart)
Digitalis purpurea (foxglove)
Eryngium species
Euphorbia species (spurges)
Foeniculum vulgare (fennel)
Fuchsia cultivars
Gaillardia aristata
Geranium species
Geum chiloense
Hemerocallis cultivars (day lilies)
Papaver nudicaule (Iceland poppy)
Pelargonium
Phlox paniculata
Physostegia virginiana (obedient plant)
Polemonium foliosissimum
Polygonum species
Potentilla hybrids and cultivars
Pulmonaria species (lungwort)
Rudbeckia fulgida
Salvia × superba
Saxifraga × urbium (London pride)
Scabiosa caucasica (scabious)
Sedum spectabile (ice plant)
Sempervivum species (houseleeks)
Sisyrinchium species
Solidago species (golden rod)
Stachys macrantha
Tanacetum coccineum (pyrethrum)
Thalictrum aquilegiifolium
Tradescantia virginiana
Tropaeolum species (nasturtium)
Verbascum species (mullein)