Slugs

Slugs are familiar slimy pests that cause havoc in the garden, eating and making holes in leaves, stems, flowers, tubers and bulbs. There are several species of slugs that are garden pests. They can cause damage throughout the year on a wide range of plants, but seedlings and new growth on herbaceous plants in spring are most at risk and may need protection.

Slug on leaf

Quick facts

Common name Slugs
Scientific name Various species, most common are Milax, Deroceras and Arion species
Plants affected Many ornamental plants and vegetables in gardens and greenhouses
Main symptoms Holes in leaves, stems, flowers and potato tubers; seedlings can be killed
Most active Year round

What are slugs?

Slugs are soft-bodied molluscs that make holes in leaves, stems, buds, flowers, roots, corms, bulbs and tubers of many plants.

Most slugs feed at night, and the tell-tale 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 all 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 leaves and stems
  • Slugs cause irregular holes in plant tissue made by their rasping mouth parts. They can kill young seedlings by completely eating them
  • Black keeled slugs (Milax spp.) 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

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.

Other non chemical controls

Preventive measures you can take include:

  • Transplant sturdy plantlets grown on in pots, rather than young vulnerable seedlings. Transplants can be given some protection with cloches
  • Place traps, such as scooped out half orange, grapefruit or melon skins, 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
  • Place barriers, such as copper tapes (e.g. Vitax Copper Slug Tape, Agralan Copper Slug Tape, Growing Success Slug Barrier Tape) around pots or stand containers on matting impregnated with copper salts (e.g. Slug and Snail Shocka, Agralan Slug and Weed Mat). Moisture-absorbent minerals can be placed around plants to create slug barriers (e.g. Westland Earth Matters Slug Blocker Granules, Growing Success Slug Stop and Vitax Slug Off). Gel repellents (e.g. Westland Earth Matters Slug Blocker Gel, Doff Slug Defence Gel) can also be used to create barriers around plants. These products are widely available from garden centres
  • Go out with a torch on mild evenings, especially when the weather is damp, and hand-pick slugs into a container. Take them to a field, hedgerow or patch of waste ground well away from gardens, or destroy them in hot water or a strong salt solution
  • Some birds, frogs, toads, hedgehogs, slow-worms and ground beetles eat slugs and these predators should be encouraged in gardens
  • Rake over soil and remove fallen leaves during winter so birds can eat slug eggs that have been exposed

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.

Chemical control

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.

There are two types of pellet available to the gardener; those that contain metaldehyde (e.g. Slug Clear Ultra Pellets, Bayer Bio Slug and Snail Killer, Doff Slug Killer Blue Mini Pellets, Westland Eraza Slug and Snail Killer) or ferric sulphate (e.g. Growing Success Advanced Slug Killer, Bayer Natria Slug and Snail Control, Bayer Organic Slug Bait, Vitax Slug Rid, Doff Super Slug Killer, Sluggo Slug & Snail Killer). Ferric sulphate is relatively non-toxic to vertebrate animals.

A liquid formulation of metaldehyde (Slug Clear) is available for watering on to ornamental plants and the soil, it should not be applied to edible plants.

Most plants, once established, will tolerate some slug damage and control measures can be discontinued.

Download

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

Biological control suppliers (Adobe Acrobat pdf)

Biology

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 live 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.

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)  

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