Slugs are familiar slimy pests that cause havoc in the garden, eating and making holes in leaves, stems, flowers, tubers and bulbs. There are about seven 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.
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, will alert you to the level of activity. Damage is most severe during warm humid periods.
Slugs 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 will be difficult to grow these plants if you have a big slug problem.
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
Slug damage on potato caused by black keel slug, Milax spp.
Slugs are so abundant in gardens that some damage has to be tolerated. They cannot be eradicated so target control measures on protecting more vulnerable plants, such as seedlings and soft young shoots on herbaceous plants.
A biological control ('Nemaslug') specific to slugs, 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 soil and soil temperatures of 5-20ºC (41-68ºF) are 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. Protect transplants with plastic bottle cloches
- Place traps, such as scooped out half orange, grapefruit or melon skins, laid cut side down, or jam jars part-filled with beer and sunk into the soil near vulnerable plants. Check and empty these regularly, preferably every morning. Proprietary traps are available from garden centres
- Place barriers, such as copper tapes (Vitax Copper Slug Tape, Agralan Copper Slug Tape, Growing Success Slug Barrier Tape) around pots or stand containers on matting impregnated with copper salts (Slug and Snail Shocka, Agralan Slug and Weed Mat). Moisture-absorbent minerals can be placed around plants to create slug barriers (Westland Earth Matters Slug Blocker Granules, Growing Success Slug Stop andVitax Slug Off, ). Gel repellents (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
Scatter metaldehyde slug pellets (Scotts Slug Clear Advanced Pellets, Bio Slug and Snail Killer Pellets, Westland Eraza Slug & SnailKiller Pellets, Doff Advanced Slug Killer or Doff Slug Killer Blue Mini-pellets) thinly around vulnerable plants, such as seedlings and young shoots on herbaceous plants.
A liquid formulation of metaldehyde (Scotts Slug Clear) is available for watering on to ornamental plants and the soil.
Pellets may harm other wildlife, pets and young children if eaten in quantity. A relatively new form of pelleted bait containing ferric phosphate (Growing Success Advanced Slug Killer, Bayer Organic Slug Bait or Vitax Slug Death XL) is relatively non-toxic to vertebrate animals.
Most plants, once established, will generally tolerate slug damage and control measures can be discontinued.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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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.
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