Slugs are widespread animals which can cause problems in the garden, eating holes in leaves, stems, flowers, tubers and bulbs. 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.
Quick facts
Common name: Slugs
Scientific name: Various species, most common are species in the Milacidae, Deroceras, and Arion spp.
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
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.