Slugs and snails
There are approximately 30 species of slug in the UK, and over 100 species of snail. And sometimes it feels like they are all munching through your precious plants!
Before we look at how to prevent their damage in the garden, let's learn a little more about them.
Slugs have evolved from snails, and both are molluscs and gastropods (literally, an eating foot). They have nearly 30 thousand teeth and both emit a mucus which helps them glide over a surface. Did you know that a snail can pass over the edge of a razor blade without damage?
The Garden Snail Cornu aspersum, has a hard, thin shell approx 30 mm in diameter. It is usually dark or golden brown, or chestnut with yellow stripes. It is a herbivore, eating vegetables, fruit, flowers and cereals, as well as rotting plant material.
- The body is soft, slimy and brownish-grey. As it moves, it secretes a mucus which helps it glide across a surface by rhythmic waves of contraction.
- Unlike a slug, the snail can climb up walls and stems, using its shell as protection.
- When injured the animal produces a defensive froth of mucus to repel enemies such as aggressive small ants.
- When it's freezing, the snail alters its blood structure to prevent the formation of ice in its tissues. In dry weather, it seals the shell opening with a thin membrane of dried mucus. This helps retain moisture and protects it from insects.
- The snail's head has four tentacles; the upper two have eye-like light sensors, and the lower two are tactile and used for smelling.
How do snails reproduce?
Snails are hermaphrodite and take up to 2 hours to mate. They lay a batch of about 80 spherical pearly-white eggs into crevices in the topsoil or sheltered under stones. In a year it may lay six batches or so. That's nearly 500 young snails, which will take one or two years to reach maturity.
How do snails move?
- The snail moves at a top speed of .012 metres per second (compare this with Usain Bolt who runs a thousand times faster, at 12 mts per second.) Hence the phrase, snail-like slowness.
- However, helped by its mucus (and unlike Bolt), it can go up a slope at any angle, including upside down; it can resist being pulled off a firm surface with an adhesive strength several times its own weight; rest on a surface at any angle without any expenditure of energy; and it can pass over the edge of razor blade without harm.
Slugs use their mucus as a navigation system, as it helps them follow the trail back to their tunnels and feeding sites.
- The slug's mantle is an area behind the head and is made of thick flesh. If a slug is frightened or not active it will retract its head into the mantle for protection. The keel is a ridge that runs the length of the back of some species of slug.
- Slugs live in dark damp places or underground. They need persistent moisture to prevent drying out. Cool weather, rain and fog are perfect for slugs. However in a long dry spell, slugs can encase themselves in a papery cocoon-like structure and attach themselves to a wall or a tree and wait it out.
- Most UK slug species are herbivorous, eating leaves, flowers, fruits, mushrooms, lichens and decaying plant material. Some, such as the Leopard slug, are carnivorous and hunt other slugs and snails.
What will eat or kill a slug?
Slugs have a variety of predators such as hedgehogs, birds (particularly thrushes), toads and ground beetles.
When a slug is attacked by a predator it will contract its body to make it a smaller target. The mucus that covers a slug’s body doesn’t taste very nice and is slippery. This is why you often see birds wiping slugs on the grass before they eat them.
There are four common species of slug:
The Netted or Grey Field slug (Deroceras reticulatum), 3 - 5 cm long, is normally a light brown above, with a chain of darker veins and blotches, and pale with a darker central zone below. It is a pest in gardens as it feeds nearly all year round, mostly on seeds and plants above ground.
The Common Garden slug (Arion distinctus/ Arion horntensis), 3 cm long, is brown and striped lengthwise, often with tiny gold spots, and with an orange or yellow underside. Attacks both leaf and root crops, and is a major pest of potatoes.
The Common Keeled slug (Tandonia budapestensis), up to 6cm long, is black or grey with an yellow-orange ridge along its body, a pale underside and colourless mucus. It lives underground, feeding on newly drilled seeds such as potatoes. As it spends most of its time underground it is hard to control.
The Large Red slug and Black slug (Arion ater) is either orange-red or black with an orange fringe. Despite it's size, up to 12cm, this species of slug is much less damaging than the other pest species. When alarmed, it contracts into a spherical shape and might rock from side to side.
Prevention and control
Slugs and snails may be the number one enemy of organic growers. They love the rich soil with damp compost, and fresh green growth. However, there are many ways to protect plants, and to prevent mollusc attack. Slug pellets should be your last resort - and only those suitable for use in an organic garden. See below .
First, aim to use several different methods. Then always check regularly - especially after wet or damp weather.
Protect young plants with a simple bottle cloche or other barrier method until they are established and growing strongly.
Choose resistant vegetable varieties: Some varieties of potato tuber, for instance, are particularly resistant to soil dwelling slugs. These include Ambo, Cara, Desiree, Romano, Sante and Valor.
Choose trouble free plants: Rather than forever waging war against slugs and snails in order to protect vulnerable delphiniums, bedding plants and hostas, perhaps a better strategy would be to choose plants that are not so high on the mollusc menu.
The plants listed below are reasonably tough:
Acanthus mollis, Achillea filipendula, Agapanthus, Alchemilla mollis, Anemone hupehensis, and A. hybrida, Antirrhinum majus, Aquilegia spp., Armeria spp, Astrantia major, Bergenia, Corydalis lutea, Cynara cardunculus, Dicentra spectabilis , Digitalis purpurea ,Eryngium spp, Euphorbia, Foeniculum vulgare, Fuchsia, Gallardia aristata, Hardy Geranium spp. Geum, Hemerocallis, Lysimachia punctata, Myosotis spp., Nasturtium, Nepeta x faassenii, Papaver orientale, Pelargonium, Polygonum spp., Potentilla, Pulmonaria spp, Rudbeckia fulgida, Sedum spectabile, Sempervivum spp., Siserinchium spp, Thalictrum, Verbascum spp.
Soil structure and drainage: Slugs thrive in rough lumpy ground which is poorly drained, so improving drainage and soil structure is important where these conditions occur.
Cultivation: Raking to create a fine tilth before sowing will help to disturb slugs and their eggs, as well as helping soil to dry out on the surface, making movement more difficult.
Encourage natural controls: Many creatures feed on slugs and snails, such as ground beetles, frogs, toads, hedgehogs and birds. Make your garden a friendly place for these beneficial creatures by avoiding harmful sprays and providing a variety of wildlife habitats and nesting boxes.
Vigilance: Inspect vulnerable plants regularly and remove any pests. Wearing rubber gloves can make this job a little less unpleasant. As slugs and snails feed after dark, night is the best time to get a torch, search and destroy! Or for the more squeamish, collect in a bucket and move them to a suitable natural habitat away from any cultivated land.
Garden hygiene: Tidy up or remove any piles of rubble or stones close to cultivated areas, as well as stacks of flowerpots, and piles of plant debris. If you find large populations of snails tucked in a corner of a veg bed or up an adjacent wall, crush them or pick them off and move them to a suitable natural habitat away from any cultivated land.
Traps: Various designs are available for all areas of the garden, either purchased or home made, free-standing or that can be buried in soil. All require some form of bait – beer is popular! To make your own, use a shallow container (plastic coleslaw or yoghurt pot) and insert it into the soil. Leave a rim 2cm above soil level to prevent beetles and other creatures from falling in. See picture.
Temporary shelter: If you place roof tiles or half grapefruit skins in the soil near vulnerable plants, this provides a shelter for the slugs and snails in the day. Inspect and empty each night. This method can be used to quickly clear an area before planting if you put succulent lettuce leaves as bait under the traps.
Barriers: There are many types, either home-made or commercial products:
Bottle cloches - cut the bottom off a clear plastic bottle and firm it into the soil around a vulnerable seedling/plant.
Slug collars - plastic rings with a lip to make crossing it difficult, placed around individual plants such as lettuce.
Grit or Granules - natural mineral products that either form a sharp, gritty repellent barrier or that suck the moisture from the slime that slugs and snails exude as they move.
Spray repellent - made from yucca plant extract, spray on to surfaces and around vulnerable plants. Especially useful in hard-to-protect-places like greenhouse window frames. Needs renewing after heavy rain.
Copper tape/rings - copper gives a natural electric charge that repels these pests. Tape is useful around pots and legs of greenhouse staging, Bran - slugs like to eat the bran, it swells inside them, reducing their appetite for your precious plants.
Biological control: There is a microscopic nematode Phasmarhabditis hermaphrodita which kills slugs - but not snails. It is a native species and can be found naturally in the soil throughout the UK. For it to be effective, however, you need large numbers and regular application. The nematodes are commercially available, often supplied in a pack of moist clay. This is mixed with water and applied to the soil with a watering can. Slugs will be controlled for up to 6 weeks, after which re-application is often necessary. Research has shown that nematodes are most effective against small and soil dwelling slugs that are difficult to control by other methods.
Important points for success with nematode controls:
- The soil should be moist for 2 to 3 weeks after application. During dry spells watering may be necessary.
- The soil temperature should be above 5°C. Use between March and October outside. In a greenhouse, use whenever slugs are active.
- Once opened, the contents must be used immediately. Unopened, the sachet may be stored in a fridge for 4 weeks or in a cool dark area for 2 days.
And finally .....
- Ferric phosphate based pellets: These pellets are classified as suitable for use by the organic grower. They contain ferric phosphate or iron III phosphate, which affects the calcium metabolism in the gut system of snails and slugs causing them to stop feeding and die within three to six days. Although the ferric phosphate is less toxic than metaldehyde, there remains the problem of the other ingredients in the tablets - known as chelators. These chemicals help bond the iron molecules and make them more toxic to the molluscs. Unfortunately they also affect earthworms, and, if consumed in large quantities, can poison pets.
Our advice is to use these pellets only as a last resort. To use them sparingly, and store them safely.
- Other slug pellets: those based on metaldehyde or methiocarb should NEVER be used in an organic garden.