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Pests and diseases

Slugs and snails

Slugs and snails can be one of the most frustrating 'pests' for organic gardeners. Here are some natural ways to deter and deal with them
A slug in soil surrounded by leaves
Slugs love young green growth so wait to plant out seedlings until they are mature and resistant.

Slugs and snails are often regarded as the scourge of the seedbed but they do some valuable jobs in the garden. They help to break down plant matter, disperse seeds and provide food for lots of other beneficial creatures.

But when they become a problem, here's a few ways to deal with them...

  1. Protect your plants. Create barriers with slug collars, grit, wool pellets, and cloches made from plastic bottles. Keep checking daily – especially after wet or damp weather.
  2. Collect slugs and snails. Put down a flat slate on the soil near your plants and check underneath it every day to remove the slugs and snails which have congregated in the dark and damp. Dispose of them as you wish – ideally putting them in a suitable natural habitat away from any cultivated land.
  3. Bio-controls i.e nematodes. These are expensive to buy so make sure you follow the instructions regarding soil temperature, watering, etc, carefully to ensure success.
  4. Choose trouble-free plants. Slugs and snails love hostas and delphiniums but won't go for foxglove or fennel. See below for a list of flowers not so high on their menu.  
  5. Wait to plant out seedlings. Slugs love young green growth, so wait until your young plants are more mature before planting out. This way they will be more resistant to attack.
  6. Choose slug-resistant varieties. Potato varieties such as Ambo, Cara, Desiree, Romano, Sante and Valor have shown resistance to slugs.
  7. Get the balance right. Attract ground beetles, frogs, toads, hedgehogs and birds to your garden, as they all feed on slugs. Make your garden a wildlife-friendly place by avoiding harmful sprays and creating a variety of wildlife habitats and nesting boxes.
  8. Remove slug homes. Ensure piles of stones, flower pots and plant debris are moved away from cultivated areas. These can all be hiding places for slugs and snails.

Barriers and traps to catch slugs and snails

Here's some useful ways to deter slugs and snails. None of them are failsafe, but they can act as a deterrent, along with other measures...

  • Bottle cloches - cut the bottom off a clear plastic bottle and firm it into the soil around a vulnerable seedling/plant.
  • Grit or granules - these natural mineral products form a sharp, gritty repellent barrier or suck the moisture from the slime that slugs and snails exude as they move.
  • Bran, oats or coffee grounds. Put a thick layer around the plant, slugs will eat it, swell up and become easy picking for birds. But you need to renew daily, especially in damp weather.
  • Spray repellent - made from yucca plant extract, spray onto 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 both slugs and snails. Tape is useful around pots and legs of greenhouse staging.

Biological controls

Nematodes. There is a microscopic nematode Phasmarhabditis hermaphrodita which kills slugs - particularly small, soil-dwelling ones - 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. You can buy nematodes online. They are 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 six weeks, after which re-application is often necessary.

Important points for success with nematode controls:

  • The soil should be moist for two to three weeks after application to enable the nematodes to live. During dry spells watering may be necessary.
  • The soil temperature should be above 5°C. If you live in the south, this is probably between March and October. Further north, your window is more limited. In a greenhouse, use it whenever slugs are active.
  • Once opened, the contents must be used immediately. Unopened, the sachet may be stored in a fridge for four weeks or in a cool dark area for two days.

Slug pellets

We would advise gardeners NEVER to use slug pellets containing metaldehyde or methiocarb, but also to avoid organic versions. So-called organic slug pellets contain ferric phosphate-based, 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 (used in other pellets), 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.

Flowering plants resistant to slugs and snails

Gastropods show little or no interest in the following plants...

Acanthus, Achillea, Agapanthus, Alchemilla mollis, Antirrhinum, Aquilegia, Astrantia, Bergenia, Corydalis, Cynara, Dicentra, Digitalis (Foxglove), Eryngium, Euphorbia, Fennel, Forget me not, Fuchsia, Gallardia, Hardy Geranium, Geum, Hemerocallis, Japanese anemone, Lysimachia, Nasturtium, Nepeta, Pelargonium, Phlox, Polygonum, Potentilla, Pulmonaria, Oriental poppy, Rudbeckia, Sedum, Sempervivum, Thalictrum, Verbascum.

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Want to know more about slugs and snails?

The garden snail (Cornu aspersum) is usually dark or golden brown, or chestnut with yellow stripes. The body is soft, slimy and brownish-grey. As it moves, it secretes mucus which helps it glide across a surface by rhythmic waves of contraction. 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.) However, unlike Bolt, the snail is helped by its mucus to 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 a razor blade without harm.

Snails are herbivores, eating vegetables, fruit, flowers and cereals, as well as rotting plant material. 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.

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.

There are forty common species of slug found in gardens, most of which are harmless. However, there are four species that may harm your plants:

  1. 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.
  2. The Common Garden slug (Arion distinctus/ Arion hortensis), 3cm 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.
  3. The Common Keeled slug (Tandonia budapestensis), up to 6cm long, is black or grey with a 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.
  4. The Large Red slug and Black slug (Arion ater) are either orange-red or black with an orange fringe. Despite its size, of up to 12cm, this species of the 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.

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