Gastropoda

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Image above: adult snail

Common snail and slug pests are introduced from other countries − no native snail or slug is a pest. Pest species usually occur in the moister areas across the southern half of Australia. The Common Garden Snail occurs through most of eastern and southern Australia, including far north Queensland and Tasmania.

Damage from snails and slugs occurs at night and the pests hide during the day in soil, under plants and detritus, or in weedy patches. Snails and slugs are usually not seen over the summer months, as they seek shelter from the hot dry weather.

When pest snails and slugs arrived here, there was little competition and they found ideal habitats in the European-style gardens that were being planted. The heavy mulching and watering that is required to maintain such moist gardens in our dry climate, creates perfect conditions for snails and slugs.

Best Treatment for Snails & Slugs

The management of snail and slug pests requires vigilance and persistence. A good place to start is to prevent them breeding as much as possible by minimising suitable breeding sites such as weedy patches. They love to lay eggs under mulch, so perhaps only mulch your garden beds when necessary, such as during hot weather.

Snails and slugs leave behind characteristic silvery slimy trails during their night time foraging. If you see this, you should search in that area for where they are hiding during daylight hours. Snails and slugs often hide under mulch, or piles of pots, timber and bricks etc. Tidying up these piles and/or rearranging them periodically will make these areas less suitable for them.

There are many other things that you can do to prevent snails and slugs chewing holes in the leaves of your plants (see Prevention). Try Yates Snail & Slug Bait, which is a pellet-bait based on Iron EDTA. This compound is of low toxicity to mammals and is also biodegradable. The pellets break down to nutrients that won't harm earthworms. Other baits based on methiocarb or metaldehyde are not so benign.

What are Snails & Slugs, &
How to Get Rid of Them

Snails and slugs fit into the largest class of molluscs known as gastropods (class Gastropoda), which include aquatic and terrestrial snails, nudibranchs and terrestrial slugs.  Gastropod translates as ‘gut foot’ or ‘stomach foot’. The entire undersurface of gastropods is flattened and muscular, forming a creeping sole or ‘foot’. The foot contains many mucous glands, which exude mucous in the form of a characteristic silvery snail trail to lubricate the ground. Snails and slugs move forward on the slippery trail by waves of muscular contractions that sweep along the foot.

Common Garden Snails (Cornu aspersum) are urban pests throughout most of Australia, and the Common White Snail (Cernuella virgata) is widely distributed in southern Australia. Some of the introduced pest slugs we have are instantly recognisable, like the Giant Leopard Slug (Limax maximus). Other common pest species include the Grey Field Slug (Deroceras reticulatum), the Brown Field Slug (D. panormitanum), the Great Yellow Slug (Lehmannia flava), and the Black-Keeled Slug (Milax gigates). These pests are found throughout Australia where conditions are suitable – there are at least 13 introduced terrestrial slug species in Australia.

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Image above: adult slug on a lime fruit
(Image courtesy of Elise Dando)

Description

The Common Garden Snail has a shell about 25 mm in diameter, brown with a distinctive striped pattern. Common White Snail has a shell up to about 15 mm in diameter and is white and usually has a brown band around the spiral. Common pest slugs usually grow to about 50 mm long, although the Great Leopard Slug can be 100 to 200 mm long. The Grey Field Slug is about 50 mm long and is usually greyish-brown with darker flecks.

Harmless native slugs can be differentiated from introduced pest slugs by a single pair of tentacles on their heads - introduced slugs have two pairs. You may need to view them from the front rather than from the top to see this. Another difference is the saddle-shaped mantle or hump seen on the backs of introduced slugs, which is reduced or absent in native slugs.

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Image above: juvenile hatchling snails
(Image courtesy of Elise Dando)

Life Cycle

Snails and slugs are hermaphrodites, possessing both male and female sexual organs. They can self fertilise, but mating with another is normal. After two individuals exchange sperm, they are both fertilised and both lay eggs, 50 to 100 at a time. Snails and slugs usually lay their eggs in the top layer of moist soil, or under leaves or organic matter such as mulch. The eggs are typically gelatinous in appearance and may be translucent or greyish-brown in colour, depending on the species. In ideal moist conditions, slug and snail numbers can build up quickly because new batches of eggs can be laid every few weeks.

Hatchling snails are miniature snails with fragile translucent shells, but as they grow the shells become harder and darker. Hatchling slugs are miniature versions of the adults. Snails and slugs take up to a year to reach maturity, depending on conditions.

Snails differ from slugs in that they have a protective shell, which is secreted from the underlying epidermis or ‘mantle’. The shell can be pulled down by a series of retractor muscles for further protection when threatened. The shell also protects land snails from hot dry conditions and has allowed pest snails to survive in drier conditions than pest slugs can. During very hot, dry weather, snails can seal the opening of their shell and enter a dormant phase known as aestivation.

Slugs usually hide in the soil over the summer months and emerge in autumn to breed and feed at night. Most snail and slug species found in gardens lay eggs in any month, if conditions are suitable and there may be up to three generations per year. Snails and slugs avoid the heat of the day and do all their damaging feeding during the night, or in the early morning, or during cool, damp overcast conditions.

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Image above: chewed holes in cabbage leaves

What Plants are Impacted by
Snails & Slugs

  • A wide range of plants including vegetables, annual flowers, shrubs and fruit trees.
  • Seedlings.
  • Strawberries.
  • Common garden Snails are a significant pest of citrus, damaging fruit, leaves and removing the bark from small branches. They sometimes cause damage in vineyards and deciduous fruit tree orchards.
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Image above: silvery slime snail trails on dichondra leaves
(Image courtesy of Elise Dando)

Symptoms of Snails & Slugs

  • Snails and slugs eat with their tongue that is a rasping organ known as a radula. Under an electron microscope the surface of the radula looks like that of a rasping tool or file.
  • Young snails and slugs skeletonise leaves, older ones chew holes.
  • Seedlings may be destroyed.
  • Chewed flower buds and flowers.
  • Gnawed fruit.
  • Slime trails.
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Image above: snail poo/faeces
(image courtesy of Elise Dando)

How to Prevent Snails & Slugs Appearing

Monitor

  • Look for slime trails.
  • Look for damage as described.
  • Look for clutches of gelatinous eggs under mulch.

Prevent

  • Tidy up piles of pots and timber to minimise daytime hiding places. Check under the rims of pots.
  • Remove weeds behind sheds and in shady back corners, as these are perfect breeding areas.
  • Barriers of diatomaceous earth, crushed eggshells, lime, copper tape or sawdust placed around seedling beds may prevent snails and slugs entering garden beds, if the material is kept dry.
  • Protect seedlings with ‘mini greenhouses’ or ‘cloches’ made from plastic drink bottles with the bottom cut off.
  • You can detect the presence of snails and slugs by placing a tile or a damp sack on the ground for them to shelter under during the day. Check your trap every few days and destroy any snails and slugs you find by dropping them into a bucket of soapy water.
  • Try a dish of beer, which they find irresistible; they will come from all directions to drink it. Collect the snails and slugs and destroy them.
  • Handpick snails and slugs off plants at night.

Natural Enemies

  • Carabid beetles, predatory (not pest) earwigs, birds, frogs and lizards.

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