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Snails and Slugs

Ravageurs et maladies
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Snail (Polymita muscarum)
Photo: Jardin botanique de Montréal (Gilles Murray)
Polymita muscarum
  • Polymita muscarum
  • Snail, Québec, Canada.
  • Slug

Onglets

Description

Summary

Snails and slugs are hermaphroditic molluscs that thrive in mild, damp climates. They do their damage at night - you aren't apt to see them in daytime except on overcast days. They eat fresh or decomposed plant matter and the remains of dead insects and other small animals. They are capable of consuming up to 40 times their body weight a day. Some species are solitary, while others live in groups, gnawing on plants and leaving slimy trails in their wake.

Signs and symptoms

  • Trails of slimy mucus and excrement on foliage and the ground. These slimy trails turn silvery as they dry.
  • Foliage full of holes and with gnawed edges. Sometimes all that is left of a leaf is the main veins.
  • Young shoots and flowers attacked; fruit and tubers gnawed and tunnelled out.
  • A severe infestation can destroy young seedlings and ruin a harvest.
  • Damage may promote the development of fungal and bacterial diseases.

Latin name (genus)

Arion, Deroceras, Helix, Limax, etc.

Host plants

Many ornamental, vegetable and wild plants, especially Chinese cabbage, daffodil, dandelion, gladiola, hosta, iris, larkspur, lettuce, strawberry, sweet pea and tulip.

Name of host plants

Development cycle

Description and life cycle

Slugs and snails are nocturnal molluscs. They are hermaphroditic, which means that they have both male and female reproductive organs. Usually, two individuals must mate for the eggs to be fertilized.

Eggs: Spherical, tiny (a few millimetres) and transparent white when freshly laid, becoming darker as
they mature.

Juveniles: Identical to the adults, only smaller.

Adults: Soft, elongated bodies, from 1 to 15 cm long, depending on the species. Snails have a spiral shell, and slugs, which are greyish or brownish, do not. They have four tentacles on their heads; the upper pair are tipped with eyes and the lower pair are tactile and olfactory organs; the mouth has tiny teeth and a file-like tongue (radula); the foot, the ventral, muscled part, is equipped with a mucus gland that helps them move about.

Slugs and snails overwinter as both eggs and adults, buried in the soil and plant litter.

Each individual can lay hundreds of eggs a year. They deposit them in batches in decaying plant matter and in the soil.

The eggs hatch in three weeks to several months, depending on weather conditions. It takes from a few months to a few years for the juveniles to mature, depending on the species.

Prevention and control

Favourable conditions

Slug and snail populations rise significantly after mild winters followed by mild, damp summers.

They are active at nighttime and on cloudy or rainy days. On sunny days, they hide in dark, cool locations (under plant litter, rocks, etc.).

Identification

Keep an eye on the foliage of susceptible plants and those growing in poor conditions, especially if you already have a problem with snails or slugs. To find out the actual extent of an infestation, check at night with a flashlight. The best time to do so is after a rain.

Prevention

  • Keep your garden free of plant litter and weeds.
  • Remove all potential shelters, woodpiles and decaying plants.
  • Avoid overwatering the soil and foliage, especially in the evening, promote good drainage by raising borders and amend heavy soil to lighten it.
  • Avoid adding large amounts of poorly decomposed organic matter; do not use or straw or sawdust as mulch.
  • Maintain adequate spacing between plants; thin seedlings; stake plants and remove lower leaves as necessary to allow air and sunlight to circulate.

Physical control

  • Install a strip of copper around sensitive plants to ward off slugs and snails.
  • Pick snails and slugs off by hand at night, using a flashlight to find them.

Biological control

Encourage natural predators in your garden: spiders, toads, frogs, garter snakes, insect-eating birds (robins, thrushes, starlings, etc.) and small rodents. Earwigs eat snail and slug eggs and juveniles.

Chemical control

As a last resort, use a low-impact pesticide with ferric phosphate as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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