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Slugs and snails

Pests and diseases
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Grey field slug (Deroceras reticulatum)
Photo: iNaturalist / Christian Grenier / CC BY-NC
Grey field slug on a leaf
  • Grey field slug on a leaf
  • A black slug on a road
  • A white-lipped nail on wood

Onglets

Description

Summary

Slugs and snails can sometimes be found in large numbers in our gardens. These terrestrial gastropod mollusks are distinguished by the presence—or absence—of a shell. Snails carry a coiled shell on their backs, while slugs are shell-less.

Both are hermaphroditic and thrive in mild, damp climates. Some species are native to Quebec, while many others were introduced from different countries and continents.

Slugs are most active at night and during cloudy or rainy weather. On sunny days, they retreat to cool, dark hiding places such as under rocks or decaying plant matter. Snails are also mainly nocturnal, though they may occasionally be seen in daylight.

These gastropods can be omnivorous, though slugs are typically herbivorous and snails mostly detritivorous. Depending on the species, their maturity, and the available food, they can be either harmful—when feeding on fresh plants—or beneficial, helping decompose organic matter. Some even feed on the remains of insects and small dead animals.

Because slugs can consume up to 40% of their body weight each day, they tend to cause more damage than snails, which aren’t always considered pests in a garden. Before taking action, it’s best to identify the true cause of the damage and assess its extent.

Signs and symptoms

The presence of shiny mucus trails and droppings on leaves or soil is a sure sign these molluscs are around. As the slime dries, it turns silvery. But these traces alone don’t always mean your plants are in trouble.

When these gastropods set their sights on plants, you’ll often find leaves riddled with holes or edged with ragged bite marks. Sometimes only the main veins remain.

Young shoots and blossoms can also fall victim to their appetites, while fruits and tubers may be chewed through or pierced with winding tunnels.

A severe infestation can destroy young seedlings and significantly reduce harvests.

The wounds they leave behind can also open the door to fungal or bacterial diseases, adding to the damage.

Latin name (genus)

Arion, Deroceras, Limax, Cepaea

Host plants

Slugs and snails feed on a wide variety of host plants—from ornamental and garden species to wild vegetation—including cabbages, lettuces, strawberries, beans, thin-leaved hostas, and basil.

Name of host plants

Development cycle

Description and life cycle

Slugs and snails are hermaphroditic molluscs, 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. They are legless and covered in mucus.

Their heads bear four tentacles: the two upper ones carry eyes at their tips, while the two lower ones serve as touch and smell organs. Their mouths contain small teeth and a radula—a toothed tongue used for scraping food. The ventral, muscular part of their body, called the foot, has a gland that secretes mucus, making it easier for them to move across surfaces.

Slugs can be brown, gray, orange, black, or mottled. Snails, in contrast, have shells that vary in colour—for exemple yellow, brown, or greenish—sometimes decorated with stripes.

Both slugs and snails spend the winter as eggs or adults, sheltered in soil or plant debris.

Depending on the species, the young reach maturity in a few months or after several years.

Prevention and control

Favourable conditions

The population of slugs and snails increases significantly after a mild winter followed by a warm, rainy spring and summer.

Identification

  • Keep an eye on the leaves of plants that are vulnerable or growing in less-than-ideal conditions.
  • Look for slugs and snails in the evening with a flashlight, ideally after rainfall.
  • Set simple traps, such as wooden boards or damp newspaper, where they can hide during the day. Check the traps in the morning.

Prevention

Slugs and snails are highly sensitive to dry conditions. If they begin to damage plants, it’s a good idea to review your gardening practices:

  • Keep the garden free of weeds and remove potential shelters such as piles of wood or decaying plant matter within planting areas.
  • Avoid overwatering the soil and foliage, especially in the evening—water in the morning instead.
  • Improve drainage by raising garden beds and lightening heavy soils with compost.
  • Avoid adding large amounts of poorly decomposed organic matter.
  • Maintain adequate spacing between plants; thin seedlings; stake plants and remove lower leaves if necessary to allow air and sunlight to circulate.

Physical control

  • Pick snails and slugs off by hand. At night, you can use a flashlight to find them.
  • Replace frequently attacked plants with species or cultivars less appealing to slugs and snails. For instance, some varieties of hostas are far less appetizing to these gastropods.

Biological control

Grow a variety of plants to create an environment that attracts slugs and snails’ natural predators such as ground beetles, spiders, toads, frogs, garter snakes, insect-eating birds (including robins, thrushes, and starlings), and small rodents. Earwigs also help control populations by feeding on slug and snail eggs and juveniles.

Low-impact pesticides

Chemical control of these gastropods is rarely necessary, as preventive measures and manual removal are usually enough to keep them under control.

As a last resort, use a low-impact pesticide whose active ingredient is ferrous phosphide (ferric phosphate). Always read the product label carefully and follow the manufacturer’s instructions.