Slugs and Snails in the Ornamental Garden

Most people are familiar with these slimy pests, that cause extensive damage to gardens, both ornamental and food. Slugs and snails while thought of as insects to most gardeners, are actually invertebrate animals known as Molluscs or mollusks (along with oysters, clams and squid). They have been around for about 500 million years (BYJU’S, (n.d.)) with about 85,000 known species (Wikipedia, (rev.  9 April 2022)). There are 2 types of snails, aquatic and terrestrial, and unless you have a pond, the type you will encounter in your garden, are the terrestrial type. These slugs and snails require moist environments in which to live and their soft bodies are prone to easily drying out. To safe guard against this they tend to feed at night, and on rainy or heavily over cast days, preferring to find cool moist areas to hide out during the day. Snails have a little more leeway in this regard, as their shells help to protect them from the sun and somewhat against predators.

Slugs and snails move about by means of rhythmic, muscular contractions, produced by what is called a foot, located on their underside. The muscles of this foot contract from the front of the foot to the back, while simultaneously secreting a layer of mucus, from glands located below the mouth, to grease the surface they are moving on. This mucus trail helps to protect their soft tissue from injury. As the mucus, left behind dries, it forms a shiny silvery trail, a sure sign the pest is present.

While they are a fascinating creature, they can cause extensive damage to ornamental and vegetable gardens; producing large, irregularly shape holes with smooth edges, on leaves, marring the surface of some fruits and vegetables and killing off young seedlings. They seem to eat almost anything, from plant debris to healthy plant material including leaves, fruits and vegetables, as well as animal waste and worms. The more you understand about the workings of slugs and snails the better prepared you will be to create an environment in your ornamental garden, that is not so attractive to them.

Meet the Pest

brown-lipped snail (Cepaea nemoralis)
A slug in autumn.
Snail shell

Slugs and Snails are very similar to one another, the main difference being, that snails have a shell and slugs do not. Both have soft, unsegmented bodies, with a head that has a pair of optical tentacles (tentacles with eyes at the tips of them), that can move independently of one another and regrow if lost; they also may have sensory tentacles located further down the face, that are sensitive to odours and sometimes taste. They have rasping mouth parts (tongue-like organ, covered with thousands of tiny tooth-like protrusions called denticles) that scrape the plant material and drink up the juices. Both move around using a flat muscular foot located down their belly, that has a sole on the bottom of it. They also oddly enough have green-coloured blood. There are many different species, with some variation in colouring, size, life span.

  • Colouring is usually brown or grey in colour but some species are yellowish, have violet over tones and some may be spotted, variegated or have distinctive bands.
  • Size can range from 2 cm to 7cm with the exception of Limax maximus, which can get to be up to 20cm long.
  • Life span is about 2-5 years but they can live longer in captivity.
  • Snail shells can vary in shape, colour and patterning. They grow along with the snail. The shell helps prevent the snail from drying out and if it become broken the snail will die. A fun fact I found on the Gov. of Canada website was that snails are able to seal off the opening in their shells and go dormant for up to 4 years.

Eggs: Are laid on the soil surface or just below, in moist soil, and hidden under garden debris and plant material. The pearl-like eggs are about 1/8″ in diameter, and are laid in batches (clutches) of about 80 eggs, then covered in a layer of mucus.

Some common species of SNAILS here in Ontario are: brown-lipped snail (Cepaea nemoralis), white-lipped snail (Cepaea hortensis), Chinese mystery snail (Cipangopaludina chinensis), Bellmouth Ramshorn (Planorbella campanulata), Marsh Ramshorn (Planorbella trivolvis). Common SLUG species are: three-band gardenslug (Lehmannia valentiana), Gray Garden Slug (Deroceras agreste), roundback slugs (Arion intermedius), worm slug (Boettgerilla pallens), and the biggest slug of all great grey slug or leopard slug (Limax maximus)

Life Cycle

Slugs and snails overwinter in the soil until about April, when the soil begins to warm and succulent new growth begins. They begin actively feeding and mating until the hot summer weather arrives at which point they cease mating and eggs laying until the cooler moist weather returns in autumn.

Most slugs and snails are hermaphroditic, which means they each possess both male and female sex organs. This makes them very proficient at reproducing. It takes slugs and snails on average, 3-5 months to reach sexual maturity, but some species required a couple of years. After the eggs have been fertilized they are laid on or in the soil, in a clutch of about 80 eggs. The eggs can begin hatching in a couple of weeks. In about 6 weeks the slug or snail is ready to reproduce again, if conditions are suitable. They can reproduce as many as 5 times per year, under ideal conditions. By the end of October they begin entering the soil again for hibernation. Their average life span in the wild is about 2-5 years.

Habits

Slugs and snails have a strong honing instinct, with the young ones preferring to stay close to the egg hatching site. If they are thrown over the garden fence they will just return again, albeit very slowly, as they only travel about 1m per hour. They must be removed more than 20 meters to nullify their homing instinct (The Guardian, (2014)).

Slugs and snails need moisture to thrive. During the heat of the day they hide out in cool dark places such as under plant material, under rocks, under boards, in the bottom of potted plants and drip trays and under pretty much any debris. After dusk and on heavily overcast or rainy days they come out to feed.

Damage Caused by Slugs and Snails

Slugs and snails feed on a wide variety of plants, tending to prefer fleshy foliage. In the vegetable garden new seedlings can be completely destroyed. Damage to leaves appears as large, irregularly shaped holes with smooth edges, both at the leaf edges and in the middle of the leaves. These holes are accompanied by the tell tale silvery mucous trails they leave behind. On fruits and vegetables with a thick skin, they use their rasping mouth parts to scrape the surfaces, greatly marring the appearance.

In addition to the damage slugs and snail can do to your plants, they can also be a host for parasites and microorganisms. One parasite in particular, that is potentially fatal to dogs, is lungworm (Angiostrongylus vasorum). Snails can also invade home ponds and depending on the species, do a lot of damage to your water plants. They can also clog filters and pipes and potentially kill fish, if the snails are diseased. On a good note though they can help control algae.

Control and Management of Slugs and Snails

Cultural practices:

  1. Attract their natural enemies. The toads, frogs, lizards and snakes eat large amounts of slugs and snails. Consider installing a toad house or is the two to encourage them in your garden. Ground beetles and their larvae, centipedes, birds, raccoons, squirrels, ducks and geese will also eat slugs and snails.
  2. Make the garden area less inviting to slugs and snails.
    • Dry up the gardens a bit, only water as much as you absolutely have to and do not water late in the day, better yet switch to drip irrigation.
    • Remove hiding places for them in the day such as bricks and boards, rocks, and any debris that makes contact with the soil.
    • Remove decaying plant material from the garden, including fallen leaves.
  3. Choose plant material that is less desirable to slugs and snails or deters them. The plants names in red are highly scented plants that tend to deter slugs and snails. These can be planted near plants you want to protect.
    • Lavender
    • Wormwood
    • Rue
    • Anise
    • Rosemary
    • Fennel
    • Chives
    • Garlic
    • Mint
    • Allium
    • Geranium
    • Aquilegia
    • Penstemon
    • Euphorbia
    • Foxglove
    • Japanese anemone
    • Ferns
    • Fuchsias
    • Nasturtiums
  4. Plant trap plants to entice slug and snails away from your valued plant material. Good chooses are:
    • Horseradish
    • Basil
    • Petunias
    • Salad leaves
    • Tomato plants
    • Squash
    • Cucumbers
    • Beans

Physical controls:

  1. Hand picking: After dusk or on heavily overcast days, use a flash light to remove the slugs and snails by hand, dropping them into a bucket of warm soapy water. Be sure to protect your self against mosquitos, for when the slugs and snails are actively feeding so are the mosquitos.
  2. Create barriers at around desirable plants or raised garden beds to deter slugs and snails.
    • Diatomaceous earth: (also known as silicon dioxide), is the finely ground fossilized remains of microscopic marine creatures called diatoms. The edges are so sharp that they scratch up the soft tissues of the slugs and snails and cause them to dehydrate and die.
    • Copper: copper collars placed around plants, a band of copper tape placed around the outside of containers or around the upper frame of raised gardens will help to deter slugs and snails by giving them a mild shock if they try to cross the barrier. Choose a copper tape that has a high percentage of copper alloy and is at least 4cm high. According to a paper by McGill university “copper was more than 95 percent effective when the top edge of the strip was bent down to form a flange” (Long B., (1996)).
    • Create a band of a very fine, dry powder, like gypsum, talcum powder, wood ash or flour around the plant. This will need to be replaced after each rain or after windy days.
    • Create a barrier of brambles or thorny rose stems on the soil around the plant.

Bait or bait and trap:

  1. Bait and trap: Burry shallow containers in the garden, so that the top edge is at ground level and fill them with either stall beer or brewer’s yeast mixed with sugar and water. Place them out in the evening when they are actively feeding and change them regularly. The slugs and snails are attracted to the odour and drown when they go in to feed on it.
  2. Trap: them by creating an ideal shelter for them in a cool dark location, such as an inverted melon, hollowed out grapefruit or inverted flower pot. Then when they congregate there during the day gather them up and drop them into a bucket of warm soapy water. Wooden boards also work well.
  3. Chemical control products: either in a pellet or a powder are not encouraged, as they are also poisonous to pets and wild life. They should only be used as a last resort. These products contain a powerful attractant and a poison that will kill them when consumed. Apply these baits when the slugs and snails are actively feeding at night or in damp weather. Baits consisting of f iron phosphate are safer and are non-toxic to pets and wild life. Using bait stations, instead of sprinkling it around makes it even safer and more effective. A bait station can be as simple as placing bait under a board in the garden or in some type of a container that has an opening at ground level. According to Utah State University Extension, “commercial baits are more attractive to slugs and snails when they are slightly moistened with apple or orange juice.
A slug moving across pavement.
A slug in autumn.

Photo credits: all photos and video by the author.

References:

BYJU’S, (n.d.). Snail Life Cycle. https://byjus.com/biology/snail-life-cycle/?msclkid=3c237842c09b11ecb854fc864930d82d

Conrad J., (n.d.). Snails and Slugs. BACKYARD NATURE HOME, https://www.backyardnature.net/snail_sl.htm?msclkid=6b8dc808c16c11ec93bb5e559e91b122

Fletcher D., (2013, rev. 23 Aug 2013). Snails can travel at one metre an hour and piggy-back on others’ slime to save energy. Mirror. https://www.mirror.co.uk/news/technology-science/science/snails-can-travel-one-metre-2208599?msclkid=767fcf28c09d11ecbd11fbcc98ac4933

Gov. of Canada, (rev. 2013-06-04). Slugs and snails. https://www.canada.ca/en/health-canada/services/pest-control-tips/slugs-snails.html?msclkid=1c3cf2cdc09111ec9fc8063987570000

GrowLikeGrandad, (2019). Snail Control Problems? Egg Shells NO, Brambles & Gypsum YES. https://www.growlikegrandad.co.uk/allotment/snail-problems-egg-shells-no-brambles-gypsum-yes.html

 iNaturalist, (n.d.). List of Slug Species of Canada. https://www.inaturalist.org/posts/20685-list-of-slug-species-of-canada?msclkid=d0e69906c1b211eca989788439faeb4a

Long B., (1996). COPING WITH SLUGS AND SNAILS. Journal of Pesticide Reform. Vol. 16, No. 1, pp. 22-23. (http://www.eap.mcgill.ca/MagRack/JPR/JPR_29.htm?msclkid=d34fd7c7c3c911ec98c420783f9b1665

The Guardian, (2014). Snails’ homing instinct can be overcome, if you move them 20m away. https://www.theguardian.com/environment/2014/may/16/snails-homing-instinct-overcome-move-20-metres-away?msclkid=767f2451c09d11ec9a60fdbc25d7f15f

Utah State University Extension, (n.d.). Slugs and Snails. Utah Vegetable Production & Pest Management Guide. https://extension.usu.edu/vegetableguide/leafy-greens/slugs-snails?msclkid=ee7110bbc09411ecad8d12a9ae557b3d

Wikipedia, (rev.  9 April 2022). Mollusca. https://en.wikipedia.org/wiki/Mollusca?msclkid=9fe957b7c16611ec885f4ef3fabca6eb

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