What is a slug? It’s a snail with a housing problem!

It’s true! A slug is essentially a snail with little to no shell. They are part of a class of molluscs called gastropods (from the Greek for “stomach-foot”), just like nudibranchs and whelks. Land slugs are a well-known slimy encounter in Pacific Northwest forests. As with many creepy crawlers, they are very misunderstood. Some of us might only know them as a nuisance to food crops and plants, yet slugs are an integral part of forest ecosystems. Although they might be considered pests in our gardens, there are many ways to protect our plants AND our native slugs who are integral to our forest ecosystems. 

Fascinating mucus 

To prevent drying out, lubricate their movement and fend off predators, slugs secrete a mucus through their skin. If you’re motivated by mucus, watch this video and read the associated article to learn what slug mucus is made of and its amazing properties. 

Banana Slugs: Secret of the Slime

Reproduction

Slugs are hermaphroditic, meaning they have both female and male reproductive organs. When mating, slugs cross-fertilize so both individuals produce eggs. Arion species can even self-fertilize when mates can’t be found. This is particularly useful with their slug-paced traveling and can also account for how successful introduced species are at colonizing new areas. Some species like Fieldslugs have elaborate courtship behaviour involving a slow dance of circling, touching and biting each other while secreting pheromones. The Leopard slug suspends itself using an elastic thread made of its mucus during reproduction. 

Mating Leopard slugs by Inge Knoff

Defensive measures

To fend off predators, slugs have various techniques. The Chocolate Arion contracts its body and twists side-to-side. The Leopard slug lifts and lashes its tail while Taildropper species literally drop their tail by self-amputation to escape. As discussed earlier, the mucus produced by slugs can be a powerful deterrent, even producing a slight tingling on human lips with Banana slug mucus. 

Ecological role of native land slugs 

Slugs have an important ecological role as decomposers. They consume decaying organic matter like fallen leaves, dung and carrion, enriching soils with their feces. Without decomposers, nutrients would take much longer to recycle. Decomposition is integral to an ecosystem’s food cycle. Many species like the Blue-grey Taildropper and banana slug are also partial to fungi (who are also key decomposers) and spread fungal spores in their droppings.

Blue-grey Taildropper

This native slug is on the Red List of species at risk in BC. It thrives in Garry oak meadows and mature Douglas-fir forest, two endangered ecosystems on southern Vancouver Island and the Gulf Islands.  

Blue-grey Taildropper by Marshal Hedin

The Blue-Grey Taildropper is a comparatively small slug (2-3 cm adults) with a distinct bluish to greyish colour variation. It lives in moist sites such as in leaf litter or moss, under ferns or rotting logs especially within dense shrubs like Ocean Spray. 

These slugs have yet to be reported on Mayne Island but are potential inhabitants. You can use this identification guide from the Habitat Acquisition Trust if you think you have found one. Reporting observations is extremely important for this species to understand its distribution and population health. Follow this link to learn more.

Pacific Bananaslug

No self-respecting article about slugs would be complete without this behemoth. These are the largest land molluscs in the province and the second-largest slug species in the world! by A.Davey

Introduced species

With their sluggish speed and inefficient mode of transport, it’s hard to believe that slugs can spread across different continents. Most non-native slugs were introduced from Europe inadvertently by human travel and trade. Introduced slugs can colonize new places rapidly especially if self-fertilized. Some can be very detrimental to crops and plants. Others like the Leopard slug can also compete aggressively with native species. 

Chocolate Arion (Black European Slug): This introduced species is widely distributed on the island and lives in gardens, along roadsides and other disturbed sites. 
by Gavin Anderson
Leopard Slug (Giant Gardenslug): With a scientific name like Limax maximus, no wonder it’s an aggressive competitor for native slugs.
by Hauke Masicaloris
Grey Fieldslug (Milky slug): Another introduced species that you might find in your garden and a very destructive plant pest in North America. 
by Bruce Marlin

Responsible slug management

Because you may find both introduced and native slugs in your garden, it is crucial to manage them carefully to avoid targeting ecologically important critters. Your priority should be to create an unwelcoming environment for any slug as opposed to trapping and killing them. 

  • Plan your watering carefully. As we’ve learned, slugs are slowed and deterred by hot, dry conditions. Take advantage of this in your garden planning and maintenance. Water early in the morning so the soil surface has time to dry before the evening. Better yet, use drip irrigation to keep the soil surface dry between plants. 
  • Reduce slug hiding spots. Keep your garden free of cover surfaces lying around where they might slip under (tarps, plywood, buckets, cardboard).
  • Plan your planting carefully. Mature plants are often less attractive to slugs and can recover with new foliage if needed. Young shoots and sprouts are more vulnerable. Start your plants indoors or plant extra seeds to plan for losses caused by slugs. 
  • Use slug barriers. Slugs avoid metals like copper and zinc. If it isn’t possible to keep them out of your garden all together, you can use metal tapes or wire to wrap around planters, shrub trunks or individual shoots. You can also use mulches that are too sharp for slugs’ skin such as eggshells, sawdust, wood ash, lime and diatomaceous earth. But make sure to keep these surfaces dry! 
  • Preserve predator habitat nearby. Garter snakes, ground beetles, ducks, geese and toads are all potential predators of slugs. 
  • Avoid using traps and bait. Although they may be effective against introduced pest slugs, they can also target native slugs that are likely not problematic for your garden. Chemical baits can poison slugs but also native predators like garter snakes and beetles. 
  • Avoid using insecticides. They might also target slug predators. 
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6 Comments

Amber Harvey · July 5, 2020 at 8:29 am

I’ll have more respect for the slugs I encounter now. Thanks for the article.

    Joël Lavigueur · July 6, 2020 at 10:41 am

    That’s great to hear Amber! A lot of strange creatures deserve more of our respect.

Don Barthel · July 5, 2020 at 8:48 am

I find that crushed egg shells to be fairly ineffective at repelling slugs. More effective is this copper mesh from Lee Valley: http://www.leevalley.com/en-ca/shop/garden/pest-control/insects/51241-copper-blocker

    Joël Lavigueur · July 6, 2020 at 10:43 am

    Good product suggestion Don! I also find eggshells aren’t sharp enough for larger slugs sometimes.

Kit Maloney · July 8, 2020 at 7:29 am

Do you think slugs having ‘homing instincts?’ I gather up all of the slugs in the garden and then ‘relocate’ them to the valley behind our house. (Ok, so I toss them one by one across the creek with the sound effect “wheee!”). I swear that I think I recognize a few of them the next night. If I could figure out how to tag one of them, I would! (Too slimey and they don’t have feet. 😆) I’d love to know how far they can travel in a day. Thanks!

    Joël Lavigueur · July 8, 2020 at 12:49 pm

    Great question Kit! The Leopard slug is known to have homing behaviour, returning to their moist, dark, hiding during the day. So it wouldn’t be surprising if other species also return to cozy or tasty spots. Large slugs won’t travel more than a kilometre in their lifetime and the fastest reach a whopping speed of 15cm/min.

    Be careful when tossing native slugs like our iconic Pacific Bananaslug. Although they might seem robust, some are heavy enough that an impact might rupture their internal organs.

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