Slugs - Friends or Foes?

This has appeared in The Grapevine, a newsletter for the Fort Bend County Master Gardeners. More recently, I posted this on the Fort Bend County Master Gardeners Blog on 4 August 2009.

I got up one morning and went out to my front yard to relax with a cup of coffee and enjoy the sunrise. I noticed several slime trails on my walk and wondered what the critter was who made those marks. I searched around and found a slug about 3 inches long, moving slowly from one point to another.

My first inclination was to take that slug and get it off my yard! But, I really didn’t know much about them. Are they beneficial or harmful? Should I encourage them or eradicate them? After being a Master Gardener (or Associate) for more than a year, I realized I needed more information before I made a decision.

So, is the common garden slug we have here in Fort Bend a friend to you or a foe to be controlled? As hard as it might be to believe, some slugs are beneficial to your garden! They can play a major part in clearing up dead and decaying material in the garden and can assist in managing the soil by incorporating humus and helping to form soil crumbs. Additionally, they are important prey for other wildlife including toads and frogs.

For most gardeners, however, the slug is a slimy, rather disgusting creature that destroys seedlings and mature plants alike. Many a grower has gone to her prized vegetables in the morning only to find them in tatters after a night of feasting by the slimy creatures.

Slugs have rasping-chewing mouthparts and cause plant damage by creating large irregular shaped holes in leaves. Slugs have a strong sense of smell and will travel substantial distances to locate a food source.

They prefer to feed on succulent foliage such as seedlings, herbaceous plants, and fruit lying on the ground. Garden slugs feed on narcissus, grasses, lilies, iris, Jimson weed, amaryllis, coleus, tulips, and vegetables and one of their favorite food plants is the hosta.

Adults vary in color from almost white to brown, grey and black. They are legless, elongate, terrestrial (land) mollusks (snails, shellfish, etc.) and prefer to feed at night. They will however feed during the day in moist shady locations. Their movement is highly dependant upon moisture availability. They move by sliding forward on a trail of secreted slime. The slime has a silvery appearance and is often used to detect an infestation.

Slug life span is anywhere from 6 months to 2 years, depending on species. They can lay between 20 to 100 eggs several times per year. Garden slugs increase in size during summer and reach sexual maturity in late autumn. Our local slugs here in Fort Bend only reproduce in early spring. Garden slugs reproduce faster on alkaline to neutral soils than on acid soils.

Slug eggs are very small (less than 3 mm or 1/8 inch in diameter), colorless and deposited beneath the soil surface, making them very difficult to detect. Newly hatched slugs are also small and transparent making their presence easily overlooked. Because of this, slug eggs or young slugs can enter the garden on new plants or in soil materials.

Transplants, perennial divisions, strawberry plants, etc, whether they are given to you by your neighbor or purchased at a retail outlet, can be responsible for infesting your garden. Even borrowed garden equipment can introduce slug eggs. Slugs, even though considered to be a slow mover, can move unaided from one garden to the next in an urban situation. Eggs, because of their size, can even be transferred on the bottom of footwear when crossing an infested area.

Slugs prefer warm, moist conditions. They hide under all sorts of debris including mulch, plant debris, rocks, boards, weeds, and ground covers and even seek shelter in cracks in the soil during dry periods. Rock piles are especially good breeding areas but also support populations of natural slug predators like garter snakes. With the onset of cooler weather, slugs will become less active or even dormant, but with the return of moist warm weather, slugs soon assume their active state.

Slug controls

Few gardeners, however, would want to willingly encourage slugs and will be looking for ways to get rid of them.

Slug management involves a combination of strategies such as hand picking, habitat modification, barriers, traps, baits, and commercial molluscicides. Spring and fall are the best times to control slugs. Feeding generally occurs at night. In the morning slime trails can often be seen where slugs were the night before. Since they have a skin covered with mucus that cannot dry out, the principal enemy of the slug is a hot, dry atmosphere. Making sure that there are as few ‘hiding’ places as possible is a start to reducing their activity. Of course, this is limited action and any light shower or overhead watering of the plants, will create ideal conditions for them.

Habitat modification is one of the most effective strategies in reducing slug populations. This involves eliminating hiding places such as mulches, weeds, old vegetation, and debris. The initial preparation of the soil is most important in a slug control regime and a soil that has a fine surface with no lumps will be less attractive to slugs. Make sure that your soil is well drained and not too alkaline. Regular cultivation and hoeing of the soil can also help as it disturbs the slugs and their eggs and exposes them to the drying conditions that are fatal to them.

There are natural predators of the slug and these include various beetles such as firefly larvae and ground beetles. In addition, toads, frogs, snakes, and lizards eat slugs. However, these predators may not be present in high enough numbers to keep slug populations below damaging levels. It is good to encourage some of these predators though, of course, some of them bring their own brand of threat!

One of the oldest methods of slug protection is the use of some kind of ‘barrier’ around the plant. Diatomaceous earth, shredded bark, eggshells, lime, and wood ash have been used as barriers to prevent slugs from feeding on plants. However, these materials generally work best during dry periods when slugs are less active. In addition, the effectiveness of these materials is reduced by rainfall, which means they have to be reapplied regularly after becoming wet. The use of some of these materials such as egg shells, lime, and wood ash is discouraged because over time, they may raise the pH of the soil. Also, never pour salt on slugs as this may burn plant foliage and roots.Mulches, too, can encourage slugs as they provide places to hide. Avoid mulching young plants and always apply mulch to a warm soil.

Another option is spraying coffee on plants that are plagued with slugs. Caffeine in any form — including a few No-Doz tablets mixed with water — is a slug neurotoxin that will kill these unwanted pests.

Another old-fashioned method that works well is the trap and one of the best is the beer trap. In this method a small container has beer put into it and then the container is placed on the ground where you know the slugs to be. They will climb in and drown and can be removed every few days. Try to ensure that the lip of the container is at least 2 cm. (approximately 1”) above the ground as, otherwise, the beetles that eat slugs will also get in and die. Slugs are attracted to the yeasty smell of beer and they fall into the pan and drown. However, beer does not have an EPA registration number, so the use of beer as a pesticide is not technically legal.

There are, of course, a number of patent chemical options available such as slug pellets and slug tape. Unfortunately, pellets can be dangerous to pets and, because of their blue color, they might look like attractive candies to small children. If the non-chemical methods work then use those as a first choice.

Finally, you can pick slugs by hand. Discard into a bag in your household garbage.

You may have a number of garden slugs in your garden. If you see the benefits that they bring, you may be willing to share your yard with them. Then again, they may be destroying your hard work by eating their way through your plants. Either way, slugs are a part of our gardens and knowing more about them makes them less disgusting!

By the way, studies from Colorado State University have shown that Kingsbury Malt Beverage is the beer most preferred by slugs. I wonder if I can get a grant from the Fort Bend Extension Office to make a study about different Texas beers and their influence on slugs.

References
Wauer, Ro, “Slugs are not the Loveliest of Creatures”, The Victoria Advocate, August 15, 2004.
“Attractiveness of Beer and Fermentation Products to the Gray Garden Slug, Agriolimax reticulatum”, Technical Bulletin TB97-1, Colorado State University, Bioagricultural Sciences and Pest Management, February 1997.
“Bringing Knowledge to Life”, Ohio State University Extension, website (http://extension.osu.edu/~news/story.php?id=1993).
“Featured Creatures”, University of Florida, Department of Entomology and Nematology, website (http://creatures.ifas.ufl.edu/misc/florida_slugs.htm).ou are in the Virginia Beach/Norfolk area, plan on spending at least a day at this wonderful place.