Establish Natural Defenses
Over the long term, a good slug and snail defense is simply keeping the planted areas on your property where they are likely to lurk cleaned up. Since these pests typically lay eggs and take refuge in leaf litter and garden debris, it is advisable to clear all this away, especially in the fall. Rake up fallen leaves and pick up twigs, rotting boards, old flower pots, labels and other stuff that accumulates over the season. While mulch is desirable as winter protection for plants, if possible, rake up and discard the existing mulch that may harbor slug eggs. Wait until the ground freezes, then replace it with 2 or 3 inches of fresh chopped leaves, with or without peat moss added, pine needles or other attractive organic material. When spring comes it will begin to decompose and nourish the soil to keep plants healthy and vigorous.
For more information see the file on Using Mulch
In addition to cleaning up planted areas and building soil rich in organic matter encourage populations of the many insects and animals that normally prey on slugs and snails. These natural predators come in all shapes and sizes, and include toads, garter snakes, spiders, harvestmen (“daddy-long-legs”), centipedes, ants, ground beetles, carrion beetles, soldier beetles, rove beetles, fireflies (predatory larvae) and birds. Some homeowners even keep ducks or geese, which are notably fond of slugs and snails. The more of these different kinds of beneficial predators live in your yard, the fewer pest problems you will have. They will take up residence in a yard that includes their favorite plants and other foods. The greater the variety of plants on your property, the greater the diversity of natural enemies of slugs and snails that will be attracted to it.
Control Slugs and Snails Early
Slug and snail activity begins in the early spring in most areas. By later in the season when it is noticeable, these pests are on their second or third generation and they are much bigger and eating much more foliage. The trick is to reduce their numbers first thing in the spring and their natural enemies will hold the remainder in check. Because they appear in your yard on about the same date every year, it is easy to estimate the best time to put out some beer-baited traps to monitor their arrival. They may be no larger than your little fingernail.
When you catch your first few, then put out lots of beer-baited homemade or commercial slug traps in the areas where you had slug problems last year. Keep changing the beer every 2 or 3 days, and make sure the traps are set flush with the soil surface so the tiny ones can easily crawl in and drown. Begun early enough, over a week or two you will severely interrupt their population growth. Do this 2 years in a row and you’ll see very few slug problems if you also maintain healthy plants, healthy soil, and encourage lots of predators.
Encourage Beneficial Insects
Make your yard hospitable to the many predators of slugs and snails by providing the rich, diverse organic environment that they need. Black rove beetles, centipedes, firefly larvae, ground beetles, carrion beetles, and soldier beetles all prey upon slugs or slug eggs. Spiders and ants particularly love their eggs. Most of these beneficial creatures reside and work in a 2 to 4 inch layer of organic mulch. Introduce some beneficial insects into your home landscape to help the ones already there patrol for slugs and snails. Many kinds are available from mail-order suppliers. Encourage the insect predators of slugs and snails to stay in your yard by providing them with a tempting variety of their favorite plant sources of pollen and nectar. One product, Border Patrol, is a seed mix of wildflowers particularly attractive to beneficial insects. See the file on Attracting Beneficial Insects
Encourage Other Slug Predators
Birds and Fowl: While birds may pass up larger slugs because of the slime on their bodies, they prey on baby ones in the early spring. Downy woodpeckers, robins, grackles, and many other birds found in home landscapes relish slugs. Chickens don't usually care for slugs, but ducks do.
Amphibians and Others: Common garter, or grass, snakes eat slugs. They live under mulch or in stone walls or under the same boards that protect slugs during the day. Other slug enemies include salamanders, shrews, toads, and turtles. If you have koi in a water garden, they will eat small slugs if you throw them into the water.
Attract Birds Year Round
Songbirds such as robins, grackles, chickadees, nuthatches, purple finches, warblers, chipping sparrows and woodpeckers consume enormous numbers of pest insects in your yard, the occasional slug among them. Even seed eaters hunt for slugs to feed their young who need protein. To enlist birds in your fight against pest slugs and snails provide food, water and shelter for bird families. See the file on Attracting Birds
Feed the Birds: Winter songbirds eagerly probe garden debris and mulch for slug and snail eggs. Encourage them to stay in the area by putting out seed and other foods such as suet blocks for them. In the spring and summer reduce the seed in your bird feeders, but provide snacks to encourage bird families to nest in or near your yard. See the file on Feeding Birds
Water the Birds: Fresh water is critical for birds. Maintain one or 2 filled bird baths for all the birds plus bees and other beneficial predators of slugs and snails. Consider acquiring an automatic self filling bird bath or a bird bath heater so that water is always available in the baths. See the file on Watering Birds
Provide Shelter: Some birds, such as chickadees, house wrens, sparrows, and purple martins will nest in a bird house if you provide one that is the correct size and mounted at the correct height. Birdhouses are easy and inexpensive to buy or build and are readily available mail order. See the file on Housing Birds
Delay Planting Spring Vegetables
For those crops such as lettuce that are routinely bothered by slugs which emerge about the same time the seedlings are growing, a good preventative measure is to delay planting for a few weeks. Since slugs are less active when air temperatures exceed 70°F crops get some relief. Combined with use of copper stripping around boxed planting beds, and other control measures this planting trick is effective. Continue using lots of organic mulch in the vegetable garden to harbor beneficial predators as another line of defense.