No garden pest elicits a more visceral response than the lowly slug. And its cousin, the snail, equally capable of destruction of prized vegetables and flowers, is only partially redeemed by its reputation among gourmets. You'll never be able to eliminate all of the snails and slugs in your garden as proved by one English zoologist who removed 400 slugs from a quarter-acre garden each night for several years with no observable effect on the population. And as snails and slugs are basically small cogs in nature's big machine, there are many good reasons why you shouldn't even try. The challenge for most gardeners is to maintain a balance.
The more familiar you are with slugs' and snails' patterns for raiding your garden, the more capable you will be of limiting damage to your plants. You're most likely to find garden snails and slugs during months with moderate but consistent rainfall and nighttime temperatures above 50°F. And, with a few exceptions, they are most active at night. Go into your garden after dark or shortly before sunrise, armed with a flashlight or, better yet, a headlamp, which will leave both hands free. Look for large, irregularly shaped holes in the leaves of your plants and telltale slime trails. Periodically conduct these informal surveys of slug populations and then try the most humane and least environmentally damaging control measures first.
The first step is making your property less desirable for these creatures. This can involve choosing plants such as rhododendrons and other hard-leaved evergreens that slugs and snails will ignore and avoiding those that they actively seek. Both snails and slugs feast on most plants, especially young, tender transplants, leafy vegetables and succulent plant parts. Slugs particularly love Campanula carpatica, Chinese cabbage, daffodil, delphinium, leopard's bane (Doronicum), gentian, horseradish, hosta, lettuce, lilies, mustard greens, perennial lobelia, marigolds, primroses, strawberries, Trillium, tuberous begonias and Viola.
Keep the garden clear of debris that offers cool, dark, moist, hiding places and don't overwater. Dense ground covers of ivy and succulents are notorious snail and slug havens. Avoid mulching, or wait until the plants are well established or the temperature is over 70°F. Cultivate frequently to destroy snail and slug eggs before they hatch.
Ground beetles, garter snakes, moles and shrews all prey on slugs. You can also invite slug-eating predators into your yard. Unsupervised chickens, geese and ducks sometimes cause more damage to plants than the slugs do, however. In some regions, you can rely upon the decollate snail (Ruminia decollata), a snail that prefers to eat snails and slugs over vegetation. They are available from some biological control suppliers.
Hand-picking snails and slugs from your plants can be an effective control. If you're squeamish about slime, wear rubber gloves or use a long-handled dandelion digger or a pair of long tweezers. Plop any captives into a jar (filled with soapy water so they don't crawl out) and use a screw top, as they have been known to push with sufficient force to pop the lid off of a yogurt container.
Traps can be either store-bought or built at home. One time-honored device is the beer trap: a shallow pan or saucer (a plastic butter tub is a good depth) sunk about halfway into the ground and filled with beer. It lures the slugs with the scent of malt and yeast. Cut a few 1-inch-square doors at the soil level and use the lid to deflect rain, thus preventing dilution. Adding a dash of baker's yeast makes a beer trap more effective. Avoid the impulse to empty the trap each day, as most slugs are attracted by the dead bodies of their own kind, but the beer will lose its potency eventually, so refresh it every two or three days. An equally potent attractant can be concocted from 2 tablespoons of flour, 1/2 teaspoon of brewer's yeast and 1 teaspoon of sugar mixed with 2 cups of warm water.
A very simple trap consists of two boards, one on top of the other, separated by a few small stones. In the morning, remove the stones and stomp on the upper board, crushing any slugs or snails that have sought sanctuary there. Other hungry slugs will be attracted by the mashed bodies.
Other effective lures for shade-loving snails and slugs include grapefruit or melon rinds, two-gallon flowerpots or green plastic leaf bags strategically placed on the soil. Check them first thing in the morning, before the inhabitants seek cooler, moister shelters.
A more refined trap, the slug hotel, can be made from an empty plastic soda bottle. Cut the bottle at its shoulder, just before it starts to taper toward the neck. Stick the piece you have just cut off into the bottle, neck first. Tape the two pieces together with duct or electrical tape. Fill the trap half full with beer or apple cider and bury it sideways in your garden, so that the entrance is level with the ground. When your hotel is full, untape the top and empty its contents into the garbage or compost. Refill it with beer or cider and post a vacancy sign.
Perhaps the best barrier is made of solid copper (available at garden centers or sold at hardware stores as copper flashing) in bands at least 3-inches wide. A snail or slug that comes into contact with one of these bands receives a slight electric shock. To increase effectiveness, bend the upper edge of each band to form a flange. Copper is expensive, so it's cost-effective to group snail- or slug-prone plants. Don't fret when your copper bands turn green with age; effectiveness isn't affected.
Many commercial snail and slug baits are available today as pellets, meals or emulsions. Most combine an attractant with an active ingredient, usually metaldehyde. Snails or slugs that consume metaldehyde are semiparalyzed, so are unable to crawl off into moist shade after a night of foraging. They die by dehydration when the sun comes up. But if weather is cool or rainy, they'll often slough off an otherwise lethal dose.
For gardeners concerned about the safety of children, pets and wild animals, there's another option. In the last few years, iron phosphate bait has made its way from Europe to the United States. This product is safer for non-target creatures, but slays slugs as well as metaldehyde baits. Sluggo and Escar-go are two common brands of iron phosphate bait. As with metaldehyde products, slugs consume the bait, immediately stop feeding, and die after a few days.