If you have a shady, moist spot in your garden, you likely have slugs. If you grow vegetables or fruit, you may have slugs. If you grow hostas, you're almost guaranteed to have slugs. If your garden is heavily mulched, you are likely to have slugs. With so many places for slugs to hang out, and so much for them to eat, it's no wonder that they are one of the top garden pests.
Nor are they picky eaters. They'll chow down on most succulent plants, vegetables, fruits, decaying plant matter, even the bark of young trees, but it's a special insult when you find a slimy little body on that ripe tomato you'd planned to pick today.
Their feeding tears jagged holes in plants, which looks unsightly. Mature plants can handle most slug feeding, though young plants without a lot of leaves can die. Slugs aren't insects; they are mollusks, relatives of oysters. They can range in length from a ¼ inch to a gross 2 inches or more. And they leave silvery slime trails in their wakes. They are most active at night, though can be spotted in the daytime in cool places in the garden. Snails have shells, slugs don't. Besides that difference, what goes for slugs goes for snails, too.
Slugs and snails' enemies are heat and light. If you are trying to control them, remove hiding places such as boards, logs, stones and debris piles, even thick groundcovers, and try to site vegetable or fruit gardens away from those materials.
Slugs can be controlled with chemicals, or organically. They are pretty easy to kill without having to resort to serious poisons (see commercial bait traps, below). Combing cultural tactics, such as removing hiding places, with active controls, such as handpicking, usually works better than doing one alone.
Handpicking is an effective and easy way to control slugs. To find them, go out at night with a flashlight, and look on the underside of leaves. Drop them into a bucket of soapy water, into the trash, or squish them. Wear gloves. Once you get the initial population down, weekly handpicking will keep them under control.
Commercial bait traps. They work well in conjunction with other control methods, but won't reduce a big population alone. Those that contain metaldehyde are poisonous to animals, especially dogs, and shouldn't be used around pets, children, or allowed to spill on plants. Iron phosphate baits, such as Sluggo, are safer. There's more to using these baits than 'just sprinkle and you're done', so be sure to read directions to know what conditions make them most effective.
Diatomaceous earth or dried eggshells have sharp edges, which cut slugs' soft underbellies and send them elsewhere. DE must be reapplied after rain. Copper strips will also repel slugs, but won't kill them. They are a good choice for containers, but impractical in larger spaces.
Salt. Sprinkling salt directly on slugs will kill them, but could affect surrounding plants as well. In large enough quantities, salt will make soil infertile. It's best to skip this one, since other methods work just as well, with less collateral damage.
Beer traps. Slugs and snails love beer (the fermented yeasts, actually). You can set a container into the ground and they'll belly up to the bar and literally drown their sorrows. Beer traps are more trouble than they're worth though, because you have to empty and replenish the beer frequently, and make sure the traps are deep, with straight sides so the slugs can't crawl out. Other methods are easier and faster. Plus, there are better uses for beer.