Slugs and snails
In cool humid climates there may be 200 slugs on every square yard of your garden. They generally prefer to eat old decaying material and important decomposing organisms, but if that isn’t available they will eat almost any crop plants (though they have their preferences). Slugs can be a real problem in cool, wet conditions. They are extremely voracious and when abundant they can devour an entire bed of seedlings in a night, or strip almost mature plants. They also reproduce rapidly and can produce up to three generations annually. They are hermaphrodites so they don’t even need to mate, and each one is capable of producing 400 round white eggs annually.
You can usually identify their mollusk handiwork by the shiny trail of mucus they leave behind as they move. They tend to chew leaves from their outer edges and may devour a young plant right down to the stem and a few of the tougher leaf mid ribs. Slugs originally had shells like snails, but abandoned them in favor of their ability to hide in crevices in the soil and under rocks (they burrow down into the soil when it gets too cold or hot). They also hide under boards, rocks, and leaves, so keep such debris out of the garden. Boards can actually be used as slug traps, so long as you check them every morning.
Snails hide in dense vegetation or cracks in your walls during the day and come out to eat at night. The easiest way to eliminate snails, is to remove all of the places that can act as a refuge for them. Cover crop beds can be completely defoliated on the side neighboring dense vegetation, while the opposite side will be untouched (though the defoliation will slowly spread). Clearing an old crop may force the snails to move elsewhere, so be prepared.
Hand picking: The most effective and low impact way to control slugs and snails is by hand picking. This is best done at night or early morning and must be done regularly if it is to have much effect. You will soon learn to find their hiding places. Slugs commonly hide down in crevices in the soil and you can often dig down around a damaged plant to find the culprit. You can squash these creatures as you pick them, or drop them in a bucket of salt water.
If you don’t want to kill them you can collect them in a bucket of leaves and transport them a few miles. If you have any gardening rivals you could drop them in the particular garden with the rivals.
Barriers: Hand picking alone won’t solve all your mollusk problems. They may move at a snails pace, but they are surprisingly mobile (some can walk a mile in a few days) and more can simply come to where there is food. Ideally you could erect some kind of barrier, such as a 2-foot wide path of cinders or crushed oyster shells. Sharp sand, wood ashes or sawdust also work apparently, but must be kept dry or replenished frequently. The razor sharp fragments of diatomaceous earth are lethal to the soft bodies of slugs and snails (they don’t do human lungs any good either - wear a mask). Other barriers are copper strips (they won’t cross it), wire screen and tilted boards with grease on the undersides.
Trapping: Cups of beer, milk or yeast (dissolved in sugar water) will catch quite a few mollusks (and a few other creatures). Put this in a cup with the rim about a half-inch above the soil surface. There needs to be a lot of these traps to be effective, at least one for every square yard. Traps can also be made from cabbage leaves, boards, grapefruit skins and cut potatoes, though these don’t kill, they merely make collecting easier (you must check them daily).
Avoid mulch: Slugs can easily hide in mulch, so it is usually avoided in areas where they are a big problem.
Ducks: These birds love slugs and snails and are one of the best controls. Chickens like them as well, but do more damage to crops. Toads, snakes, birds and Ground Beetles (probably the most important) all kill slugs and snails. There are even predatory slugs (Testacella species) which eat other slugs (cannibal slugs).
Metaldehyde: This poison is widely used for killing slugs, but probably will only kill about 10% of those present, it's not very effective. It is also poisonous to Ground Beetles (a main predator) and mammals, so must be used with caution (poisoned slugs may poison slug predators). It works best in the greenhouse, where predators are usually absent.
Image: Joseph Berger, Bugwood.org