Slugs are cousins of snails, and seem to have no shells. They actually have much smaller shells, hidden beneath the flesh of their backs. In the U.S. alone, there are more than 40 species of slugs. Most common is the common garden slug, which grows to be about an inch in length with dark skin and a light stripe on one side. These slimy critters can damage lawns and gardens, yet can prove difficult to exterminate, as they come out to feed only at night and hide during the day in dark, damp places like under decking.
Find a way into the underside of your deck such as an opening you may be able to fit through. Otherwise, you will need to remove some boards to create access.
Crawl into the underside of your deck with a trash bag in hand. If you are easily grossed out, do this during the day, when the slugs are burrowed into the soil and you are less likely to come face-to-face with any of the critters.
Pull up any weeds growing under your deck and eliminate any debris you find under there. Weeds and litter are a slug's dream. Slugs may also hide among shrubs and other plants near your deck.
Lay down a layer of dehydrating agent. Many household items can be used for this, including crushed egg shells, Epsom salt, talcum powder and diatomaceous earth. Cedar and oak bark chips are also effective at dehydrating slugs. This will kill grown slugs and prevent eggs from hatching.
Use chemical baits for particularly difficult slug infestations. Iron phosphate compounds are non-toxic and safer touse around children, pets and wildlife, which can confuse grain-based baits with feed. Meal, pellets, granule and liquid baits are all available commercially.
Lay down more dehydrating agent periodically, as rainwater can wash it away, allowing eggs buried deep in the soil to hatch. Just because you haven't seen a slug in a while doesn't mean your problem is completely gone. Slug eggs can survive for years, waiting for conditions to be just right for them to hatch.
If you get slimed while fighting a slug infestation, simply wash the slimed area in a solution of water and vinegar. The same efforts you make to eliminate slugs also work for garden snails.