Diatomaceous earth (DE).
After testing all kinds of slug barriers, Jeff Gillman, author of The Truth about Garden Remedies and professor of horticulture at the University of Minnesota, concluded that DE is the most reliable. "DE is a white powder made from the fossilized remains of diatoms, one-celled algae that have a skeleton made of silicon," Gillman says. "To a slithering slug, this lethal powder is extremely sharp and cuts their undersides, causing dehydration." DE does have to be replenished each time it rains, making it a better choice for climates where it does not rain frequently. (Note: Buy only untreated diatomaceous earth formulated for garden use, and wear a dust mask when applying it. DE made for swimming pools is chemically altered and not suitable for use in any garden, much less an organic one.)
Beer trap. This type of trap works because slugs are attracted to the fermented yeast in beer. In a study by Whitney Cranshaw, Ph.D., professor of entomology at Colorado State University, nonalcoholic Kingsbury Malt Beverage was found to attract the most slugs. Michelob and Budweiser placed second and third out of the 12 beverages tested. (We're still waiting for the results from the gardeners' taste test.) "Take a shallow container (such as a sour cream or yogurt cup) and bury it so that it is even with the soil level," explains gardening expert Willi Evans Galloway, who gardens in Seattle. "Then fill the container with beer to within an inch of the rim. The slugs crawl in and drown." For best results, change the beer every few days.
Copper barrier. When a slug crosses a copper barrier, its moist, mucusy body reacts with the copper and it receives an electric shock. Copper barriers can be pushed into the soil to make a vertical fence around a plant or bed, or laid flat. Either way, use 2- to 3-inch strips. Yes, this product can be costly, so save it for your most prized plants or where the slugs are congregating.