This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
It takes only a little rain to bring garden slugs out of hiding. Feeding at night, slugs chew irregular holes in a long list of vegetables, and they are especially fond of salad greens, beans and cabbage family crops. Deep organic mulches provides habitat for slugs, so problems are most severe in heavily mulched gardens during periods of prolonged rain. Good organic slug control methods include handpicking, habitat modification, traps, copper barriers and commercial baits based on iron sulfate.
What Are Slugs?
Soft-bodied mollusks often described as snails without shells, slugs exude sticky slime as they move across leaves. Rarely seen on sunny days, slugs feed at night and during periods of wet, overcast weather. Slugs damage plants primarily by feeding on foliage, though they will also make holes in tomatoes, beans and other vegetables.
Several species of different sizes often inhabit the same garden. Small gray milk slugs are frequent problems on lettuce, Chinese cabbage and other leafy greens. Larger gray garden slugs climb up taller plants to damage leaves and fruits. During the day, slugs hide in mulch or inside the boards used to structure beds.
What Slug Damage Looks Like
Slugs chew irregular holes with smooth edges, which often appear overnight. In spring when little other food is available, slugs may chew young seedlings down to the ground. Frequently slugs return to the same plant to feed for several nights. Faint shiny trails of slug slime are easily seen early in the morning and confirm the presence of garden slugs. In leafy greens, tiny slugs can often be found hiding in leaf crevices near the bases of the plants.
Some slugs overwinter as adults, but buried eggs are the primary way slugs survive from year to year. Egg laying takes place mostly in spring and fall, though slug populations invariably follow on rainfall. Slugs cannot move unless the soil and their feeding areas are moist.
Natural Slug Predators
Numerous natural predators consume slugs, including frogs and toads, small snakes and turtles. In the soil, eggs and young slugs are eaten by ground beetles and firefly larvae. Among domestic animals, ducks are highly rated for their slug control talents.
Organic Slug Control
The most fundamental way to impact slug populations is by making your garden as inhospitable as possible. Delay mulching until the beginning of your dry season. Irrigate with soaker hoses, which do not wet the foliage, and avoid using hoses or sprinklers late in the day. Thin leafy greens to help the sun dry out the soil between plants. When a period of dry weather is predicted, sprinkle diatomaceous earth (DE) atop the soil beneath high-risk plants like lettuce or bush beans. Until rain washes it into the soil, DE deters slugs by abrading their bodies as they move across it.
You can lure many slugs away from vegetables using slug traps. These can be as simple as orange or grapefruit rind halves placed on the soil at night, or you can make pit traps. One popular and effective version involves placing an inch of beer (or sugar water with a pinch of yeast) in a small container such as a tuna can or yogurt tub, and burying it up to its rim among plants. At night, slugs crawl in and drown. These slug traps must be emptied and refilled daily for maximum effectiveness.
Another good slug trap can be made by digging narrow holes, four inches wide and six inches deep, and covering them with a roofing shingle or piece of plywood wrapped in aluminum foil. Leave the trap alone for two to three days; as the moist conditions in the pit trap are discovered by slugs, they invite others to join them. Another popular method is to place wet newspapers beneath boards laid on the soil’s surface. Slugs use these traps as daytime shelter. Gather slugs caught in various traps and drown them in soapy water before dumping them in the compost.
Need more organic slug control methods? Caffeine can be an effective spray, whether you use cold coffee or caffeine tablets dissolved in water. When slugs are a problem in containers or small raised beds, encircling the perimeter with copper tape can stop the invasion. Slugs suffer an electric shock when they must cross copper.
All of these methods should be supplemented with attentive handpicking. This often can be done on cloudy mornings or during the day in drizzly weather.
As a last resort, try slug baits that use iron phosphate as their active ingredient. This is often a good solution in perennial beds that are kept mulched year round.
More Advice on Organic Slug Control
Look for secret slug habitats around your garden and move them away from your crop plants. Logs, upturned flower pots, boards, and compost piles placed along the edge of the garden can keep enough slugs around to satisfy natural slug predators, which also consume other garden pests.
At the end of the season, allow chickens to feed in beds where slugs were present. They will eat large numbers of slugs and slug eggs. In climates where high rainfall favors slugs, ducks may be enlisted as excellent slug slayers.
Historically, salt was applied to slugs to kill them, but any salt sprinkled on slugs will end up in your soil, where it may prove troublesome for sensitive plants.
More information on organic slug control is available from the University of Illinois, Ohio State University and Cornell University.