Slugs

Garden crops have a lot of enemies, but few are as non-selective and destructive as slugs. Unlike plant diseases and many insect pests, slugs are not host-specific and have a very diverse diet from asters to zinnias, with hostas being one of their favorites.  As much as we hate them for the damage they do to our garden plants, we need to recognize slugs as an important part of the ecosystems because many species are decomposers and feed on fallen leaves, dead insects, and dead worms. They are also food for snakes, toads, turtles, and birds.

Description & Life Cycle

Slugs are close relatives of snails, clams, and oysters in the phylum Mollusca. These animals have soft, unsegmented bodies usually protected by a hard calcareous shell. Slugs look very similar to snails but without the external shell. The shell of a slug is greatly reduced and located internally beneath the fleshy mantle on their backs.

Depending on the species, slugs range in size from less than an inch to 10 inches long, such as the banana slug, native to the West Coast. One of the most common slug species found in Iowa gardens is the gray garden slug. They are typically less than an inch long, and their plump, slimy bodies range in color from light gray to brownish black.

Slugs require a damp environment to survive. Periodic drying that occurs in Iowa gardens may be one of the factors that limits this animal to relative obscurity except in wet years. Unfortunately, they don’t die in the middle of the summer when conditions get hot and dry. Slugs bury themselves in the soil or find a moist, well-protected spot where they remain in a state of suspended animation. They secrete a mucous-like cocoon around themselves and wait it out until there is enough rain or moisture to dissolve the mucous and soak the water into their bodies. They can lose as much as 50 percent of their weight during a dry spell and regain it after only two hours of rehydration. 

Most slug species overwinter as adults. In the spring or early summer, eggs are laid in moist areas near the soil surface, such as under dead leaves, rocks, mulch, or flower pots.  Baby slugs hatch when there is plenty of moisture present and begin feeding immediately. They resemble adults but are smaller and lighter gray in color.

Damage Caused

Slugs are nocturnal and feed at night when we can’t see them. They prefer cool, dark, moist hiding places during the day to protect them from drying out. They are usually found in the daytime in soil crevices or under boards, rocks, mulch, debris, or the foliage of low, dense plants.

Cool, wet spring conditions are ideal for slugs, resulting in early, serious damage to plants. They destroy young seedlings and chew holes in hosta foliage, rasping large, irregular holes and leaving them unattractive the entire season. Slug damage can be as serious in late summer as in the spring if cool, wet conditions return.

Slugs secrete a slimy mucous trail as they move across plants and smooth objects. This prevents them from drying out and provides a protective track for gliding across sharp surfaces. It can be seen as a silvery trail on leaves. The silvery slime trails left as they travel help distinguish slug damage from that of other leaf-feeding pests such as variegated cutworms.

Management 

Slug treatment may be occasionally necessary, but not always. Several integrated pest management strategies can be used to control slugs. 

Cultural Control

One tactic is to reduce the favorable habitats where slugs live and reproduce. Since they require moist soil to lay their eggs and cool, moist, sheltered sites to hide during the day, open up the garden to allow more sun and air circulation so that the soil isn’t damp for extended periods. Keep mulch layers shallow; a uniform, one-inch layer will prevent rapid drying around the plants and will not retain excessive soil moisture. Since slugs also feed on decaying plant material, do not mulch with fresh grass clippings where slugs are a problem, and rake leaves from the garden beds in the fall.

Mechanical Control

Traps are a mechanical form of slug control. Trap boards or moist newspaper or carpet samples, about a square foot in size, can be placed around plants where slugs have been feeding. After a couple of days, check the underside of the traps and remove and destroy the slugs that have gathered there to hide. Inverted melon rinds set on the soil will also attract slugs but may make your garden look like a waste site or compost pile.

Slugs are also attracted to beer, so it is often used to trap slugs.  A beer trap consists of a shallow container, such as a yogurt cup, buried to within a half inch of the rim and filled with beer. Slugs will find it irresistible, crawl in, and drown. You may want to put a loose cover over the beer trap to shade it and prevent rain from diluting it.

Some gardeners get great satisfaction from stalking slugs early in the morning, sprinkling salt on unsuspecting slugs, and then watching them shrivel up as the salt removes water from their bodies. This rather sadistic method is neither an efficient nor effective control method.

Chemical control

Since slugs are not insects, they are not controlled with insecticides but rather with molluscicides applied as bait. Apply slug baits in the spring or fall when slugs are active. It is a good idea to irrigate before applying bait to promote slug activity and apply it in the late afternoon or evening. Many baits contain metaldehyde. Although it is effective, it has its faults. It is rapidly inactivated by sunlight and water, so it has to be reapplied frequently. It cannot be used in vegetable gardens and can be toxic to pets that ingest it. Alternative baits are available that contain iron phosphate (ferric phosphate) as the active ingredient. Trade names include Sluggo and Escar-Go! Schultz Slug and Snail Bait and others.  Although some experts consider iron phosphate baits slightly less effective than baits containing metaldehyde, they have several advantages. They can be used more effectively under high moisture conditions because iron phosphate doesn’t readily dissolve in water, they can be used around edible crops, and they do not pose a threat to pets, birds, and other non-target species.