Slugs are usually not a problem in Iowa vegetable gardens and flower beds, although prolonged periods of wet weather like we have had for the past year may change our luck. Slugs are familiar pests because of the frequent write-ups they get in gardening publications from eastern U.S. where conditions for slugs are more favorable.
Slugs are close relatives of snails, clams and oysters in the phylum Mollusca. These animals have soft, unsegmented bodies that are 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.
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. Slugs are protected from drying by hiding during the day and feeding at night. They are usually found in the daytime in soil crevices or under boards, rocks, mulch, debris or the foliage of low, dense plants.
Both snails and slugs move by gliding on a secretion of mucous or slime. As they move about at night they feed on tender plant parts by rasping large, irregular holes. When abundant, they may cause considerable damage to newly transplanted bedding plants and large-leafed ornamentals such as hostas. The silvery slime trails left as they travel help distinguish slug damage from that of other leaf-feeding pests such as variegated cutworms.
Slug treatment may be occasionally necessary, but not always. Cleaning up the garden and eliminating slug hiding places as practical may help. Heavy leaf litter, boards, bricks and other piles of damp debris in contact with the ground should be removed. Dense ground covers that are harboring slugs can be thinned to promote sunshine, air circulation and drying.
Minor slug problems can be controlled by handpicking. Check carefully around the base of damaged plants and favored hiding places. Night checking with a flashlight may improve your efficiency, or you can leave 'trap sites' to be checked on a regular basis. Good slug 'traps' or convenient hiding places under which the slugs will retreat include overturned pots and a piece of board or asphalt shingle.
Beer is a well-known trap attractant for slugs, though any fermenting or yeast-containing liquid appears to work. The traditional trap design is to bury a shallow pan in the soil with the top edge level with the soil surface. Renew the beer or attractant regularly and empty the pan of trapped slugs frequently.
Chemical control of slugs will require a special molluscicide. The most commonly available product contains metaldehyde. Formulations available are liquid, granular or pelleted baits with labels for both vegetable gardens and ornamental beds. Read and follow all label directions.
This article originally appeared in the June 30, 1993 issue, pp. 1993 issue, pp. 106-107.