The gray garden slug (Deroceras reticulatum), introduced from Europe, is the most damaging slug species in the United States. These slugs vary in color from cream to a dark mottled gray and can reach a length of two inches. Milky slug is another common name for this species, as it produces copious milky slime when irritated. Juveniles and adults feed on a wide range of edible and ornamental plants. Young plants are especially vulnerable to attack.
BIOLOGY: Mollusks without shells, slugs glide on a large, fleshy, muscular “foot” that secretes mucus. The slime provides physical protection, and the trail of slime helps slugs return to a particular feeding spot. The head has two pairs of retractable tentacles—the longer pair has light-sensitive eyes located at the end of each stalk, and the smaller pair is used for feeling or smelling. Rasping mouthparts scrape ragged holes in leaves and flowers.
Adults are hermaphroditic, having both male and female sex organs, although cross-fertilization is the norm. Most egg-laying occurs in the spring and fall. The 20 to 30 gelatinous eggs are laid in a clutch just beneath the soil surface. Eggs hatch in 15 to 20 days, but will not hatch during long, dry periods.
The juveniles take three to four months to mature. Adults live up to a year and can lay up to 300 eggs during that time. There may be one or more generations per year, depending on conditions. Wet springs and summers lead to large populations.
SYMPTOMS: Silvery slime trails and irregularly shaped holes in the leaves are the most characteristic signs of slugs. Most of the damage occurs close to the ground. A close inspection of plants at night, early in the morning, or on cloudy, wet days is the best way to catch slugs while they are feeding.
CONTROL: Where slugs can be handpicked, they can be dropped into soapy water. Setting out an inverted clay pot, board, or grapefruit rind near favorite plants will give the slugs a place to congregate. Pitfall traps baited with beer or a simple mix of sugar water and yeast will attract and drown slugs, although these must be filled and emptied regularly.
Their protective layer of slime makes many physical barriers ineffective, although bands of wood ashes and diatomaceous earth have proven useful. More effective are strips of copper foil (such as Snail-Barr) or even copper screen attached to the edges of pots, raised beds, and tree trunks.
Poison baits containing iron phosphate (such as Sluggo) are a newer and safer alternative to the now-restricted metaldehyde slug and snail baits. Finally, since slugs prefer moist conditions, eliminating trash and other debris in the garden, limiting irrigation, and delaying mulching until seedlings have grown larger will reduce problems with slugs.