With the rainy weather that we’ve had in many parts of Illinois, slugs are very numerous and causing heavy damage. Slugs, which are shell-less snails, are usually a problem on thin-leaved plants growing in shady areas--hosta, violets, and impatiens. With this summer’s prolonged, heavy rainfall, slugs are also numerous in sunnier areas, feeding on a wide range of plants--including such bedding plants as petunias, chrysanthemums, daisies, and lobelia. They also like lilies, daffodils, narcissus, gentians, primroses, tuberous begonias, hollyhocks, irises, and strawberries.
The most common species is the gray garden slug, which is usually about 3/4 inch long but may be up to 1-1/2 inches long. Although called gray, they come in white, yellow, lavender, purple, or blackish with brown specks and mottled areas. A less common species in Illinois is the spotted garden slug, which can be seven inches long but is more commonly three to five inches in length. Although small individuals tend to be dark gray to black, large ones are handsome: yellow to brown with black mottling and three rows of black spots running down the posterior half of the body.
Depending on the species, slugs live from one to six years. They lay eggs in protected areas such as under dead leaves or pieces of bark mulch. Slugs are hermaphroditic, having both male and female sex organs. A slug can mate with itself if another isn’t present; more commonly, a slug both provides and receives sperm while mating with another.
Slugs have two pairs of tentacles extending off the front end of the body. The upper, longer pair are optic tentacles with eyes on the tips. There is also a shorter pair near the ground that are sensory tentacles for feeling and smelling. Near the front of the body on the right side is a hole or slit called a pneumostome, which is the opening that leads to the slug’s single lung. The largest structure is the foot, which runs the length of the slug. The underside of the foot is called the sole.
At the front of the sole, under the slug’s head, is a gland that produces two types of slime or mucus. One mucus is very free-flowing; the other is more viscous. These two substances combine to form the slime trails that slugs are famous for. These slime trails remain in the morning and will glisten, reflecting the sunlight before they dry up. Mucus is produced in smaller quantities over the entire outside of the body. Different types of mucus are produced for moisture control, mating, and defense. When attacked, a slug produces an extra-thick mucus that makes the slug hard to pick up. The defense mucus is also capable of sealing the mouths of such predators as snakes and shrews and can cause dogs and ducks to gag.
Slugs feed by using a radula, a structure in the mouth that is covered with tiny teeth, which slugs use to scrape away the surface and then the plant material underneath. This feeding mechanism causes damage to appear most commonly as holes in the leaf, although windowfeeding (when slugs eat only partly through) on the leaf is common. On some plants or in heavy numbers, slugs will eat the leaf margins. Because chewing insects usually eat the leaf margins, large holes in the leaves are a good clue that slugs are present. You can verify that slugs are responsible by checking for their presence at night or on foggy mornings, or by searching the ground beneath the plants. Slime trails on the plant are also good scouting clues.
Slugs need a moist environment to survive, and they feed on decaying organic matter. The best long-term control involves reducing this supply. Under less rainy conditions, spacing plants farther apart or pruning them back allows better air circulation and creates drier conditions that are difficult for slugs. Eliminating fallen leaves, bark mulch, and other dead organic material will reduce slug numbers by reducing food sources.
Slug baits containing metaldehyde or mesurol are effective for controlling slugs. Mesurol can be used only around ornamental plants, not edible ones. Be careful using slug baits where dogs or cats are present. Slug-bait poisoning is a major source of calls to the National Animal Poison Control Center located in Urbana. Copper strips that extend an inch or more below and above the soil line will keep out slugs. The copper apparently generates an electrical charge that is large enough to deter slugs.
Most other slug remedies are not reliably effective. The mucus that slugs produce allows them to cross a razor-blade edge without harm, making the use of sharp gravel, broken glass, and cinders of dubious value. Beer in shallow dishes is effective sometimes, although my experience is that this method allows the harvest of slugs but doesn’t actually control them. Diatomaceous earth is not very effective in the damp environment that slugs inhabit. Salt, lime, and other chemicals may disrupt the soil’s fertility until nothing will grow, which defeats the purpose. Because slugs and snails are mollusks (very distantly related to insects and other arthropods), insecticides are not effective.