Slugs

Slugs can be very damaging pests in moist, shady gardens. They feed on the  leaves of many plants, especially seedlings. Later in the season they can feed on ripening fruits and vegetables. Slugs are especially numerous during rainy seasons and in wellirrigated gardens. If slugs are abundant one year, it does not mean they will be as common the following season; the relative number of slugs in a given season depends on how moist  the growing conditions are. The gray garden slug is the most common and most destructive slug in Rhode Island.

Slugs are best described as snails without shells. They are a type of mollusk, that is, related to clams and oysters. Slugs are soft bodied with eye stalks and they vary in size from 1/4 inch to 2 inches or longer. They leave a silvery slime trail that they secrete as they move.

Damage

Slugs use file-like mouthparts to rasp and chew plant tissue. Because of their mouthparts, they create irregularly shaped holes. The grating action produces a large wound on the leaf surface nearest the slug that gradually tapers to a smaller hole through the  opposite surface. Slug injury to cabbage appears ragged compared with the clean-sided incision typical of caterpillar feeding. The presence of a glistening slime trail can sometimes be used to distinguish slug injury. Feeding damage can be cosmetic, however, extensive  feeding can result in plant stress or even death.

Slugs attack the fruit of tomatoes and strawberries leaving  small, shallow holes in the fruit's surface.

During wet growing seasons, large numbers of slugs survive the summer months and may move into gardens or cultivated fields from weedy borders, drainage ditches, or other moist, sheltered areas. Landscape or crop damage is usually most severe in late summer or early autumn of cool, wet growing seasons, especially when preceded by a mild winter.

Monitoring

A covered pit can provide a humid, sheltered hiding place for slugs during daylight  hours. The pit should be 4 inches in diameter and 6 inches deep. An aluminum foil-covered shingle or a board can be used as a cover to provide a cool refuge from the  sun. Slugs tend to congregate in large numbers in these shelters and may be counted and destroyed during daylight hours. Set monitoring traps near field and garden borders. The traps will not function as well in weedy areas or with crops such as cabbage which provide adequate shelter for slugs beneath large leaves close to the ground. Control measures should be applied when one to five slugs per trap are found.

Cultural Control

• Maintain good weed control within the garden or field and along borders to eliminate daytime refuges of slugs.

• Rake in early spring to remove leaves, plant debris, and slug eggs.

• Prune lower leaves or stake large plants to reduce potential hiding places for slugs and to allow better air circulation that helps keep the soil surface drier.

• Thin or divide plants if they are too crowded.

• Create a border from anything dry, dusty or scratchy, such as lime, road dust, diatomaceous earth, cinders, coarse sawdust, gravel, or sand. (Never use salt, as it ruins soil for most plant growth.) When slugs crawl over these materials they secrete mucus to free themselves which soon exhausts them and they die.

• Water your garden only when necessary. Irrigate in the morning so plants are dry by evening.

• A fly screen, 4 inches wide, placed on edge and partly embedded in soil for support, will keep slugs out of an area.

• Boards, bark, or other materials not less than 6 inches square make effective traps when placed in gardens. Each morning slugs can be gathered from under the traps and destroyed.

• Slugs can also be hand-picked off garden plants at night using a flashlight and a pair of disposable gloves. However, since only a portion of the population is active on a given night, it can take quite of bit of slug harvesting before there is a noticeable impact on the population.

• Slugs can be kept from potted plants by supporting pots over a pan of water.

• Slugs are attracted to and drown in a shallow dishes containing beer or baker's yeast dissolved in water. Set the top edges of the dish at ground level and cover loosely with a board so slugs can easily get into the mixture.

• Choice of mulch can have a large impact on slug problems in flower gardens. Large bark chips and wood chips provide excellent hiding places and favor slug outbreaks. Shredded pine bark is less attractive to slugs; cocoa hulls seem to repel them.

Toads are the most important natural enemy of slugs.

Chemical Control

Iron phosphate (e.g. Escar-Go, Sluggo), applied to the soil as granules, is a less toxic bait for slugs. Iron phosphate is mixed with a food product that draws slugs to the bait. Once slugs consume this bait, they stop feeding and die three to six days later.

Chemical slug baits often contain metaldehyde, available as a granular or liquid paste. When metaldehyde is eaten by slugs, it destroys their ability to move and digest food. Apply it to the soil near slug-infested plants. Metaldehyde is more effective during warm, dry weather. It is best to apply metaldehyde after a rain storm but when sunny weather is predicted.

Copper compounds (copper silicate and copper sulfate) are effective repellents. They are usually mixed with water and then sprayed on plants. Copper products repel slugs but do not usually kill them. Do not spray copper compounds near baits; slugs will avoid baits contaminated with them.

Always use insecticides strictly in accordance with label statements and directions.

Adapted from the University of Connecticut Integrated Pest Management Program; University o Maine Cooperative Extension, 1999; University of Minnesota Extension