integrated pest management


Slugs are mollusks, related to oysters and clams. Slugs are referred to a "naked snails," as they possess no shell. Slugs lay clusters of translucent, pearly-shaped eggs under debris or buried beneath the soil surface. They can lay between 20 to 100 eggs several times per year, taking approximately two years for slugs to reach maturity. The gray garden slug, Peroceras reticulatum, is the slug generally encountered in Illinois. It is approximately 3/4 to 1-1/2 inches long, ranging in color from pale yellow, lavender, to purple. In addition, they are covered with black or brown spots and mottling. Less commonly encountered is the great gray garden slug, Limax maximus. It is up to 7 inches long and is yellow-gray to brown with black mottling.

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Gray Garden Slug
Figure 1. Gray Garden Slug

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Spotted Garden Slug Damage
Figure 2. Spotted Garden Slug Damage

Slug activity is highly dependent on soil moisture, requiring moisture to move around. They secrete a slimy mucus substance, which they use to move about. This then dries up into a shiny noticeable trail. Slugs are active at night (nocturnal) when humidity is high from evening rains or irrigation. They hide during the day under mulch, plant debris, rocks, boards, weeds, and ground covers.

Wet weather generally favors an increase in slug populations. Slugs are destructive pests feeding on many plants in landscapes, gardens, and greenhouses including annuals, perennials, bulbs, ground covers, trees, and shrubs. In addition, slugs can devour young seedlings overnight. One of their favorite food plants is hosta. Slugs have rasping-chewing mouthparts and cause plant damage by creating large irregular shaped holes in leaves. They prefer to feed on succulent foliage such as seedlings, herbaceous plants, and fruit lying on the ground. Slugs have a strong sense of smell; they will travel substantial distances to locate a food source.

Slug management involves a combination of strategies such as hand picking, habitat modification, barriers, traps, baits, and commercial molluscicides. Monitoring is important to determine the effectiveness of slug management strategies. Monitoring involves going out in the evening with a flashlight and looking for the presence of slugs. During this time, handpicking can be performed to reduce slug populations. Handpicking is especially effective during moist weather conditions. Placing slugs into a jar with soapy water will kill them. For heavy slug infestations another possibility is to employ some neighborhood children to collect slugs and kill them. Various beetles such as firefly larvae and ground beetles feed on slugs. In addition, toads, frogs, snakes, and lizards eat slugs. However, these predators may not be present in high enough numbers to keep slug populations below damaging levels.

Habitat Modification
Habitat modification is one of the most effective strategies in reducing slug populations. This involves eliminating hiding places such as mulches, weeds, old vegetation, and debris. Planting a diversity of trees and shrubs, especially those not preferred by slugs such as rhododendron and hard-leafed evergreens, can reduce slug populations and minimize plant damage. Cultivating the soil around plants may reduce slug populations by destroying eggs. Proper watering practices can also keep slug numbers low. Avoid watering late in the day as this creates moist conditions conducive to slug activity. Instead, water plants early in the morning. Research has demonstrated that morning watering provides protection from slug injury comparable to some slug baits. In addition, the use of drip irrigation systems where water is directed toward individual plants may lead to fewer slugs. Spacing plants far enough apart to allow air movement to dry the soil is also helpful.

Barriers and Traps
Copper barriers can be placed around the base of shrubs, flowerbeds, and trees that are being fed upon. They can also be placed around flowerpots. Slugs receive a slight electric shock when their moist bodies contact the copper; repelling them. However, widespread use of this method may not be feasible. In addition, copper bands have sharp edges, which can harm children and pets. Diatomaceous earth, shredded bark, eggshells, lime, and wood ash have been used as barriers to prevent slugs from feeding on plants. However, these materials generally work best during dry periods when slugs are less active. In addition, the effectiveness of these materials is reduced by rainfall, which means they have to be reapplied regularly after becoming wet. The use of some of these materials such as egg shells, lime, and wood ash is discouraged because over time, they may raise the pH of the soil. Also, never pour salt on slugs as this may burn plant foliage and roots. Traps, such as wooden boards, rolled-up newspapers, grapefruit rinds, and inverted one-gallon plant containers can be placed where slugs are feeding. Check traps daily, early in the morning. Place slugs into a jar with soapy water to kill them.

Baits are available that attract slugs into traps where they then drown. Beer is one popular bait. Some die-hard practitioners swear that beer reduces slug populations. Beer is poured into a shallow pan, which is sunk into the ground with the pan edges sticking up 1/8 to 1/4 inch. Slugs are attracted to the yeasty smell of beer and they fall into the pan and drown. Aldehydes in the beer may also be toxic to the slugs. Studies from Colorado State University have shown that Kingsbury Malt Beverage (from Heileman Breweries) is the beer most preferred by slugs. However, beer does not have an EPA registration number, so the use of beer as a pesticide is not technically legal.
Slug baits that are commercially available are applied to areas that need protection. However, some baits may be toxic to non-target organisms such as dogs, birds, and earthworms. Some baits should not be applied around food crops. Check the label on the bait. Baits are less effective during hot, dry times of the year when slugs are less active. Irrigate before applying these materials to promote slug activity. Make spot applications as opposed to broad scale applications.

Prepared by Raymond A. Cloyd and Philip L. Nixon, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois, in cooperation with the Illinois Natural History Survey. For additional copies, contact your local University of Illinois Extension Office.