The recent wet weather throughout Illinois favors an increase in slug populations. Slugs are destructive pests, feeding on many plants in landscapes and gardens including annuals, perennials, bulbs, ground covers, trees, and shrubs. In addition, slugs can devour young seedlings overnight. One of their favorite plants to feed on is hosta.
Slugs are mollusks, related to oysters and clams, and are referred to as “naked snails,” as they have no shell. Slugs lay clusters of translucent, pearl-shaped eggs under debris or buried beneath the soil surface. They can lay between 20 to 100 eggs several times per year. The gray garden slug, Deroceras reticulatus, is the slug generally encountered in Illinois. It is approximately 3/4 to 1-1/2 inches long. It ranges in color from pale yellow, to lavender, to purple. In addition, it is covered with black or brown spots and mottling. Less common is the spotted garden slug, Limax maximus. It is up to 7 inches long and is yellow-gray to brown with black mottling.
Slugs have chewing mouthparts and cause plant damage by creating large irregularly shaped holes in leaves with tattered edges. They prefer to feed on succulent foliage such as seedlings, herbaceous plants, and fruit lying on the ground.
Slug activity is highly dependent on soil moisture, as they require moisture to move around. They secrete a slimy mucous 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.
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, hand picking can be performed to reduce slug populations. Hand picking is especially effective during moist weather conditions. Placing slugs into a jar with soapy water will kill them. Another possibility is to employ some children to collect slugs (this will keep them off the streets). There are also two other methods of killing slugs once they have been collected. However, both are quite macabre. One method simply involves stepping on the critters. Slugs make a squishy sound when stepped on. The other method involves scattering slugs on a driveway and simply driving back and forth over them with a truck or car. Although this method is not environmentally sound, it does appear to be psychologically satisfying to most individuals. A more environmentally sound method is to have children run over the slugs with bikes (it is important that they run over the slugs more than once).
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. Proper watering practices can also minimize slug populations. 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 metaldehyde pellets. In addition, the use of drip irrigation systems where water is directed toward individual plants may lead to fewer slugs.
Copper barriers can be placed around the base of shrubs and trees that are being fed on. Slugs receive a slight electric shock when their moist bodies contact the copper; this then repels 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, and wood ash have been used as barriers to prevent slugs from feeding on plants. These materials work best during dry periods when slugs are less active. However, the effectiveness of these materials is reduced by rainfall, which means they have to be reapplied regularly. These materials also lose their effectiveness after becoming wet. The use of some of these materials such as eggshells 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 or rolled-up news- papers, can be placed where slugs are feeding. Check traps early in the morning. Do this once or twice per week.
Baits are available that attract slugs into traps where they then drown. One popular type of bait is the use of beer. Although there are better uses for beer, some diehard practitioners swear that beer reduces slug populations. Beer is poured into a shallow pan (although some may “accidentally” be poured into the individual’s glass), which is then 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. (Some folks claim that slugs have a massive beer party, and eventually they get very “sluggish” with a high enough alcohol content that eventually kills them.) 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. The use of beer to manage slugs may also be a problem on college campuses that don’t allow alcoholic beverages.
Commercial poison baits include metaldehyde (Deadline) and methiocarb (Mesurol). These materials are applied to areas that need protection. Metaldehyde does not kill slugs directly. It works by paralyzing and causing slugs to secrete excess amounts of mucous. Death generally occurs from water loss and/or exposure to direct sunlight. However, in cool, moist weather, slugs may recover. Metaldehyde is very sensitive to environmental conditions, as it breaks down very rapidly in direct sunlight and under moist conditions. However, the new formulation of Deadline is more resistant to breakdown when exposed to sunlight. Methiocarb is a true nerve poison as it interferes with nerve-impulse transmission. However, methiocarb may be toxic to nontarget organisms, such as earthworms. These materials are less effective during hot, dry times of the year when slugs are less active. Irrigate before applying materials to promote slug activity. Make spot applications as opposed to broad-scale applications. A new molluscicide called Sluggo is also available. The active ingredient is iron phosphate. Sluggo is a bait that attracts and kills slugs. Once they consume the material, they crawl away and die. It will kill slugs within 3 to 6 days. Sluggo can be used around pets and wildlife, and it remains potent for a longer time compared to metaldehyde.
Various beetles such as firefly larvae and ground beetles feed on slugs. However, these predators may not be present in sufficient enough numbers to keep slug populations below damaging levels.