Snails and slugs are destructive pests of landscapes and gardens, feeding on such plants as annuals, perennials, bulbs, ground covers, trees, and shrubs. They can also devour young seedlings overnight. They especially like to feed on the hosta plant.
Snails and slugs are mollusks, related to oysters and clams. Snails transport a protective shell on their backs, basically carrying their homes with them. They are generally brown and from 1 to 1-1/2 inches long. Slugs are referred to as “naked snails” because they have no shell. Gray garden slugs are brownish and from 1-1/2 to 2 inches long, while spotted garden slugs are yellowish with black spots and may be 6 inches long. Both snails and slugs lay clusters of translucent, pearly-shaped eggs under debris or beneath the soil surface. They can lay from 20 to 100 eggs several times per year.
Snails and slugs have chewing mouthparts. They cause plant damage by creating large irregularly shaped holes with tattered edges in plant leaves. They prefer to feed on succulent foliage such as seedlings and herbaceous plants and on fruit that is lying on the ground.
Snails and slugs require moisture to move around, and they secrete a slimy mucus trail, which they use for this purpose. These secretions then dry up into a shiny, noticeable trail. Snails and slugs are active at night when humidity is high from evening rains or irrigation. During the day, they hide under mulch, plant debris, rocks, boards, weeds, and ground covers.
Managing snails and slugs involves a combination of strategies, such as handpicking, habitat modification, barriers, traps, baits, and commercial molluscicides. Monitoring is important to determine the effectiveness of these strategies. This involves going out in the evening with a flashlight and looking for snails and slugs. At this time, you can handpick snails and slugs to reduce their populations. Handpicking is especially effective during moist weather conditions. Placing snails and slugs in a jar with soapy water kills them. Another possibility is employing children to collect snails and slugs. (Keeps the kids off the streets and is more educational than midnight basketball.)
There are two other methods of killing snails and slugs after they have been collected, but both are quite macabre. One method simply involves stepping on the critters. Snails give off a crunchy sound; slugs do not. The other method involves scattering snails and slugs on a driveway and driving back and forth over them with a truck or car. Although this is not the most environmentally sound method, it seems to be psychologically satisfying to most individuals.
Habitat modification is one of the most effective strategies in reducing snail and slug populations. This method involves eliminating hiding places such as mulches, weeds, old vegetation, and debris. Proper watering practices can also minimize their populations. Avoid watering late in the day as this creates moist conditions conducive to snail and slug activity. Water plants early in the morning. Also, using drip irrigation systems, with water directed toward individual plants, may lead to fewer snails and slugs.
Copper barriers can be placed around the bases of shrubs and trees that are being fed upon. Snails and 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 that can harm children and pets.
Diatomaceous earth, shredded bark, eggshells, and wood ash have been used as barriers to prevent snails and slugs from feeding on plants. These materials work best during dry periods when snails and slugs are less active; however, their effectiveness is reduced by rainfall, which means they have to be reapplied regularly. They also lose their effectiveness after becoming wet. The use of some of these materials, such as eggshells and wood ash, is discouraged because they may raise the pH of the soil over time. Also, never pour salt on snails and slugs as this may burn plant foliage and roots.
Traps, such as wooden boards or rolled-up newspapers, can be placed where snails and slugs are feeding. Check traps early in the morning once or twice a week, and place snails and slugs in a jar with soapy water to kill them.
Also available are baits that attract snails and slugs into traps where they then drown. One popular type of bait is beer. Although there are better uses for beer, some die-hard practitioners swear that it reduces snail and slug populations. Pour beer into a shallow pan and sink it into the ground with the pan edges sticking up 1/8 to 1/4 inch. Snails and slugs are attracted to the yeasty smell of beer, and they fall into the pan and drown. (Some folks claim that slugs and snails have a massive beer party and, after a while, they become very “sluggish,” with an alcohol content so high that it eventually kills them). Beer does not have an EPA registration number, however, so the use of beer as a pesticide is not technically legal.
Commercial poison baits include metaldehyde (Deadline) and methiocarb (Mesurol). These materials are applied to areas that need protection. Metaldehyde does not kill snails and slugs directly. It works by paralyzing snails and slugs and causing them to secrete excess amounts of mucus. Death generally occurs from water loss and/or exposure to direct sunlight; however, in cool, moist weather, snails and slugs may recover. Metaldehyde is very sensitive to environmental conditions. It breaks down very rapidly in direct sunlight and under moist conditions. 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. It may also be toxic to nontarget organisms, such as earthworms. These materials are less effective during hot, dry times of the year when snails and slugs are less active. Irrigate before applying materials to promote snail and slug activity. Be careful where these baits are applied because dogs can be harmed if they eat them. Make spot applications as opposed to broadscale applications.
Various beetles, such as firefly larvae and ground beetles, feed on snails and slugs. However, these predators may not be present in large enough numbers to keep snail and slug populations below damaging levels.