The recent wet weather experienced throughout most of Kansas will likely increase slug population activity. Slugs are destructive pests that feed on many plants in landscapes and gardens including annuals, perennials, vegetables, bulbs, ground covers, trees and shrubs. Slugs can devour young seedlings overnight. One of their favorite plants to feed on is hosta.
Slugs are mollusks, related to oysters and clams. Slugs are referred to as “naked snails,” because they possess no shell. They are brown in color and 1-1/2 to 2 inches long. Slugs lay clusters of translucent, pearly-shaped eggs under debris or beneath the soil surface. They can lay between 20 and 100 eggs several times per year. The gray garden slug, Deroceras reticulatum, is the slug generally encountered in the Midwest. It is approximately 3/4 to 1-1/2 inches long. Colors include pale yellow, lavender or purple. In addition, they are covered with black or brown spots and mottling.
Slugs have chewing mouthparts and cause plant damage by creating large irregular shaped holes in leaves with tattered edges. They feed using a structure called a radula, which is in the mouth and covered with small teeth. Slugs tend to feed on succulent foliage such as seedlings, herbaceous plants and fruit lying on the ground.
Slug movement depends on soil moisture because they need it to produce a slimy mucus substance, which allows them to move around. It dries, leaving a shiny, noticeable trail. Slugs are mainly active at night (nocturnal) when relative 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 implementing a combination of strategies such as handpicking, habitat modification, and use of barriers, traps, baits and commercial molluskicides. Monitoring is important to determine the effectiveness of slug management strategies. Monitoring involves going out in the evening with a flashlight and looking for slugs. Handpicking is recommended to reduce initial slug numbers. It is especially effective during moist weather conditions. Placing slugs into a jar containing soapy water will kill them.
There are two other methods of killing slugs once they have been collected. But both are quite macabre. One method involves stepping on the critters. Slugs make a squishy sound when stepped on (although this depends on the intensity of the step). The other method is to scatter slugs on a driveway and drive back and forth over them with a truck or car. Or have children with bikes run over the slugs. But they will have to do this more than once.
Habitat modification is one of the most effective strategies in reducing slug problems. This involves eliminating hiding places such as mulches, weeds, old vegetation and plant debris. Proper watering practices can also alleviate slug problems. For example, avoid watering late in the day, which creates moist conditions conducive to slug activity. Instead, water plants early in the morning. Research has shown that morning watering provides as much protection from slug damage as metaldehyde (Deadline) pellets. In addition, the use of drip irrigation systems, in which water is directed toward individual plants, may reduce slug populations.
Copper barriers may be placed around the base of shrubs and trees subjected to constant feeding. Slugs receive a slight electric shock when their moist bodies contact copper; this then repels them. However, widespread use of this method may not be feasible. Furthermore, copperbands have sharp edges, which can harm children and pets.
Diatomaceous earth, shredded bark, egg shells, and wood ash have been used as barriers to prevent slugs from feeding on plants. Overall, 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. Moreover, these materials lose their effectiveness after getting wet. Constant 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 because it may burn plant foliage and roots.
Traps, such as wooden boards or rolled newspaper can be positioned where slugs are feeding. Check traps early in the morning. Perform this procedure once or twice per week. Afterwards, place slugs into a jar containing soapy water to kill them.
Baits are available that attract slugs into traps where they then drown. One popular bait is beer. Although there may be better uses for beer, some die-hard practitioners insist that beer reduces slug populations. This can be done by pouring beer into a shallow pan and placing it in the ground with the pan edges extending about 1/8 to 1/4 inches. Slugs are attracted to the yeasty smell and fall into the pan and drown. Some folks claim that when slugs “party” they consume too much beer, get sluggish from the high alcohol content, and it eventually kills them. I have not seen any data that supports this claim. But a study from Colorado State University showed that Kingsbury Malt Beverage (from Heileman Breweries) was the beer most preferred by slugs. (I guess they conducted a slug taste test.) But beer does not have an Environmental Protection Agency registration number, so using it as a pesticide is technically not legal.
Most commercially available poison baits include metaldehyde (Deadline) as the active ingredient. Metaldehyde does not directly kill slugs. Instead, it works by paralyzing and causing slugs to secrete excess amounts of mucus. Death generally occurs from water loss and/or exposure to direct sunlight. Nonetheless, under cool, moist weather slugs may actually recover. Metaldehyde is sensitive to environmental conditions, degrading very rapidly when exposed to direct sunlight and excessive moisture. However, the new formulation of Deadline is more resistant to degradation. Metaldehyde is toxic to cats and dogs if large quantities are ingested.
The molluskicide, Sluggo is also available. The active ingredient is iron phosphate. Sluggo contains a bait that attracts and kills slugs. Once they consume the material they crawl away and die. This is why people do not typically see dead slugs around the area. Sluggo kills slugs within 3 to 6 days. It can be used around pets and may remain potent for a longer time compared to metaldehyde.
There are various beetles such a firefly larvae and ground beetles that feed on slugs. But these predators may not be present in sufficient enough numbers to maintain slug populations below damaging levels.
Articles reprinted from the K-State Research & Extension Horticulture Newsletter
Old Mill Nursery 