All About Slugs
Slug injury is sometimes difficult to diagnose because slugs hide during the day. Smaller leaves may be eaten entirely, while only the edges of larger leaves may be consumed. Thick leaves often are rasped from the leaf underside. The presence of slime trails often is the best indication of slug activity.
History and Habits
Slugs and snails are classified as gastropods. They are more similar to clams and mussels than to other common garden pests such as insects. Slugs feed on a variety of plant materials that they eat by rasping with specialized mouthparts. They feed mostly at night, although they occasionally are active on overcast or rainy days. Slugs seek shelter during the day in soil cracks and under debris. During hot, dry weather, they may be temporarily inactive. Slugs lay small masses of eggs in soil cracks. The young slugs that hatch from these eggs travel through the soil and damage germinating seeds and root crops. Under favorable conditions, slugs complete development in a few months.
Slugs are mostly water and produce large amounts of protective mucous. Consequently, slugs are susceptible to drying. The level of a slug population depends on the moisture conditions in a garden. Any practices that decrease moisture will reduce slug problems, although effects may not be seen immediately. To eliminate potential shelter for slugs, remove surface debris in and around the garden and avoid organic mulches (straw, grass clippings). Also, increase air movement around plants and reduce high moisture conditions with trellises and wider plant spacing. Use drip irrigation, soaker lines or other techniques to limit water and decrease the humidity around plants. Overhead irrigation should be done early in the day to allow more time for leaves and soil to dry before the nightly activity of slugs.
Attractants and Traps
Slugs are attracted to chemicals produced by many fermenting materials. These materials can be used to make attractant traps. For example, pans of beer or sugar-water and yeast mixtures can effectively attract, trap and drown slugs. A single baiting can remain effective for several days, as long as sufficient liquid remains. However, because the range of such traps is only a few feet, they must be placed throughout the planting to significantly reduce slug populations.
Repellents and Barriers
Slugs often avoid traveling over acid, alkali or abrasive materials. Diatomaceous earth, wood ashes and similar materials placed around plants provide some protection. However, moisture reduces the effect of these treatments. Salt also is toxic to slugs and direct applications of table salt to a slug can kill it. This technique has limited usefulness because excessive salt can affect plants. Certain metal ions also are highly repellent to slugs. Barriers of copper foil exclude slugs from greenhouse benches and raised bed plantings. Other copper-based materials, such as copper sulfate, repel slugs.
Pesticides effective against slugs and snails are known as molluscicides. These often are different chemicals than those used to control insects and other garden pests. Slugs are not susceptible to poisoning by most insecticides.
Metaldehyde is the most commonly used and effective molluscicide. It is sold often in the form of granular baits (Bug-Geta, etc.) or as a paste or gel (Deadline, etc.). Metaldehyde is attractive to slugs and causes intense irritation to them upon contact. After contact, slugs often become immobile and secrete large amounts of mucous. Death usually results from loss of water. All metaldehyde products are labeled for use around flower gardens and ornamental plants. Currently available products do not allow direct application among vegetables and edible crops.
Successful use of metaldehyde baits requires careful application and favorable weather conditions. Slugs often can recover from metaldehyde poisoning if high moisture conditions occur. Also, control is poor during cool periods because slugs are relatively inactive. Apply metaldehyde when the evening is warm and the forecast for the following day is hot and dry. Young slugs are more susceptible than older slugs.
The range of metaldehyde attraction is limited, so spread it throughout the infested area. Use 10 or more pieces of bait (flakes, pellets, liquid drops) per square yard. The amount needed for effective control varies depending on the attractiveness of other food in the area. Metaldehyde is rapidly inactivated by sunlight so spread it underneath leaves late in the day. Please note that Metaldehyde is attractive to and potentially hazardous to dogs. Do not allow dogs access to metaldehyde packages. An alternative bait that recently has become available includes iron phosphate (ferric phosphate) as the active ingredient. Trade names include Sluggo, Slug Magic and Escar-Go!, among others. Data suggest iron phosphate generally is comparable in effectiveness to metaldehyde. Furthermore, iron phosphate products can be used around edible crops and do not pose special hazards to dogs.