Question of the Week
Answer: Slugs are one of the more common and serious pests in the vegetable garden. These soft-bodied insects are mollusks and feed on decaying plant debris and on a number of vegetable, flower, fruit and herb species. Although slugs have a range of favorite foods, including cabbage, cereals, lettuce, green vegetables, strawberries and root vegetables, slugs will attack any and all seedlings, even if the mature plant is unpalatable. (1) Crop damage from slugs is greatest at the seedling stage.
Lacking a protective shell cover, slugs prefer to stay out of the sun hiding under rocks and boards, mulch, compost piles and other garden debris during the day. They forage only at night and on wet, sunless days, and are easy to identify by the telltale trails of silvery, sticky mucus they secrete to smooth their path. In most cases, it is temperature, not light, that will most influence slug feeding.
Compared to snails, slugs spend more time on the ground or in the soil. Lacking a shell, they require more contact with moist environments to avoid desiccation. Conditions that favor slug development include the following (2):
• No-till practices that provide crop residue on the soil surface for slug habitat
• Dense weed cover, or addition of organic matter such as livestock manure
• Mild winter temperatures that enhance overwintering success
• A prolonged period of relatively cool temperatures (63-68° F) combined with evenly distributed rainfall that maintains wet soil moisture conditions
• Either heavy-textured soils or coarse-textured soils, neutral to high pH, and excessive nitrogen levels.
Accompanying this letter are lists of articles that describe the biology and control of this mollusk. Not all slugs are pests. Native species like the banana slug recycle nutrients, produce humus and build soil. Other insects and birds prey on them for food. Species that attack garden or field crops are those that were inadvertently introduced from Europe—like the gray garden slug (Deroceras laeve), the spotted garden slug (Limax maximus), the tawny garden slug (Limax flavus) and the European black slug (Arion ater ater).
Slug biology and habits must be understood in order to implement an effective management program. Correct identification and regular monitoring are necessary to assess the severity of the infestation and determine what kind of controls to most appropriately implement. The trick is to find weak points in the slug life cycle at which management measures can be most effectively applied. For example, baiting is most effective when the slugs emerge to mate and lay eggs (see the article Stopping Slugs and Snails by Joel Grossman).
Management measures for slugs generally fall into two general categories: proactive and reactive. (Biological controls may be either proactive or reactive, depending on the context of their implementation). Proactive, or preventative measures are various cultural controls that can be taken prior to an outbreak of slugs in order to decrease the chances of an economic infestation developing. These are often manipulations of the farm ecology that somehow increase the environmental pressure against an organism. A program that integrates preventative and reactive measures will be the most likely avenue to successful slug management.
Recent research in Switzerland (3) found that slug damage to lettuce plots that were watered in the morning was significantly less than slug damage to plots that were watered in the evening. Evening watering creates a very slug-friendly environment. Drip irrigation may be of some benefit (compared to overhead irrigation), since its use will decrease the general moisture of the crop environment.
Rototilling the ground during warm weather in early spring will reduce slug populations through physical destruction and exposure of slugs and slug eggs to predators and the weather. Three passes of a rototiller in an area may decrease slug population by 75%. (4) The downside is that the cultivation will expose the organic matter of the soil to an increased rate of oxidation.
A posting on the Gardening in Harmony website message board noted that:
Rhode Island Red hens are great slug hunters, they eat virtually all the slugs they can find. Other chickens don't seem to care for slugs very much. My number one vote is for ducks! They devour slugs like mad. I have heard it said that they will eat half their body weight in slugs a day. Here is a quote that says it all: "You don't have a slug excess, you've got a duck deficit!"—Bill Mollison, Permaculture expert. I have several friends with organic farms who put their ducks on slug patrol with great success! (5).
Khaki Campbell or Indian Runner duck varieties are especially good snail and slug hunters.(6) A word of caution about ducks and chickens. Ducks are browsers and may eat the salad plants, particularly if they are young. Chickens may take a few bites out of the foliage as well. Of greater concern may be the presence of duck or chicken feces contaminating the lettuce leaves. One option is to use these animals to control slugs (and snails) after harvest. It may be worthwhile to experiment with ducks or chickens on a small scale to observe the practicality of integrating them into your farming system as slug controls.
Other biocontrols include various species of ground beetles in the genus Scaphinotus and other genera. These beetles are not available commercially and must be conserved through habitat manipulation. However, the habitat that favors Scaphinotus spp., such as mulch, may also favor slugs.
Some research has been done using parasitic nematode, Phasmarhabditis hermaphrodita, as an applied biocontrol for slugs. Field research in 2004 reported that slug damage on Chinese cabbage was reduced by about one-third by the nematode treatment. (7) Sources for these nematodes are provided at the end of this letter. The same research (7) found that slug damage was significantly (c. two times) greater after red clover or vetch than after ryegrass. Damage on plots without cover crop was intermediate and not significantly different from either extreme.
Harvestmen (Opiliones) prey on slugs. The sciomyzid flies, Tetanocera plebeia Loew and Tetanocera valida Loew prey on brown slugs. (8) Other general predators include birds (up to 6 percent of the diet of starlings), moles, toads, shrews, rove beetles and firefly beetles. (9)
Treatment: Barriers and Baits
Reactive measures generally take the form of some kind of treatment. A variety of baits, traps and barriers have been tried with varying degrees of success. Barriers reported to be effective against slugs and snails include sand, diatomaceous earth, ashes, crushed eggshells and others. (10) Rain or moisture will decrease the effectiveness of many of these. Another technique, supposedly very effective, is to surround the plant, bed or garden with a 6" wide copper foil barrier. The mollusks get a mild shock when they encounter the metal and stay away. Wooden surfaces treated with copper sulfate also act as a slug barrier. (9) Copper sulfate is toxic to slugs as are inorganic salts of aluminum and iron. There have been some observations that aluminum sulfate, which is commonly used as a soil acidifier, repels slugs well at the rate of 5-10 lbs per 1000 square feet. However, noting that the compound is inactivated very quickly when in contact with wet soil, researchers speculate that its effectiveness could be enhanced by application to inert materials like plastics that surround the plants. (11) Another method of protecting barriers consisting of aluminum and iron ions is to combine (chelate) these ions with an organic compound to protect against environmental degradation and inactivation. (1)
Other options include use of pennyroyal as a slug barrier or perhaps a mulch. Pennyroyal and some other mint oils contain pulegone, a toxic monoterpenoid that acts as a feeding deterrent. (12) Sources of mint oils are provided below.
Research in Great Britain in 2003 investigated the repellent and irritant effects of cinnamamide, copper ammonium carbonate, a mulch, a horticultural ground-cover matting impregnated with a copper formulation and urea/formaldehyde. In the no-choice experiments the products had a stronger irritant effect on the snails than on the slugs. All products tested except the mulch significantly reduced the locomotor activity of both the slugs and snails. The most effective product, cinnamamide, reduced snail locomotor activity by 94% and track length by 96%. The overall repellent effect of the treatments in the choice experiments was stronger in the slugs; where presence, locomotor activity and track length in the treated area were significantly reduced by all products. The avoidance of treated areas exceeded 95% with the mulch (for slugs) and with copper ammonium carbonate (for snails). (13) An earlier study in 2002 lab-tested 4 materials, metaldehyde, methiocarb, cinnamamide and 3,5-dimethoxycinnamic acid (DMCA), as a seed treatment on oilseed rape against slug damage. The researchers found that all doses of cinnamamide and DMCA were phytotoxic to the young plants, and that all compounds reduced slug damage, but metaldehyde and methiocarb consistently performed better than cinnamamide and DMCA. (14)
Beer baits are often recommended but must be closely spaced in order to be effective. The slugs are attracted to the yeast products, not to the alcohol or hops. Recommended spacing is one bait per 10 sq. feet. To create your own slug bait, add 1Tb of granulated yeast in three cups of water and 2 Tb of sugar. Put it in a pan with the edge at least ½" above soil surface to keep beneficial insects from inadvertently falling in.
Baits made with iron phosphate (i.e., Sluggo and Escar-Go) are an environmentally friendly way to manage snails and slugs. The allowed use of this material in organic production systems remains undecided at this time, so it is important to discuss this material with your certifying agency prior to use. Organic standards does not allow use of synthetic forms of iron phosphate, which may leave the door open for the use of mined iron phosphate. However, "inert ingredients" make up 99% of the formulation of these products, and it may be that the composition of the inerts must be determined and pronounced "allowable" before these products are declared allowable for use in organic production systems.
In summary, the slug has several weak links in its life cycle that can be exploited to manage slug populations. It seeks moist environments, so actions to reduce moisture, particularly during the night, will provide some relief from this pest. Morning (vs. evening) irrigation, use of drip irrigation, and use of raised beds all may help reduce slug populations. The penchant of slugs to seek protected environments might be taken advantage of by providing such environments in order to trap them. For example, long sheets of plastic laid in between rows might be an attractive shelter in which slugs will congregate come daylight. The plastic sheets could be collected during the day and the slugs harvested.
Weaving the options available to you for slug control into an integrated pest management program that fits your operation requires creativity, flexibility and a willingness to experiment, observe the results and experiment again.
1) Quarles, William. 1997. Slugs and snails in gardens and fields. Common Sense Pest Control. Winter. p. 5-15.
2) Van Dyke, J. 1998. Slime trails in the moonlight. Iowa State Dept. of Entomology Website. www.ent.iastate.edu/ipm/icm/1998/6-29-1998/slime.html
3) Speiser, B., and M. Hochstrasser. 1998. Slug damage in relation to watering regime. Agriculture, Ecosystems and Environment. Vol. 70. No. 2-3. p. 273-275.
4) Symondson, B. 1990. Slug control. Webpage at www.cardiff.ac.uk/biosi/research/biodiversity/staff/wocs2.html
5) Michaels, F. 2001. Organic slug and snail control. http://www.greenharvest.com.au/pestcontrol2/slug_and_snail_info.html
6) Judi. 1999. Posting on Gardening in Harmony message board. www.raingardens.com/forum/messages/400.html
7) Vernavá, M.N., Phillips-Aalten P.M., Hughes L.A., Rowcliffe, H., Wiltshire C.W., and D.M. Glen. 2004.Influences of preceding cover crops on slug damage and biological control using Phasmarhabditis hermaphrodita. Annals of Applied Biology, December 2004, vol. 145, no. 3, pp. 279-284(6)
8) University of Florida Institute of Food and Agricultural Sciences, Mid-Florida Research and Education Center (MREC) Webpage. Derocerus slugs.
9) University of Florida Institute of Food and Agricultural Sciences, Mid-Florida Research and Education Center (MREC) Webpage. Introduction to Slugs and Snails. www.mrec.ifas.ufl.edu/Foliage/entomol/ncstate/slugintr.htm
10) Organic Gardening (eds.) 1997. 18 nifty ways to negate slugs and snails. Organic Gardening. May-June. p. 46-48.
11) Anon. 1990. Iron and aluminum molluscicides. Hortideas. September. p. 102.
12) Quarles, W. 2000. Mints and pest control. Common Sense Pest Control. Vol 16. No. 2. Spring. p. 17-19.
13) Schuder, I., et al. 2004. The Behavioural Response of Slugs and Snails to Novel Molluscicides, Irritants and Repellents," Pest Mgmt. Sci., 60(12), 1171-1177, December 2004.
14) Simms L.C.1; Mullins C.E.1; Wilson M.J. 2002. Seed dressings to control slug damage in oilseed rape. Pest Management Science, July 2002, vol. 58, no. 7, pp. 687-694(8)
Grossman, Joel. 1991. Stopping slugs and snails. Fine Gardening. November-December. p. 56-59.
Organic Gardening (eds.) 1997. 18 nifty ways to negate slugs and snails. Organic Gardening. May-June. p. 46-48.
Quarles, William. 1997. Slugs and snails in gardens and fields. Common Sense Pest Control. Winter. p. 5-15.
Suggested Further Reading:
Long, Becky. 1996. Coping with slugs and snails. Journal of Pesticide Reform. Spring. p. 22-23.
Lynas, Bryam D.T. 1992. More on iron sulfate as a molluscicide. Excerpt from a report. Hortideas. April. p. 43.
McLeod, Edwin J. No date. Snail and Slug Biology and Management: A Review. Organic Agriculture Research Institute. 6 p.
Verbena, Melissa and Jean Yates. 1991. Success secrets of super slug slayers. Organic Gardening. April. p. 44, 46-48.
Sources for slug control products
Barriers and exclusion devices:
PO Box 4247 CRB
Tucson, AZ 85738
800-827-2847 or 520-825-9785
5100 Schenley Place
Lawrenceburg, IN 47025
2806 Oakhurst Avenue
Los Angeles, CA 90034
2723 116th Avenue
Allegan, MI 49010
Nemaslug, a garden product containing the nematode Phasmarhabditis hermaphrodita, is available from:
Unterdorf, 6146 Grossdietwil, Switzerland, 062-927-2840
Ilse Velden, 18
2260 Westerlo, Belgium
Hoyle Depot, Graffham, Petworth
West Sussex GU28 OLR, UK 01798-86754
Mint Oil Pesticides:
69 N. Locust St
Lititz, PA 17543
« How do you suggest controlling aphids in organic broccoli? :: How can I manage Fusarium on tomatoes? »