Slugs can damage many types of crops, especially when soil moisture and relative humidity is high. Leafy vegetation, seedlings and strawberries are commonly consumed by slugs. Their feeding results in irregular holes in foliage and fruit, stripped stems and slime trails. While damage is usually cosmetic rather than lethal, it can greatly reduce marketable yields.
Slugs are related to clams and other mollusks—they are basically snails without a shell. Their soft bodies are covered with a protective mucus layer that also helps them move around. Slugs like dark, cool areas and typically emerge from their hiding places in the evening or at night.
A slug’s body consists of a head, neck, mantle, tail and foot. It has two pair of tentacles, one large pair with eyes and a small, lower pair for smelling. Its mouth is located in the center of its head below the lower tentacles and is equipped with chewing and rasping mouthparts. It moves around using its muscular foot that slips along a shiny trail of slime.
Slugs are hermaphrodites, able to cross-fertilize as well as self-fertilize. Mating takes place in the late summer and early fall with egg-laying a month or so later. Each slug lays an average of 20 to 30 eggs several times per year. Eggs are laid on or near the soil surface and are usually concealed and in a moist area. They are held together with a sticky substance and the young crawl and feed immediately upon hatching. Eggs can remain viable over an extended time if it’s dry and will then hatch when it gets damp. Slugs overwinter as adults or nearly mature young, and will generally survive one year outside, and even longer in a greenhouse.
Altering the environmental condition in a field is one way to manage slug populations. Since moisture is key to slug survival, try to reduce the level of excess moisture available. Instead of watering late in the day, water in the morning to allow water to evaporate from the soil surface, and use trickle irrigation to limit the application of excess water.
Light tillage to create a dry, rough soil surface between crop rows may also deter slugs.
Using organic mulches, such as straw, may encourage slug populations, but they have other benefits that may outweigh this factor, such as the management of diseases like leather rot on strawberries or suppression of weeds. Mulches can also help build up populations of natural slug predators such as ground beetles and rove beetles.
Drinking to death
Fermented liquids—like a mixture of yeast, sugar and water—or beer, can be used as traps for drowning slugs. Place the liquid in smooth plastic containers sunk into the soil. To avoid drowning beneficial ground beetles, set the rims slightly above the soil’s surface; the slugs can crawl up and over the rims. For effective control, you’ll need a lot of beer traps, perhaps one every few feet or so, and the liquid has to be refilled every few days, so this technique is practical only on a small scale.
Bait and wait
Slug baits are probably the most reliable and efficient method of slug control. Commercially available baits or pellets contain molluscicides, poisons that kill snails and slugs. Until recently, there were no organically allowed materials registered as baits in crops, but now there are products containing iron phosphate. This is far less toxic, but comparable to metaldehyde baits for slug control.
Iron phosphate is considered GRAS (generally regarded as safe) for food crop use by the U.S. Environmental Protection Agency. No toxicity has been seen in mammals, birds or fish. It is applied to soil around the crop plants in a pellet form that also contains bait to attract snails and slugs. It is not volatile and does not readily dissolve in water, which minimizes its dispersal beyond where it is applied. This also means that it will remain effective after it rains.
Snails and slugs are more sensitive to the effects of iron phosphate than are other organisms. When they eat the pellets, the iron phosphate interferes with calcium metabolism in their gut, causing the snails and slugs to stop eating almost immediately. They die three to six days later.
Organic slug baits
The Organic Materials Review Institute (OMRI) lists several brands of iron phosphate slug and snail bait on their list of brand-name materials approved for use by certified organic growers (see www.omri.org): Scott’s EcoSense Slug and Snail Bait, Sluggo Slug & Snail Bait, and Worry Free and Ferramol Slug & Snail Bait from Lilly Miller Brands. As with other restricted materials, these should only be used if other management practices are insufficient to prevent or control the slugs. Before using any pesticide, always check with your certifying agency to make sure it is allowed for use on organic farms, and always follow the label instructions.
Some of the information presented here comes from an article by JulieWeisenhorn, Department of Horticultural Science, University of Minnesota, and some comes from Becky Grube, vegetable specialist at the University of New Hampshire Extension.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.