CORVALLIS, Ore. – Stymied by slugs that can plod through your chard and cabbage, leaving a slimy trail of destruction?
Put away that salt shaker, advises Robin Rosetta, an entomologist for the Oregon State University Extension Service. Table salt can build up in the soil over time and damage plants.
"I have always believed that the best way to control a pest is to make the environment where you're trying to grow plants less advantageous to their happiness," Rosetta said. "So try to figure out what makes them happy. You have to think like a slug."
Invasive slugs such as the European red slug often make their homes in Oregon gardens, Rosetta said. Some slug species are partial to warm temperatures—others prefer it cool. All slugs thrive in moisture and low light, often hiding in places underneath objects such as clods of dirt.
Slugs leave evidence of their presence in that infamous trail of slime. They scrape the center of leaves with their rasping rows of teeth, creating irregularly shaped holes. They are fond of plants in the brassica family, such as kale and cabbage, and also feast on strawberries and melons.
Rosetta suggested the following methods to make your garden less hospitable to slugs. It is generally more effective to combine an arsenal of strategies and try less-toxic approaches first, she advised.
- Water plants in the morning instead of the evening. Rosetta cites a study by Bernhard Speiser and Marcel Hochstrasser with the Research Institute of Organic Agriculture that showed slugs consumed a far greater percentage of the leaf area when plots of lettuce were watered in the evening than when they were watered in the morning.
- Design your garden to separate groups of plants that need more watering at a distance away from drought-tolerant plants. This will limit the moist areas that slugs call home.
- Copper barriers can be used to deter snails and slugs when placed around garden boxes or plant containers. The barriers should be at least 3-4 inches wide.
- As you plant young seedlings in August and September for your fall garden, use a hoe to break up the soil to expose any hidden slugs to the sun. Also keep your plants free of weeds, which provide dewy shelters that prevent slugs from drying out.
- Hand pick slugs about two hours after sunset. "Handpicking is a viable method if you have the time and a small area," Rosetta said.
- Make traps out of margarine containers halfway full of beer to drown slugs. "You don't have to use your expensive microbrews; any beer will work well," Rosetta said. "They're attracted to the fermenting yeasty-sugar mix in the malty beverage." People can skip the beer and mix sugar and yeast together too. Instead of either of those options, you can make homemade board or corrugated cardboard traps that have wood panels with ridges that can be lifted slightly so that slugs and snails can hide. These are useful methods that are much less messy than beer.
- Iron phosphate bait is less toxic to mammals than metaldehyde baits but still take care to apply it when pets are not around, Rosetta advised. When slugs eat sufficient bait, it damages their digestive system and they stop eating, eventually starving to death in three to six days.
- Metaldehyde is synthetic chemical bait that may be applied in fall to break the slug life cycle. Note that its pellets may affect some insects and it can be harmful to children and fatal to dogs if ingested in large quantities. Make sure to read and follow the label for instructions on using both metaldehyde and iron phosphate baits.
- Some pesticides are available for homeowners, including boric acid, chelated iron and botanically-based pesticides such as those containing cinnamon, garlic oil and clove oil. These products act both as repellants and are directly toxic to slugs and snails. They can be hard to find in stores but can be found online in many instances, Rosetta said.