CORVALLIS, Ore. – After years of studying the effects of baits on slugs for the Oregon grass seed industry, Oregon State University entomologist Glenn Fisher has learned a lot about battling the slugs.
Only two effective chemicals are licensed and formulated into slug and snail baits for use on home gardens and on food and seed crops in the United States, Fisher pointed out – metaldehyde and iron phosphate. He and his colleagues found that the less toxic slug baits containing iron phosphate are as effective as metaldehyde baits for controlling the slugs that damage gardens and landscapes, including the common gray garden slug.
Iron phosphate slug and snail baits, originally used in Europe, have been registered in the United States since 1997. Products containing iron phosphate include: "Sluggo," "Escar-Go!" and "Worry Free" slug and snail bait. These are sold as pelleted bait, typically applied to the ground around plants or crops. Iron phosphate baits have proven to be relatively non-toxic around children and pets, unlike those baits containing metaldehyde, according to the U.S. Environmental Protection Agency.
Metaldehyde has been an active ingredient in slug and snail baits since the 1930s. Products containing varying concentrations of metaldehyde include: "Cory's Slug and Snail Death", "Deadline," and "Slug-Tox." These products are sold as granules, sprays, dusts, pelleted grain or bait and are typically applied to the ground around plants or crops, to attract and kill slugs and snails.
Classified by the EPA as a "slightly toxic compound," metaldehyde may be fatal to dogs or other pets if eaten. The death of birds feeding in metaldehyde-treated areas has been reported in the scientific literature. These deaths were from the birds eating the slug bait, not dead slugs. The 4 percent pelleted metaldehyde bait, a concentration commonly sold to home gardeners, is reported to be toxic to wildlife, according to the EPA.
Gardeners who are frustrated with battling slugs need to keep in mind that pest control using baits is very different from using an herbicide or insecticide.
"With baits, the slugs must actively encounter and ingest the bait for it to work," said Fisher. "Since more than 90 percent of the slugs are underground at any one time, total eradication is impossible.
"Even when 'good control' is achieved with only a single baiting, only about 60 percent of a given slug population is destroyed," he said. "This may suffice to protect the crop or garden, but often allows the population to recover over time."
A crop or vegetable garden is at its greatest risk of slug damage when plants are young. It is best to bait at planting time or at seeding.
"Gray garden slugs will even feed in the furrow on some vegetable seeds, as well as the seedlings before they emerge from the soil," said Fisher. "If you wait until your vegetables or flowers get big, the slugs are less likely to come down off their plant food sources to consume bait, and the plants may have already sustained damage.”
By baiting just before seeding, your slug bait will be the major food on the soil surface, so slugs won't be attracted away from the bait by foliage from plants, said Fisher.
Cereal-based "mini-pellet" metaldehyde or flour-based iron phosphate baits such as "Sluggo" give the best performance record in western Oregon's rainy weather. The "tastier" the bait, the better the kill rate. In other words, cereal-based baits generally attract and kill more slugs than non-cereal-based baits, such as liquid slug baits.
Liquid slug bait, registered for ornamental use only, is metaldehyde in a paste form that is diluted with water and applied as a spray to leaves of landscape plants. It is most effective on young, small slugs as they are reluctant to search far from plants for baits.
For vegetable and fruit plants, pellet baiting is recommended. When baiting food gardens for slugs, apply baits at a density of about four to eight pellets per square foot of soil surface. Pick a time when daytime temperatures will be at least 50 degrees, with night temperatures not below 42 degrees. If the soil is not wet, sprinkle it with water just before applying the bait to make it more hospitable for slugs to come to the soil surface where the bait is.
Don't forget to bait in the spring and the fall. Autumn is an important time to keep controlling slugs because you can kill many of them before they lay eggs. But wait for the mornings to get damp, so the slugs will come out of their underground hiding places. Fisher recommends applying additional bait once more a little later in the fall to kill those little ones that just hatched.
Remove yard and garden debris, leaf litter and other excess vegetation, as these all prove to be refuges for slugs.
Rototilling the soil prior to planting a vegetable garden is an excellent way to reduce slug populations, said Fisher. The mechanical tilling action crushes slugs and their eggs and disrupts the cracks and worm holes they travel in.
When using baits, follow all label instructions and heed all label warnings. Don't allow baits to contaminate the edible portions of plants.