What's the scoop on slug bait formulations?

CORVALLIS - Two chemicals are currently licensed and formulated into slug and snail baits for use on home gardens and on food and seed crops in Oregon - metaldehyde and iron phosphate.

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 typically applied to the ground around plants or crops, to attract and kill slugs and snails.

"They work by destroying the mucus-producing system unique to slugs, which severely reduces mobility and digestion," explained Glenn Fisher, Oregon State University entomologist who has researched slugs as agricultural pests for many years.

Metaldehyde is classified by the U.S. Environmental Protection Agency as a "slightly toxic compound that may be fatal to dogs or other pets if eaten." The deaths of birds feeding in metaldehyde-treated areas have 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 U.S. EPA.

Sometimes pesticide manufacturers add insecticides such as carbaryl to a metaldehyde bait to provide additional control of certain pest insects such as cutworms, army worms, pillbugs and earwigs, said Fisher. But then earthworms, harvestmen ("Daddy Long Legs") and beetle predators of slugs that feed on these dual-purpose baits are killed as well.

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 than those baits containing metaldehyde.

According to Fisher, scientists do not yet completely understand how iron phosphate kills snails and slugs. According to the U.S. EPA's Office of Pesticide Programs, iron phosphate has "no unreasonable adverse effects to human health," and there is a "known lack of toxicity of iron phosphate to birds, fish and non-target insects."

A third chemical, called methiocarb (sold in products such as "Mesurol") has been available for limited use on ornamentals as a repellent spray, but cannot be used on food crops in the United States.

In recent trials, OSU entomologists found the metaldehyde bait more effective at killing gray garden slugs in the field than baits containing iron phosphates. But they also determined that iron phosphate baits are much better at killing slugs than using no bait at all.

By: Carol Savonen
Source: Glenn Fisher