With the rain we've had, the local slug population seems to be thriving and local gardeners seem to be wringing their hands.
Oregon has about 10 pest species of slugs, yet nine of those 10 are exotic species, introduced from somewhere else. Most came in accidentally, hidden on imported vegetation. One species, the brown garden snail, was intentionally imported as a gourmet treat. Slugs are hermaphrodites, capable of laying eggs, which they do in the soil, among leaves or under boards. Most lay eggs in the late summer and fall; some lay again in the spring. The pearl-like eggs will hatch in three to six weeks, depending on weather conditions and food supply.
Only two 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 - metaldehyde and iron phosphate. 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 nontoxic around children and pets in comparison with those baits containing metaldehyde, according to the U.S. Environmental Protection Agency. Now we have news from Oregon State University entomologist Glenn Fisher that iron phosphate slug bait proves as effective as metaldehyde baits for controlling our common gray garden slug. Fisher has studied the effects of baits on slugs for the Oregon grass seed industry for years.
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.
Classified as a "slightly toxic" compound, metaldehyde 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 E.P.A.
With baits, the slugs must actively encounter and ingest the bait for it to work, said Fisher in an OSU report. "Since more than 90 percent of the slugs are underground at any one time, total eradication is impossible. Even when 'good control' is achieved, only about 60 percent of a given slug population is destroyed."
A crop or vegetable garden is at its greatest risk of slug damage when plants are young - as in right now. It's best to bait at planting time or just before the seedlings sprout. 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.
Here are some other things to keep in mind:
The gray garden slug is common in fields and landscapes. Adults are 1 to 11/2 inches long. This slug is tolerant of low temperatures. Undoubtedly, sudden cold snaps catch many of them in exposed areas, and winter kill may be higher when the soil freezes to two or more inches. Gray garden slugs can live as long as a year, but six months is more typical.
Most of the time, slugs do beneficial things. They cycle organic matter, contributing to our rich soils, and they are an important food source for other wildlife.
There are alternatives to baits. You can trap slugs under boards. Place scrap boards on the ground under plants and between garden rows. Slugs seek shelter under the boards and you can collect them each morning, disposing of them in a bucket of soapy water. Do not put salt on slugs, as adding salt to the soil makes it unsuitable for gardening. Drown slugs in beer. Slugs love yeasty odors, and cheap beer seems to work the best. Take a container with a plastic snap-on lid, and cut a hole in the container about half way up. Bury it to the level of the hole, and pour about 2 inches of beer or yeasty water in the container. Cover with the lid to reduce evaporation and to keep out curious pets. Check and remove slugs daily and refill with solution. Use copper strips to blockade your garden patch. These materials are sold at lawn and garden stores. Take care not to trap slugs inside your garden plot. Take advantage of slugs' natural enemies. Snakes, mallard ducks, bantam chickens and some ground feeding wild birds feed on slugs.
The "tastier" the bait, the better the kill rate. Cereal-based baits attract and kill more slugs than non-cereal-based baits, such as liquid slug baits.
Mature slugs are more easily killed with baits than young slugs.
Apply bait after rain showers, as slugs like to come out and feed then.
In addition to spring, fall is a good time to bait, 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.
Follow all label instructions and heed all label warnings. Don't allow baits to contaminate the edible portions of plants.
Remove yard and garden debris, leaf litter and other excess vegetation, as these all prove to be refuges for slugs.
If you work the soil, it crushes slugs, buries them and disrupts their pathways.
Cathy Peterson belongs to the Clatsop County Master Gardener Association. "In the Garden" runs weekly in Coast Weekend. Please send comments and gardening news to "In the Garden," The Daily Astorian, P.O. Box 210, Astoria, OR 97103 or online to firstname.lastname@example.org