Tuesday, May 12, 2009

Slug control in hoophouses

Photo credit: hortipm.tamu.edu

Slugs are mollusks that are closely related to snails, but without the shell. The common garden slug is 1/2 to 1 inch long depending on the age of the slug and brown or gray in color. Slugs can thrive in the hoophouse environment-their food source is close to the ground, there are many places to hide and take a good nap, and the humidity is just right. Unfortunately, slugs can cause serious problems in vegetable, fruit and ornamental plants by feeding on fruit and leaves.

There are many tips and tricks floating out in cyberspace, many of which are interesting but wouldn't hold up in a commercial setting. Beer traps for example, may work great in a home garden but not when you have many hoophouses to protect and limited hours in the day.

In a nutshell, slug control revolves around three concepts: 1) Modifying the environment to make it less slug-friendly, 2) setting up barriers to physically keep slugs away from your crops and 3) baits which attract the slugs but also give them a poison dose.

In terms of modifying the hoophouse environment: Look at removing places where slugs can shleter during the daylight hours such as crevices and under boards or other equipment. Reducing the humidity may make the environment less appealing: If it's warm enough outside, run the fan to replace the humid air with drier air, fix leaky hoses, consider switching to drip irrigation instead of overhead or handwatering.

Physical barriers can help prevent slugs from coming in contact with crop plants. Diatomaceous earth sprinkled around the plants 1 inch high and 3 inches wide can help-until it gets wet! This might work on a small-scale but not in a larger setting. The same thing goes for sprinkling lime or salt. (And we already have enough salinity in our Colorado soils!) One option that shows promise is copper strips or flashing. This can be expensive if you have large areas to protect, but flashing can be bought in bulk rolls which will bring the cost down a little. Copper flashing (about 10 inches wide) can be circled around the susceptible plants and buried 3-4 inches. If you have benches in your hoophouse you can wrap the copper around the base of the bench or the legs. The flashing needs to form a complete enclosure to be an effective barrier. While the exact mechanism isn't known, it is thought that the slime produced by the slug interacts with the copper and produces an electrical charge which repels the slug from the area. There are various copper strips and tapes on the market. This won't keep existing slugs within circled area from feeding, but will keep new slugs out.

The third control option is baits. The most effective baits contain either metaldehyde or iron phosphate. Metaldehyde is very attractive to slugs, but is also poisonous to children and animals (especially dogs). Metaldehyde should also not be used around food crops as they can leave toxic residues on fruits and veggies. If you have dogs on your property, consider the iron phosphate option. This is sold as Sluggo and Escar-go (and others) and is considered non-toxic to pets. These products can be used to prevent slugs from entering a crop area (by encircling with the granules) or to lure slugs out of crop plants (by sprinkling within crops and between rows). Research out of Oregon State University showed that iron phosphate containing products were as effective as metaldehyde containing products.

For more information, see this CSU Extension factsheet: Slugs by W.S. Cranshaw.

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