Western Washington is famous for slugs. With lush vegetation and six or more months of rain each year, what more could a slug wish for? 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. Despite this, most gardeners are not very sympathetic to the damage they do when they get into our gardens and landscape plantings.
Slugs are closely related to snails, and both are related to common shellfish such as clams. They all belong to the animal phylum Mollusca and are termed mollusks. About 40 species of land slugs live in the 48 contiguous states. Roughly half of these are considered economically important. Of the pest species, all but one were introduced into this country. Introduction from foreign lands was no doubt, by accident. However, at least one, the brown garden snail, was intentionally imported as a potential gourmet food item.
The gray garden slug is common in our coastal gardens and landscapes. Adults are 1 to 1.5 inches long. The slug has two principal breeding seasons: from the start of fall rains until the advent of freezing weather, and in the spring when temperatures warm. Slugs are hermaphroditic. This means that each slug has both male and female reproductive systems. Offspring are produced by cross fertilization or mating.
Slugs can live from 12 months to two years. They reach sexual maturity before attaining full development, and they can begin to lay eggs when they are as young as three months, depending on moisture and food conditions. Eggs can be laid anytime during the year; however laying decreases during drier or colder weather. The clear, slightly oval eggs are laid in batches under dead leaves, in soil cavities, and in other protected places. Gray garden slugs can lay up to 400 eggs per year which hatch in three to six weeks.
Slugs glide over a slime trail they produce. The mouth houses a hornlike rasping organ used to scrape at food. Slug damage generally is quite distinctive because of this unique feeding structure. A small slug can easily eat as much as it weighs in one night. When slugs' teeth wear out, new rows move forward and replace them, conveyor belt style. Only 5 percent of the slug population appears above ground at any one time. Slugs are largely nocturnal. They feed during the night and on overcast, cool, cloudy days. They can travel as far as 100 feet to find food. If the weather is hot and dry, slugs hide beneath stones, boxes, boards and under dense plant cover such as ivy. Slugs avoid dry, dusty areas.
The gray garden slug is tolerant of low temperatures. Slug movement has been seen at or near freezing. A typical garden slug weighs about .01 ounces - it would take nine slugs to equal the weight of a penny. The showy California banana slug can weigh up to 70 times more than a garden slug. Although man can run a 100-yard dash in less than 10 seconds, it would take a slug almost two hours.
Many predators, including some birds, squirrels and mice stay clear of slugs because they taste terrible. Typical predators include frogs and garter snakes. Slugs are territorial and will attack caterpillars and other slug intruders.
In a heavily infested field, there are approximately nine slugs per square foot. This means 392,040 slugs per acre. In one night alone, slugs will eat approximately 87 pounds of crop, in a two-week period this is a ton per acre.
One of the most effective ways for home gardeners to reduce slug populations is to simply eliminate rocks, boards and other places which provide protection for slugs. Stale beer attracts slugs and many insects as well. Cans of beer sunk into the soil in and around the garden area have proved useful in slug control. The slugs crawl into the liquid and drown. Over the years, homeowners and popular garden articles have stated that cinders, wood chips, or sand used as borders along gardens provide effective barriers against slug invasion. This probably works because it eliminates favorable slug habitats.
One of the more recent slug control products registered for use in home gardens is iron phosphate. Sold under various trade names including Sluggo and Escargot, this material simply causes slugs to lose their appetite after feeding. They slither away and die. Unlike older slug baits, iron phosphate does not pose a risk to pets, birds and other wildlife when used according to labeled directions. (Sometimes it is necessary to use trade names for educational purposes. No product endorsement is intended or implied.)
EDITOR'S NOTE: For answers to local gardening questions, contact Master Gardener Earl Miller at 642-0541 or e-mail him at firstname.lastname@example.org.