In the moister areas of Washington, slugs are probably the most frequent pests of home ornamental and vegetable plants. There are several slug species in Washington. The most common are the gray garden slug, Deroceras reticulatum, and European slug, Arion ater.
Host Plants and Damage
Slugs eat many kinds of plants. Corn, lettuce, beans, and flowers
of all kinds are but a few of the hosts. Ripened strawberries
are particularly attractive to slugs. Slugs may totally consume
young plants or chew large, ragged unsightly holes in foliage.
When slugs move or glide across surfaces, they leave a slime trail
which appears silvery when dry. This is a good indication of what
did the damage, even if the "guilty party" is no longer
present. Slugs are particularly dependent on moisture for development
and extent of activity; thus, losses from slugs will be more serious
during rainy, cooler weather.
Slugs belong to a large group of animals called the Molluscs.
This group includes clams, oysters, squid, and octopus, as well
as slugs and snails. Slugs are most closely related to snails;
in fact, slugs are generally thought of as snails without shells.
Snails cause similar damage but usually are not as serious a pest
in the Pacific Northwest.
Slugs vary in length, depending on species and age, from 1/4 inch
to as much as 10 inches. They are elongate with a prominent dorsal,
fleshy mantle that encloses the vital organs.
They move or glide on a ventral organ called the foot. The
foot secretes the slime which helps them move. They also possess
conspicuous retractable eye stalks on the head.
Slugs are hermaphroditic. This means that each slug has both male
and female reproductive systems. Offspring are produced by crossfertilization
or mating. Slugs can live from 12 months to 2 years. Slugs 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.
Each slug is capable of laying up to 400 eggs in a year's time.
They are laid in small groups of 3 or 4, or as many as 30 or 40.
Eggs are laid in moist places, such as under stones, in plant
debris, or under boards. Eggs are round or oval, about 1/8 to
1/4 inch in diameter, and colorless to milky white. If conditions
are right, they hatch in a few weeks, but if the weather is hot
and dry, hatching may be delayed until weather becomes favorable.
Baby slugs resemble adults in shape. They may not assume adult
coloration until later in development.
Slugs are largely nocturnal. They feed during the night and on
overcast, cool, cloudy days. 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.
Natural enemies. Garter snakes, birds, and frogs are some
of the common predators of slugs. Among domesticated animals,
ducks and geese are effective predators.
Sanitation. Many objects, such as rocks, boards, and compost
piles, provide shelter for slugs. Remove such sites to reduce
both slug and snail numbers. Gardens next to areas of tall grasses
and weeds are particularly susceptible to slug invasion. Eliminate
these sites since they provide excellent protection for slugs.
Mechanical control. 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. Picking, stabbing, cutting, and other
laborious methods can be used to control slugs. However, this
will be an endless task unless hiding places, such as tall grasses
and debris, are eliminated.
There are a number of commercially sold apparatuses such as barrier
fences or slug inhibitors of some kind. Many of these have not
yet been tested by Washington State University; however, some
do show promise.
Chemical control. You may use baits in accordance with
labels in ornamental plantings and in edible crop plantings if
the material does not come into contact with edible portions of
the plant.
Apply baits to the soil in the evening during moist weather when slugs are active. When the weather has been hot and dry, it is helpful to sprinkle the plants and surrounding area lightly to activate slugs. Apply the material as the label directs. Do not place bait in piles unless the piles are covered with boards or other objects; otherwise, dogs or cats may be attracted to and eat the bait. Baits lose their attractiveness to slugs when they become moldy. Various other formulations, such as sprays, dusts, and foams are available for slug control. Apply these as the label directs.
