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Gray Garden Slug - Deroceras reticultatum
Live adult slugs and slug eggs photographed at Hawk Hollow
Forest Preserve, near Bartlett IL, USA
Taxonomy: Kingdom: Animalia / Phylum: Mollusca / Class:
Gastropoda / Subclass: Orthogastropoda / Superorder:
Heterobranchia
Order: Pulmonata / Suborder: Eupulmonata / Infraorder:
Stylommatophora
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Fig. 1 Slug Eggs |
All slugs lay eggs. Each
species requires a different length of time for the development
of its eggs and the maturing of its young. The number of eggs
laid at one time by one slug may be up to 100, but average 20 to
30. Young adult slugs apparently lay fewer eggs than older ones.
Though slug eggs may be found outdoors during any month of the
year, most of the eggs are laid in the spring and early summer.
Most species overwinter as adults or nearly mature young. In the
spring, eggs are laid in moist areas and the new slugs normally
reach maturity by fall. During periods of particularly warm and
wet climatic conditions, the rate at which the slugs develop may
allow for eggs to be laid in mid-summer, thus making possible a
second generation. Mating usually takes place from August until
mid-October and eggs can be laid from 30 to 40 days after a
successful mating.
Eggs are generally laid on or near the soil surface, but are
usually deposited in places of concealment, such as underneath
mulch, dead leaves, rocks, flower pots, trash, and boards.
Particularly preferred are spots where the nature of the cover
keeps the surroundings relatively cool and moist. These eggs
were found underneath tree bark lying on the forest floor during
the first week of April; they have overwintered and are near
hatching. New slug eggs are transparent.
The minimum temperature at which egg development will take place
varies with the species of slug but is in the general range of
32 to 42 degrees F. At the minimum temperature, as long as 100
days may be required for the eggs to develop. At higher
temperatures, development is usually completed in ten days to
three weeks.
As soon as slugs hatch, they are active and begin to crawl or
feed if the temperature and humidity are right. They are mainly
nocturnal and remain motionless and concealed until nightfall
provides suitable conditions for activity. |

Figure 2. Adult Garden Slug
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The gray
garden slug was introduced to North America from Europe during
the 1800s. It has become a voracious pest of garden vegetables,
field crops, and ornamentals throughout continent. Gray garden
slugs attack seedlings of a number of crops, particularly
no-tillage corn and alfalfa, and strawberries. Large numbers of
slugs can be found in a wet year when the preceding winter was
mild. A slug is often described as a snail without a shell. The
head of the slug has two sets of tentacles. The eyes are on the
tips of the upper tentacles. The lower tentacles, which are
shorter, are used for tasting and smelling. The mouth is located
between and below the lower tentacles, and is equipped with a
radula, a tooth-covered rasp that the slug uses to grate plant
tissue. The slug glides along a path of mucus that is secreted
by the pedal gland, located just below the mouth. The mantle is
a fold of skin on the back of the slug. At the edge of the
mantle is an opening called the breathing pore, or respiratory
pore, which can be seen opening and closing rhythmically as air
passes in and out.
The gray garden slug is about 5 cm (2 in.) long when fully grown
(figure 1.) It is variably colored, ranging from cream colored
with irregular gray spots to dark brown with dark spots. The
mantle is situated forward, near the head, and the respiratory
pore is behind the mid-point of the mantle. The keel is located
only at the tail. The gray garden slug produces a milky slime
when disturbed; this slime distinguishes it from other species,
many of which produce clear slime.
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Fig. 3 -
Pneumostome |
Figure
3. The respiratory pore, or
pneumostome, is clearly visible at the edge of the mantle (the
apparent division along the midline of the slug). Select
thumbnail for larger image. |
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Fig. 4 -
Skirt |
Figure 4. The slug's anus and radula
(mouth) are located on the underside, or foot. The slugs glides
slowly along on a lubricating layer of slime, using rhythmic
contractions of its foot. The slime is produced by the pedal
gland, located just ahead of the mouth. (None of these
structures are readily apparent in this photograph.) The skirt
is the curled lip between the topside and the foot. |
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Fig. 5
- Optical Tentacles |
Figure 5. The head of the slug has two
sets of tentacles. The eyes are the black dots on the tips of
the upper tentacles. The lower tentacles, which are shorter, are
used for tasting and smelling. |
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Figure 3. Slug Anatomy
Drawing:
Michal Manas
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Control
If the area is small, for example, a
garden, removing shelter
for the slugs can help to control them. The gardener
should remove debris, logs, and
stones, and trim grass and weeds along
the edge of the garden. For high populations (more than
five slugs per sampling hole), it may be necessary to
remove slugs by hand from plants at
night. Daytime collection is also
possible if the gardener lays a trap of boards or shingles on
the ground near the most susceptible plants. The slugs
will crawl under the boards at night
and can be crushed or removed in the
morning.
Other methods of control are to search for egg masses and
destroy them; to encircle the most susceptible plants
with wood ashes, diatomaceous earth,
or even screen to exclude slugs; and
to encourage natural enemies of slugs,
toads, garter snakes, and predacious
ground beetles. In field crops, avoid
planting corn, alfalfa, beans, or other susceptible
crops in wet fields where slugs have been a problem.
Slug populations are often highest in fields that were in
sod the previous year, or that had
crop debris on the ground. If corn or
alfalfa is to be planted following sod, plow as early as
possible in the fall. If high
populations of slugs are detected, till the soil or
use a molluscicidal bait to prevent slug damage.
A molluscicidal bait is more effective than a chemical
spray for controlling slugs in crops.
The bait works best if applied in the
evening when the ground is damp and slugs are active. Further
information about molluscicides
may be obtained from local Cooperative
Extension agents.
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More Environmentally Sound Control Methods
Rake your garden in early spring to remove leaves, plant debris
and slug eggs. Also remove boards and other material to reduce
favorable areas for slugs. Avoid using large wood chips as they
provide hiding places for slugs. Do not place mulch any thicker
than three inches. This helps protect plants from weeds and
helps maintain plant moisture while minimizing a favorable
environment for slugs.
Water your garden only when necessary. Irrigate in the morning
so plants are dry by evening. Prune lower leaves or stake large
plants to reduce potential hiding places for slugs and to allow
better air circulation that helps keep the soil surface drier.
Thin or divide plants if they are too crowded.
Some plants have been observed to be less affected by slugs. A
list of those that do well in shaded areas includes Astilbe,
Dicentra, Lobelia, Ranunculus, Vinca, and Viola. Plants
resistant to slugs that grow well in partial shade include
Phlox, Campanula, Hemerocallis, and Mentha.
Trapping
Trapping and handpicking helps lower slug numbers. To be
effective, traps must be checked and cleaned out several times a
week (more when slugs are abundant). Be sure to put out enough
traps to adequately protect the entire garden.
You can trap slugs by setting out several flat boards, shingles
or damp newspapers. Check under these traps the next morning and
kill any slugs that are hiding. You can drown slugs in soapy
water, crush them, or spray them with household ammonia diluted
to a 5 percent or 10 percent solution. Traps containing beer or
other fermenting food are popular. You can purchase commercially
available slug traps or make your own. Sink jars, cans, pans or
similar containers into the ground so the top is level with the
ground (some commercially available traps are placed on the
ground). Pour beer or a water and yeast mixture (one teaspoon of
yeast to three ounces of water) or similar fermenting liquid
into the container. Slugs are attracted to the odors, fall in
and drown.
Barriers
Copper is an effective barrier to slugs. Copper strips or tape
sold specifically for slug control can be purchased from garden
suppliers. Caution: The sharp edges of some products may cause
safety problems, especially for young children. Copper barriers
are most practical for small gardens and containers.
Diatomaceous earth (tiny fossilized skeletons of ancient
aquatic diatoms) is moderately effective as a slug barrier. When
slugs come in contact with diatomaceous earth, it is abrasive to
their skin. Diatomaceous earth is most effective when used in
dry conditions and has little effect when it absorbs moisture.
You can purchase diatomaceous earth at any
pool supply store, as it is most often used in swimming pool
filters. |
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