Slugs and Snails                                  EL 14

by G.R. Nielsen, Former Extension Entomologist, Plant and Soil Science Department
 

Slugs and snails are not insects but mollusks* related to oysters, clams, and other shellfish, and characteristically have
soft, unsegmented bodies, usually protected with a hard calcareous shell. A slug is more or less a snail without a shell,
or with a shell reduced and located internally. Thirty-two species of slugs have been recorded in the United States.

Description: Most snails are gray, but their shells vary from white to brown or nearly black and are often striped or
mottled. The body of a snail consists of the head, neck, visceral hump, tail, and foot. The head bears two pairs of
tentacles, or feelers--a large pair above, upon which the eyes are borne, and a smaller pair below, which are used for
smelling. The mouth is in the center of the head, below the lower pair of tentacles. Below the mouth is the opening of a
large mucous or slime gland. The visceral hump, containing most of the internal organs, is contained in the shell. The
shell is secreted by the mantle, which forms a fold where the shell joins the body or "foot," of the snail. Under the edge
of the mantle, on the right side, is the breathing pore, and the muscles by which the animal crawls. When disturbed, the
entire animal may withdraw into the shell.

Slugs are similar to snails in general structure, except that they have no external shell or visceral hump, the mantle being
a smoother area occupying a forward fourth or third of the back. Slugs range in length from 1/4 inch to 10 inches,
depending on species and age. They vary in color from whitish yellow through various shades of gray to black, usually
more or less mottled or marked with darker shades. At least one species has a period when it is rusty orange in color.
Both snails and slugs have a definite mouth, which is equipped with a horny file, or radula, with which they rasp away
the substance to be eaten.

Damage: Slug damage to crops varies considerably with the season and year. Slugs can cause serious farm, home
garden, and greenhouse losses. They damage plants by eating the leaves and tender shoots, attacking almost all field
and garden crops, including grains, clovers, corn, young vegetables, ornamentals, and small fruits. They are especially
attracted to ripe strawberries and tomatoes. Head cabbage provides excellent shelter and food (especially savoy).

Some slugs are also injurious in mushroom houses. Slugs leave a slime trail of mucus on the surfaces on which they
crawl, and, on drying, silvery marks result which are objectionable, especially on floral or ornamental plants. Very
humid environments favor the development of slugs and snails.

Habits: Snails and slugs are mainly nocturnal, but they come out of their hiding places and feed in the evening or on
dark days. Their favorite hiding places are under old decaying boards and logs; under boardwalks; in cellars,
springhouses, and rock piles; along hedgerows; and beneath damp refuse. Snails are less particular in this respect than
slugs, since they can seal the opening of the shell with a mucous sheet, the operculum, which soon hardens to a
leathery texture. Under adverse conditions, the snails become dormant for up to 4 years. When conditions favor, the
"door" of the shell is rasped away and the snail resumes its normal activity.

Life History: The eggs are laid in masses, in damp places about the greenhouse benches, underneath boards and
flowerpots, or in the soil. They are held together by a sticky secretion that turns yellow before the eggs hatch. In about
1 month, the eggs give rise to very small young, which closely resemble the adult slugs except in size. They develop
slowly and probably live for a year or more. In the early spring when temperatures are consistently above 5 degrees C
(40 degrees F), survivors begin to move, hatch, feed, and lay eggs. The ensuing growth in the population is directly
related to temperature (speed of development increasing with increasing temperature) and humidity. In the hot, dry
months of summer, the slug population and related feeding apparently declines. However, with a return to cool wet
conditions in autumn, numbers of adult slugs can be found, which points to an efficient method of summer survival,
probably in habitats similar to those adopted for winter survival.

Control

Cultural - Mechanical: In gardens and fields, get rid of plant rubbish, boxes, sacks, and other materials where slugs
can hide and lay eggs. Trim rank grass and weeds along fences and ditches. In greenhouses, where it is always damp,
sanitation is most important. Do not let any plant matter collect under or near benches. Flowerpots, flats, and other
supplies on or beneath greenhouse benches harbor these pests and should be stored elsewhere. Trapping and hand
picking are fairly effective under greenhouse conditions. In home gardens traps or barriers are useful. Pieces of boards,
bark, or shingles (6 inches by 6 inches or larger) make effective traps when laid on the ground in the area to be
protected. Collect the slugs each morning from under the traps and destroy. The best barrier is fly screening about 4
inches wide. Set on edge into the soil around the plant. For a flat or cold frame, tack the screen around the top edges,
leaving the cut edges of the screen sharp.

Chemical: Many commercial baits, dusts, and sprays are available. All are effective when used according to the
directions. For best results, apply on a warm, moist night, when the slugs are most active. Because not all slugs come
out on any one night, repeat the treatment 5 days later. More applications may be needed if slugs are abundant and
continue to damage your plants. Place the materials as close as possible to the plants. Do not spray or dust the edible
parts.

*Stylommatophora, Gastropoda, Mollusca
 

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