Garden Slugs - Taxonomy, Anatomy, and Control

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.

Fig. 1 Slug Eggs
Fig. 1 Slug Eggs, 2mm

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.

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).

slug 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.

The head of the slug has two sets of 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.

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.

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.