NC State Extension Publications

Introduction

Slugs and snails belong to the phylum Mollusca and are more closely related to octopi than insects. Mollusks are a large and diverse group of animals of worldwide distribution. The slugs and snails are much like some insects in their biology. Their damage to ornamental plants resembles that done by caterpillars or wireworms. Slugs and snails are in a subgroup called the Gastropoda, the members of which have a head, a ventral muscular foot, and a shell (internal in slugs and external on snails).

Approximately 725 species of land snails and about 40 species of slugs are now known from North America. Most of these have been introduced accidentally. With few exceptions, native species are solitary in habit and do little or no damage. The introduced slugs and snails are usually gregarious and may cause serious damage as they build up large populations in a local area. In many places, slugs and snails have caused as much damage as insect pests. Eradication and control of slugs and snails is difficult and costly.

Slugs and snails are usually nocturnal so their damage is noticed before the pests are. Slugs and snails leave silvery slime trails on the ground and over plants. Slugs are able to crawl over the sharp edge of a razor blade without harm. Whenever the plant damage done by snails and slugs is easily observable, it has been determined the live weight of the slugs infesting the area may be around 70 pounds per acre! Slugs and snails may consume several times their own body weight each night; damage can be serious within a short time.

Movement of plant material facilitates the dispersal of these pest snails and slugs. Growers need to examine the plant material as it arrives at the facility. If live snails or slugs in the plant material are noticed, the local Department of Agriculture inspector should be called immediately. Failure to take quick action before the slugs or snails become established at the facility can be costly. For example, in a greenhouse complex in Maryland where the brown garden snail was detected as plant material was being unloaded, the grower took no action. One year later the snail was found in a routine inspection by the Department of Agriculture. The grower was then required to have all outgoing shipments inspected for the snail and to begin eradication treatments. It took over three years for the snail to be eradicated from the premises.

As the slugs mature, they become functional males and then true hermaphrodites (self fertilization has been observed in some species). Older slugs are females. Slugs are apparently not repelled by light, but are repelled by rising temperatures. As temperatures rise, slugs crawl down to their hiding places on the soil surface to rest and absorb water through their skin. As temperatures start to fall, slugs actively begin foraging. Thus slugs may be active during the day after a cooling shower as long as the temperatures decline or remain steady. Slugs are very sensitive to ambient temperature and can detect temperature changes as gradual as 2°F per hour! Slugs prefer to remain at 17 to 18°C although they lay eggs and develop normally (but slower) at lower temperatures. Development ceases below 5°C. Slugs can withstand slight freezing temperatures although their tendency to take shelter in cold weather protects them from freezing. Slugs try to escape from temperatures higher than 21°C. Slugs are also sensitive to air currents. Gentle breezes elicit a positive response in which the slug turns toward the source and extends its antennae. As the breezes become more brisk, the slugs turn away from the source evidently to escape dehydration. Slugs can withstand brief periods of immersion under water, although they drown after several hours.

Birds (up to 6 percent of the diet of starlings), ducks, moles, toads, shrews and carnivorous ground beetles, rove beetles, and firefly beetles feed on slugs. Sciomyzid flies and nematodes also parasitize slugs. In addition, slugs are preyed upon by omnivorous slugs such as the spotted garden slug. Dry weather may kill up to 90 percent of slug eggs and young per year.

Metaldehyde baits have been shown to attract slugs up to 1 meter away. The toxic effects of metaldehyde seem to be primarily due to dehydration as metaldehyde elicits excessive mucus production (mucus is 98 percent water and 2 percent mucoproteins.) Thus in dry weather, metaldehyde is more effective. In wet weather, slugs sometimes can absorb enough moisture to compensate for the water lost in mucus production and therefore recover from the effects of metaldehyde. However, if slugs consume too much metaldehyde, they do not recover. Slugs seem to become more susceptible to carbamate pesticides as they mature. Copper sulfate is toxic to slugs and slugs will not crawl across a barrier of copper metal or wooden surfaces treated with copper sulfate.

Key to Common and Potential Slug and Snail Pests of Flowers and Foliage

  1. With external shell - SNAILS (GO TO 2)

    Without external shell - SLUGS (GO TO 5)
  2. Shell an elongate spire that is often broken off (Figure 129); these snails mainly feed on other snails; they are not considered a pest - RUMINA SP.

    Shell not an elongate spire - GO TO 3
  3. Shell a flattened disc spiraling toward the center (Figure 130); these snails mainly feed on other snails, they are not considered a pest - OXYCHILUS SP.
  4. Shell oval, with a large teardrop-shaped aperture which is approximately two-thirds the total shell length (these snails feed on algae and soft vegetable matter, they are not considered a pest (Figure 131) - SUCCINIA SP.

    Shell globular, with the aperture approximately the shell diameter (Figure 132) - BROWN GARDEN SNAIL

  5. Breathing pore located in anterior half of mantle; back never keeled; posterior end rounded when viewed from above (Figure 133); these slugs can do considerable damage - ARION SP.

    Breathing pore located in posterior half of mantle; back keeled at least at posterior end which is pointed in dorsal view - GO TO 6

  6. Back strongly keeled from the mantle to tip of tail; mantle grainy with center part bound by a groove (Figure 134); usually burrows in the soil and feeds on roots - GREENHOUSE SLUG

    Back keeled only near posterior end; mantle wrinkled in concentric circles, without a groove - GO TO 7

  7. Mantle and body with black spots; large slug (up to 150 mm) (Figure 135) - SPOTTED GARDEN SLUG

    Mantle and back without well defined spots - GO TO 8

  8. Mantle and back with well defined, dark stripes; breathing pore not surrounded by a pale ring (Figure 136); exudes watery slime when irritated - LEHMANNIA SLUG

    Mantle and back without well defined stripes; breathing pore surrounded by a pale ring - GO TO 9

    Medium sized slug (up to 50 mm) (Figure 137); gray to reddish brown; exudes milky adhesive slime when irritated - GRAY GARDEN SLUG

  9. Relatively small slug (up to 25 mm) (Figure 138) light gray to blackish brown; exudes clear, watery mucus when irritated - BROWN SLUG

Figure 129. Rumina sp. Figure 130. Oxychilus sp. Figure 131. Suc

Figure 129. Rumina sp. Figure 130. Oxychilus sp. Figure 131. Succinia sp. Figure 132. Brown garden snail.

Figure 133. Arion sp. Figure 134. Greenhouse slug. Figure 135. S

Figure 133. Arion sp. Figure 134. Greenhouse slug. Figure 135. Spotted garden slug. Figure 136. Lehmannia slug. Figure 137. Gray garden slug. Figure 138. Brown slug.

Arionid Slugs

Garden slug (Figure 139), Arion hortensis, Arionidae, STYLOMMATOPHORA
Garden slug (also black field slug), Arion hortensis Ferussac, Arionidae, STYLOMMATOPHORA
Hedgehog slug, Arion intermedius Normand, Arionidae, STYLOMMATOPHORA

DESCRIPTION

Adult

Garden slugs grow up to 25 to 30 mm. The mantle is on the anterior part of the slug and the breathing pore is in the anterior half of the mantle. Garden slugs have a band along either side which is not distinct on the lower border. There is no dorsal row of tubercles, and the creeping sole is yellow to dark yellow or orange. The respiratory pore is in front of the midpoint of the mantle shield. There is no keel, and the sole of foot is not divided. The skin is rough and wrinkled, gray to dark brownish gray and often with small yellow-brown dots (color lighter toward the foot). Hedgehog slugs are about 20 mm long, and are greenish to yellowish gray with one prominent dark lateral band on each side and on the mantle. The head is dark gray or black with dark gray tentacles, and the skin is covered with conical tubercles with transparent tips when contracted giving the slug a spiky appearance. The sole is yellow at the rear.

Mucus

When irritated, the slime on the body of the garden slug is yellow to dark yellow and the slime on the sole is colorless. The mucus of the hedgehog slug is golden yellow.

Young

Young garden slugs are slate gray on top. There is a dark band on each side (the band on the right enclosing the respiratory pore). The sole is yellow to orange.

BIOLOGY

Distribution

The arionid slugs are introduced from Europe. They can survive only in areas of moderate to high humidity. In the United States, arionid slugs occur naturally only in the North. Hedgehog slugs are found in the British Isles, Spain, Central Europe, and North America.

Host Plant

The garden slug is a pest of narcissus. The hedgehog slug feeds on narcissus, grasses, lilies, (leaves, brood bulbs, flowers), and iris in greenhouses. Jimson weeds are highly favored, amaryllis (leaves, bulbs), coleus, tulip (germinating bulbs, leaves, flowers), and vegetables.

Life History

The garden slug (also called the black field slug) creeps very slowly (covers 90 cm per night) and becomes inactive at 5°C. The life span is 7.5 to 12 months and females lay 158 to 205 eggs. Garden slugs increase in size during the summer and reach sexual maturity in the autumn and winter. This slug only reproduces in the early months of the following spring. Crowding increases mortality. These slugs forage primarily from 0530 to 0600 hours although activity starts at 1900 hours. The garden slug feeds more frequently than the gray garden slug, but eats the same amount. Maximum seasonal feeding and mobility occurs in October and November. Frost and dryness drives these slugs deeper into the soil. Garden slugs reproduce faster on alkaline to neutral soils than on acid soils. The garden slug can consume 56 percent of its body weight in one day. Hedgehog slugs oviposit in July and August. Eggs of hedgehog slugs hatch in September and October. These new slugs mature the following spring. The life span is about 12 months.

CONTROL

Hedgehog slugs become infected with the ciliate, Tetrahymena rostrata (Kahl) by contact with cysts in the soil during winter. A sciomyzid fly, Tetanocera elata Meigen, parasitizes this slug in North America and Europe. Hedgehog slugs are also infected with the cestode, Davainea proglottina (Davaine).

Figure 139. Garden slug.

Figure 139. Garden slug.

Brown Garden Snail

Brown garden snail (Figure 140), Helix aspersa Miller, Helicidae, STYLOMMATOPHORA

DESCRIPTION

Adult

An adult shell has 4 to 5 whorls and is 28 to 32 mm in diameter. The shell is large, globose, rather thin, and has fine wrinkles on the surface. It is yellowish or horn-colored with chestnut brown spiral bands that are interrupted by yellow flecks or streaks. The aperture is crescent-shaped or oval-crescent-shaped, approximately one half the shell diameter, and has the tip turned back.

Young

Young brown garden snails are similar in appearance to adults, but are smaller.

Eggs

Brown garden snail eggs are white, spherical, and about 5 mm in diameter.

BIOLOGY

Distribution

The brown garden snail is native to Europe and the Mediterranean Region. It was introduced by French immigrants into California, Kentucky, Louisiana, New Mexico, Oregon, South Carolina, Texas, Utah, and Washington.

Host Plants

The brown garden snail is a general feeder and will probably attack anything in a greenhouse.

Damage

The brown garden snail eats large ragged holes in leaves and may totally consume seedlings. Low growing plants generally suffer the most damage, but this snail climbs trees to feed and has been reported as a pest in citrus orchards.

Life History

Eggs are laid in a nest 2.5 to 4 cm deep in the soil. Each snail lays an average of 85 eggs. Eggs hatch in 2 to 4 weeks depending on soil moisture and temperature. The egg mass is concealed by a mixture of soil and secreted mucus, and is then covered with excrement. The number of times a snail oviposits is dependent on temperature, humidity, and soil conditions. Low temperatures (less than 12°C) and low humidity inhibit the activity of the snail. Dry soil is unsuitable for nest preparation.

During warm, damp weather, ovipositions may be as frequent as once a month. Peak activity period is February to October. Each adult snail during this period will oviposit about once every six weeks. During these five ovipositions each snail lays an average of 430 eggs.

When dry conditions prevail, the snail may seal itself to a object or close off the aperture of the shell with parchment-like material. With the return of moist or humid conditions, the snail will again become active.

CONTROL

One control option is to hand pick the snails from plants. Place the snails in a jar, fill it with water, put the lid on securely, and let them drown. The addition of dish detergent to the water will kill them faster. Another big help is to clean up the area by removing the hiding places in which the snail spend the day. Sometimes bands of wood ashes or lime an recommended to discourage snails from entering an area. Lime may work as long as it is kept dry, but lime may adversely affect the soil pH. Wood ashes appear to be somewhat effective. Snails will avoid crossing copper screening or banding because the mollusc apparently receive a minute electric shock when it contacts copper.

Figure 140A. Brown garden snail. Figure 140B. Shell opening.

Figure 140A. Brown garden snail. Figure 140B. Shell opening.

Deroceras Slugs

Brown slug (Figure 141A) (also marsh slug or smooth slug), Deroceras laeve (Muller), Limacidae, STYLOMMATOPHORA
Gray garden slug (Figure 141B), Deroceras reticulatum (O. F. Muller) (also netted slug or gray field slug), Limacidae, STYLOMMATOPHORA

DESCRIPTION

Adult

The brown slug (also marsh slug or smooth slug) grows up to 25 mm long. It is very soft and translucent and is covered with watery slime. The brown slug is light gray or brown to blackish brown with only slight marks if any. The mantle is central and half of the body length. The sole is pale brown or light gray. The gray garden slug grows to about 50 mm long. It is pale cream or yellow white, gray to reddish brown; with a network of mottled, dark brown grooves, spots, and streaks (these rarely absent). The mantle is rounded at both ends with dark dots and flecks; the mantle and body are not banded; the rim of the respiratory pore is slightly paler than the mantle and the center point of the concentrically ridged mantel lies to the right of the midline. There is a short, truncated keel at the rear. The sole is pale or light brown;

Mucus

The mucus of the brown slug is thin, clear, and colorless. When the gray garden slug is irritated, the mucus is milky (the white pigment is lime) and sticky.

BIOLOGY

Distribution

The brown slug is found in Europe, England, the United States, and in the former USSR. Introduced from Europe, the gray garden slug has spread throughout most of the United States except perhaps for the Coastal Plains of the Southeast.

Host Plants

The brown slug feeds on coleus, lilies, iris, narcissus, chrysanthemums, Fittonia verschaffeltii, jimson weed, strawberries, and vegetables (especially lettuce), and (in greenhouses) amaryllis leaves and bulbs, cyclamen, ferns, and orchids. The gray garden slug feeds on narcissus and other ornamentals.

Damage

These slugs may eat 60 mg (about 2 sq cm) or up to 40 percent of its total body weight per day.

Life History

The brown slug is well known for its great mobility. It is most active from 6:30 am to 2 hours after sunrise and most activity ceases about 9 am. There is only one peak of activity per day. Self fertilization occurs in this species. It emerges from hibernation 3 weeks before the gray garden slug and it is resistant to frost down to - 8°C. Gray garden slugs live 9 to 13 months and lay about 300 eggs. Gray garden slugs cover up to 90 cm per night and are active from 17.5 to 20.5°C although they may forage at 1 to 2°C. The gray garden slug forages even in mild winters and may not be completely immobile even at 0°C. This slug can survive -8°C for several days. They regulate body temperature by evaporation of water from the skin (up to 12°C difference in body temperature and environment). When it is too warm, field slugs immediately seek shelter to conserve moisture. In damp soil, slugs move singly; in dry soil they bunch together. This slug can lose up to 50 percent body weight and recover within two hours of exposure to water. September is the period of maximum feeding and mobility. Field slugs tend to reside in coarse soils, and tend to oviposit in fine soils. They prefer to retreat into holes that touch all sides, and they penetrate much deeper into coarse soils (12 to 14 cm). Field slugs may oviposit on the surface of damp soils but oviposit deeper into dryer soils. They prefer soil moisture content of 60 to 85 percent water. These slugs may drown in excessively wet soils. Field slugs reproduce faster on alkaline to neutral soils. They have one or two generations per year depending on weather. There are usually two generations per year in England and one in central Europe. Sometimes there are three overlapping generations (one from a late population of the preceding year). This slug covers significant distances in search of a new source of food only when the old supply has been exhausted.

CONTROL

Brown slugs are infected by the ciliates, Tetrahyema limacis (Warren), and T. Rostrata (Kahl). The latter is so highly pathogenic that it is a possible biological control organism. The trematode, Rhabditis lambdiensis (Maupas), also infects the brown slug. This slug also is sensitive to Angiostrongylus cantonensis Chen (a cause of eosinophilic meningoencephalitis of people so do not eat slugs raw). Brown slugs are hosts of Pneumostrongylus tenuis Dougherty, a lungworm parasite of Virginia deer. About 25 percent of brown slug populations are infected from spring to June. By July only 1.5 percent are infected apparently due to mortality in the population. The slug is the main overwintering host of this trematode. The trombidiid mite, Riccardoella limacum (Shrank), lives in the mantle cavity of brown slugs and retreats to the respiratory pore when disturbed. Harvestmen (Opiliones) prey on this species. The sciomyzid flies, Tetanocera plebeia Loew and Tetanocera valida Loew prey on brown slugs. Slices of raw potato can be used to monitor brown slug populations. Better control of the brown slug is obtained by scattering baits near the edges of gardens or near shelters, and greenhouse benches. Metaldehyde is fairly effective, but metaldehyde at high concentrations is repellent to brown slugs. Carbamates are much more effective if slugs can get to water after treatment.

The effectiveness of baits for the gray garden slug is greatly increased by placing the bait under a board, pot, or flat. For specific chemical control recommendations, see the current Cooperative Extension publications on ornamental plant pest management or consult your Extension agent.

Figure 141A. Brown slug. Figure 141B. Gray garden slug.

Figure 141A. Brown slug. Figure 141B. Gray garden slug.

Greenhouse Slug

Greenhouse slug (Figure 142), Milax gagates (Draparnaud), Limacidae, STYLOMMATOPHORA

DESCRIPTION

Adult

An adult greenhouse slug is 50 to 70 mm in length. The body color is gray or black without distinct markings or patterns. The mantle is slightly granulose but not concentrically wrinkled, the central portion bounded by a horseshoe-shaped groove. The breathing pore is on the right posterior half of the mantle. The body is strongly keeled from mantle to posterior end. The mucus is colorless.

Eggs

Greenhouse slug eggs are opaque to whitish, oval, and about 2.5 mm in diameter.

BIOLOGY

Distribution

The greenhouse slug was introduced into the United States from Europe. It is found throughout the temperate regions of the world.

Damage

The greenhouse slug differs from most pest slugs in that it is a burrowing species. It can cause serious injury to roots and stems. A few seen on plants and stems means that many more are below the soil surface.

Life History

Eggs are laid in a tunnel 3 to 5 cm from the soil surface, either singly or in clusters of up to 16. The adult slug plugs the end of the tunnel with mucus material which dries to resemble the soil surface. Eggs hatch in 11 to 24 days depending on the temperature. Parthenogenesis has been observed in the greenhouse slug. There is little information available on the life history of the greenhouse slug. One study noted that slugs preferred drier sites during the day and moved to more humid areas at night.

CONTROL

Control of the greenhouse slug consists primarily of placing baits in likely areas the slugs would encounter. The effectiveness of such baits is greatly increased by placing the bait under a board, pot, or flat.

For specific chemical control recommendations, see the current Cooperative Extension publications on ornamental plant pest management or consult your county Extension agent.

Figure 142. Greenhouse slug.

Figure 142. Greenhouse slug.

Lehmannia Slugs

Lehmannia slug (Figure 143), Lehmannia poirieri (Mabille)(= Limax marginatus of Pilsbry), Limacidae, STYLOMMATOPHORA
Tree slug, Lehmannia marginatus (O. M. Muller) (= Limax arborum Bouchard-Chantereaux), Limacidae, STYLOMMATOPHORA

DESCRIPTION

Adult

Lehmannia poirieri is a light brown to yellowish gray slug with dark lines along the top of the body. There is a pair of faint grooves just above the foot. The mantle does not have a groove or fold although it is faintly concentrically wrinkled. When irritated, the mucus secreted is colorless. This slug grows to about 60 mm long and can stretch to about 75 mm.

The tree slug, Lehmannia marginata (Muller), is a gray to reddish slug with a lighter median stripe down the back. It usually has a large, translucent water reservoir (body) at the tail. The respiratory pore is behind the middle of the mantle, and the body is keeled only at tail. The mantle is one-third of the body length just behind the head. Each side has two dark bands lengthwise (lower band sometimes forming a network, and often faint). The mantle has three dark, lengthwise bands (median faint) and the pale areas between the bands forming a lyre shape. The sole is gray, often with a darker median area. The mucus is colorless and watery.

Egg

The eggs of the tree slug are oval to somewhat pointed on one end. The eggs tend to be opaque when first laid and become more transparent as the embryo develops. The surface of the eggs is covered by tiny wavy wrinkles. The size of the eggs may vary from one slug to the next although the eggs in one clutch (up to 63 eggs per clutch) are fairly uniform (2 to 2.5 mm).

Young

No description found.

BIOLOGY

Distribution

There is some confusion in the literature about the identity of Lehmannia poirieri. In the American literature this slug was at one time called Limax marginatus. There is now an old world slug called Lehmannia marginatus (Muller), the tree slug, which is apparently similar to if not the same species as Lehmannia poirieri. However, the tree is an arboreal slug in Europe whereas Lehmannia poirieri seems to be terrestrial in the United States. It is thought that

Lehmannia poirieri was introduced from Europe and now occurs throughout the United States where it tends to occur around dwellings, greenhouses, and gardens. In Europe, the tree slug inhabits woodlands and damp rocks on open hillsides. The tree slug is found in England, Europe, Australia, Japan, Mexico, and Venezuela.

Host Plants

Lehmannia poirieri feeds readily on living plant material and has been associated with damage to pepper plants, carnation, ivy, vinca, and Cymbidium. Tree slugs feed on hosta, vegetables, wheat, jimson weed, and usually prefer a vegetable diet to casein or blood meal. In the greenhouse it feeds on amaryllis, asters, mums, orchids, andTradescantia.

Damage

Aside from the direct damage Lehmannia poirieri does to bedding plants and vegetables, this slug (or a very closely related species) has been found to be an intermediate host of a parasitic nematode of rats. Slugs have been implicated as intermediate hosts of the fowl tapeworm, Davainea proglottina (for this reason it is suggested slugs not be eaten raw). The tree slug can transmit tobacco mosaic virus.

Life History

Lehmannia slugs require a higher moisture substrate than other limacid slugs. Lehmannia poirieri also tends to burrow into soft soil. During the day this slug tends to rest under boards, logs, flats, and other debris. Lehmannia slugs space themselves so they do not touch in the daytime resting sites in summer, but cluster tightly during the winter. Lehmannia slugs have an elaborate courtship behavior which lasts from a half to 2 hours. Eggs are laid sometime later (perhaps many weeks later). Young slugs hatch from the eggs about 2 weeks later at 68°F. Tree slugs live 24 to 26 months, and lay 105 to 132 eggs. This slug searches the edges of strange environments and can learn the position of food.

CONTROL

Tree slugs are infected with the sporozoan, Pfeifferinela impudica Leger & Hollande, and with the ciliates, Colpoda steini Maupas, Tetrahymena limacis (Warren), and Tetrahymena rostrata (Kahl) that infect this slug during the winter probably from cysts in the soil. The trematode, Strongylus spp. has been found in eggs of tree slugs. The trombidiid mite, Riccardoella limacum (Schrank), can considerably reduce a slug population especially in captivity. The mites crawl rapidly on the surface of the slug and disappear into the respiratory pore when mites are disturbed.

Tree slugs are intermediate in resistance to molluscicides. Tree slugs habituate to the attractiveness of metaldehyde so that broadcasting of baits tends to give better control than clumps. When sprayed, these slugs crawl onto each other to avoid the residue. Metaldehyde only gives 35 percent mortality if the weather is cool and rainy. Moisture does not seem to improve the efficacy of carbamates for tree slug management. One method of controlling Lehmannia poirieri is removal of its hiding places. Picking up flats, boards, pots, and debris will force the slugs to crawl elsewhere for a suitable resting spot. For specific chemical control recommendations, see the current Cooperative Extension publications on ornamental plant pest management or consult your county Extension agent.

Figure 143. Lehmannia slug.

Figure 143. Lehmannia slug.

Spotted Garden Slug

Spotted garden slug (Figure 144), Limax maxumus, Limacidae, STYLOMMATOPHORA

DESCRIPTION

Adult

An adult spotted garden slug is 80 to 150 mm in length. The body color is usually yellowish gray, but may be brown. There are two or three darker rows of spots or strips on each side. The mantle is only on the anterior end of the animal, it is concentrically wrinkled, without a groove, the breathing pore is located in the posterior half of the mantle. The body is keeled only near the posterior end. Both the body and the mantle have large black spots. The mantle and body are often banded, and the foot fringe has small black grooves. The mucus is colorless and sticky.

Eggs

The light yellow eggs are laid in clusters of about 25 in the soil.

Young

Young slugs are about 14 mm long at hatching. They are usually dull white when they hatch. A few hours after hatching the body begins to darken and in about two days it is brownish in color. The body darkening lasts about a month, at which time the black spots begin to appear (the slug is about 25 mm long now). Immature slugs remain together in a colony near where the eggs are deposited for four to five weeks.

BIOLOGY

Distribution

The spotted garden slug was introduced into the United States from Europe. In this country it is recorded from Massachusetts south to Georgia and west to Oregon and California.

Host Plants

Spotted garden slugs feed on lilies, iris, and narcissus in greenhouses and bedding plants outdoors.

Damage

This slug leaves a trail of slime wherever it goes. It eats large ragged holes in the leaves of mature plants and may completely devour small seedlings. They usually are most serious on plants growing close to the soil surface.

Life History

The spotted garden slug lays clusters of about 25 light yellow eggs covered by mucus (674 to 834 total). Eggs are usually deposited under objects on the ground such as stones and boards. Eggs hatch in about 30 days at 24°C (sooner at higher temperatures). Immature slugs remain together in a colony near where the eggs are deposited for four to five weeks.

Activity is greatest during the night and on damp, cloudy days. Spotted garden slugs prefer temperatures of 21 to 27°C. They can survive 30 to 34°C for a short while, but they immediately seek shelter to conserve moisture. These slugs regulate body temperature by evaporation of water from the skin (maintaining up to 12°C difference). They often feed and hide among the leaves of large-leafed plants. Outdoors they are among the first pests to begin feeding in the spring and among the last to stop feeding in the fall. Indoors they will feed as long as environmental conditions are favorable. In damp soil they tend to be solitary, but in dry soil spotted garden slugs bunch together. They may live 30 to 36 months.

CONTROL

Spotted garden slugs can lose up to 50 percent body weight and recover after two hours of exposure to water. Thus if poisoned with metaldehyde, spotted garden slugs may recover if they have access to water.

For chemical control recommendations, see the current Cooperative Extension publications on ornamental plant pest management or consult your county Extension agent.

Figure 144. Spotted garden slug.

Figure 144. Spotted garden slug.

Publication date: Jan. 1, 1994
AG-136

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