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.

Fig. 141: Deroceras slugs. The brown slug (also marsh slug or smooth slug), Deroceras laeve (Muller); gray garden slug, Deroceras reticulatum (O. F. Muller) (also netted slug or gray field slug), Limacidae, STYLOMMATOPHORA

Deroceras slugs. A, Brown slug. B, Gray garden slug.

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 at1 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 loose 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.

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 per cent 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.