Slugs

Slugs are not insects but mollusks, a group that includes snails, clams, scallops, oysters, squid, and octopus. Slugs are omnivorous but prefer to eat vegetation, and they thrive in moist places like the humid greenhouse environment. Several species of both native and imported slugs may cause damage in greenhouses on most vegetable and ornamental crops, especially orchids.

Damage

Slugs are nocturnal feeders that eat seedlings or chew holes in succulent leaves, stems, or roots. During the day they hide in dark, damp places beneath benches, pots, or litter on the ground. Like caterpillars, slugs damage plants by rasping away plant tissue when young and by eating irregular holes when older. The slime trails they leave on plants reduce the salability of ornamentals.

Description and life cycle

Slugs range in length from V2 to 4 inches (1.3 to 10 cm) and are usually gray or brown.They leave behind a characteristic mucous trail as they move.The hermaphroditic adults (individuals have both male and female organs) lay clusters of 20-100 gelatinous eggs in moist soil crevices protected by debris or covered by containers. Eggs hatch in

Available natural enemies and their potential for control

Few effective natural enemies are available for slugs, symphylans, spring-tails, and sowbugs. Cultural or other controls can alleviate problems with these minor greenhouse pests.

Slugs

Phylum Mollusca, Class Gastropoda

Order Stylomatophora: Snails and slugs

Family Limacidae: Slugs Symphylans

Class Symphyla: Symphylans

Family Scutigerellidae: Symphylans

Garden symphylan, Scutigerella immaculata

Springtails

Order Collembola: Springtails Various families Sowbugs

Class Crustacea: Crustaceans Order Isopoda: Pillbugs and sowbugs Family Asellidae: Sowbugs

10 days or less, and the slugs mature in 3 months to a year, depending on the species. Eggs are resistant to desiccation and will persist in the soil for a long time.

Monitoring

Silvery slime trails indicate the presence of slugs. Relative population levels can be determined by counting the number of slugs that are attracted each night to shallow pans of beer placed at ground level or boards placed on moist soil. Growers should determine their own threshold levels for initiating control measures.

Natural enemies

Although there are many parasites, predators, and pathogens of slugs and snails, few have been investigated for commercial control of these pests.

Predators

Euthycera spp. The larvae of several species of flies in the family Sciomyzidae are known to be slug-killers, but none has been investigated for use in green-houses.The European Euthycera cribata lies in wait, poised for attack.When it encounters a slug it rapidly attaches itself with its mouth hooks, climbs onto the prey and enters into the tissues. In a few minutes it disappears almost completely, with only its breathing tube remaining visible. First-instar larvae feed until the slug is dead, then move on to another prey; later instars continue feeding on the slug after it dies.This species has only one generation per year.Two other European species of Euthycera also attack slugs. No Euthycera species are commercially available.

Rumina decollata. This predatory snail attacks, kills and consumes slugs, common brown snails, and garden snails. It is a burrowing species, normally occurring in the upper inch of the soil. It has a brown, elongated spiral shell tapering to a blunt end.These snails prefer to feed on slugs and organic matter but will eat living vegetation if these foods are not available. It may take a few years for the snails to provide control outdoors.Their use in greenhouses has not been investigated but decollate snails are commercially available.They are legally sold only to residents of certain southern California counties but might be obtained with a letter from your county agricultural commissioner or similar local official stating that possession of the decollate in your county or state is not illegal and that interstate shipments of decollates are permissible.

Staphylinus (=Ocypus) olens.This

European rove beetle, often called the devil's coach horse in England, is a promising predator of slugs and of the brown garden snail in California. Both the 11/4-inch (3.2-cm) black adults and the black bristly larvae, which grow to 1 inch (2-2.5 cm) at maturity consume at least their weight in slugs or snail bodies every day.They pupate in earthen cells in the soil. Little research has been conducted on this insect as a biological control agent of slugs and it is not commercially available.

Tetanocera spp. The larvae of the widely distributed species of the sciomyzid fly, T.elata, uses fresh slime trails to track slugs and immobilize their prey with a toxic injection.The larvae are host-specific as first-instar larvae, but this selectivity later disappears, and third instars attack many different genera of slugs and even snails. Each larva can kill four to nine slugs during its develop-ment.There are two to three generations per year.Three other North American species of Tetanocera have the same feeding method.This fly may be commercially available.

Pathogens

Nematodes. The nematodes Steinernema carpocapsae, S. feltiae, and Heterorhabditis bacteriophora will infect, kill and develop in slugs under laboratory conditions. However, other species of nematodes may be more effective for use against slugs.The predatory nematode Phasmarhabditis sp.that attacks snails is commercially available in Europe. Research is underway to find slug-parasitic nematodes that could be commercialized in the United States.

Alternative control methods

Sanitation

Keep areas beneath benches dry and clean. Remove plastic, boards, and debris that may serve as hiding places for slugs. Eliminate excess moisture. If empty pots or flats must be stored under benches, stack them on a clean, dry, wooden pallet on their sides, and keep them dry. Gravel under the benches allows for good drainage and is not a suitable habitat for slugs.

Inspect all new plant material carefully before bringing it into the greenhouse. Quarantine any possibly infested shipments to prevent contamination of the entire range.

Barriers

Slugs receive a mild shock when they come in contact with copper, so they usually do not cross copper bands. Tack or staple copper strips or bands to the frames of raised beds or greenhouse benches, or wrap the copper around containers to prevent slugs from moving into the beds. Slugs are less likely to cross gravel than they are moist organic soil,organic mulch,or soil overgrown with vegetation.To capitalize on this, spread gravel around the perimeter of the greenhouse to discourage entry.

Repellents

A repellent called Snailproof, composed of sawdust and shavings of the incense cedar, repels snails and slugs.They do not like to travel over this material but will if it becomes packed down.

Hand picking

Removing all visible slugs by hand will have a noticeable effect on the population,although this method would not be practical on a large scale. Since slugs feed at night, that is the best time to collect them.

Traps

Place shallow pans of beer or rubbing alcohol at ground level in the evening to attract slugs.They fall into the liquid and die.You can also place boards or bricks in damp spots on the soil.The slugs that collect under these can be smashed or killed with hot water. Commercial traps are also available.

Poison baits

A bait that contains an extract of quackgrass, identified previously as a slug-specific molluscicide, was very effective in field tests against the slug Arion subfuscus, an important pest species. However, at present no commercial product contains this material.

Metaldehyde or methiocarb pelleted bran baits are commonly used to control slugs, but these baits have several drawbacks.They mold rapidly and must be replaced frequently. Also, some slug populations have developed resistance to metaldehyde. Predatory snails are susceptible to poison baits, so you must quit using baits at least 2 months before releasing predatory snails.\

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