Slug

For other meanings see Slug (disambiguation)

gastropods without or with very small shells, in contrast with snails from which they evolved, which have a prominent shell. Although they undergo torsion (twisting) during development, their bodies are streamlined and worm-like, and so show little external evidence of it. This same basic design developed independently in several different groups, the largest being the sea slugs or nudibranchs. Other slugs are found on land, but their soft, slimy bodies are prone to dessication, so they are confined to moist environments. Among the various species are the grey field slug, Deroceras reticulatus, the garden slug, Arion hortensis and the banana slug, Ariolimax columbianus.

Like snails, slugs have two pairs of 'feelers' or tentacles on their head. The upper pair optical tentacles are light sensors the lower provide the sense of smell. Both pairs are retractable and can be regown if lost. On top of the slug behind the head is saddle-shaped the mantle, and under this is the genital opening and anus. The mantle also has a hole, the pneumostome, for respiration. The slug moves by rythmic muscular action of its foot

Most slugs eat leaves, fungus and decaying vegetable material, but some are predators and most also eat carrion including dead of their own kind. Slugs eat using a radula, a rough tongue-like organ with many tiny tooth-like denticles.

Mucus

Slugs produce two types of mucus: one which is thin and watery, the other which is thick and sticky. Both are hygroscopic. The thin mucus is spread out from the centre of the foot to the edges,the thick mucus spreads out from front to back. Mucus is very important to slugs as it helps them move around, and contains fibers which prevents the slug from sliding down vertical surfaces. Mucus also provides protection predation and from drying out. Some species use slime cords to lower themselves on the ground.

Reproduction and life cycle

Slugs are hermaphroditic: having both female and male reproductive organs. Once a slug has located a mate the encircle each other and sperm is exchanged through their protruding genitalia. A few days latter hundreds of eggs are laid in holes in the ground. Although some species hibernate over the winter in temporate climates, in most species the adults die in the autumn.

Predation and defense

Frogs, toads and hedgehogs, also some birds and beetles are natural slug predators. Slugs, when attacked, can contract their body making themselves harder and more compact and thus more difficult for many animals to get a hold. The unpleasant taste of the mucus is also a deterant.

Slugs as pests

Some are notable garden pest and there are various methods for controlling them. Commercial slug pellets containing metaldehyde or methiocarb are available, but are not approved for organic gardening as they can poison creatures further up the food chain. Aluminium sulphate can also be used and its proponents say that it is less toxic to the environment. Salt will cause a slug to dry out, but this is probably cruel to the slug and also washes away easily.

Slugs have an aversion to copper. Copper pipe can be used and commercial slug rings are available. A six inch (15 cm) length of plastic pipe with copper foil tape stuck round it can be used to protect individual plants. Slugs also find rough surfaces such as crushed eggshells irritating.

Beer traps can be used to drown slugs. Put a jar containing half an inch of beer (12 mm) into a hole in the ground. The slugs will be attracted to the beer and will drown.

Comfrey can be used as a decoy. Put comfrey leaves round plants that need protecting. The slugs will eat the comfrey and leave the protected plant alone. (This is the honeypot approach.)

Encouraging natural predators will help to keep the slug population under control. Nematodes are parasites that can be used to kill slugs.

Finally, night time patrols can be useful.

For best results, use several methods together.

See also

• Gastropod
• biological pest control
• pesticide