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Slugs and Snails (Gastropoda)
Slugs and Snails are molluscs (Mollusca). They have a chitinous-like friction plate with numerous teeth, the so-called radula helps them to literally graze their food from the surface. Snails are crepuscular or nocturne, during the day they live under barks, pots or between plants. All snails produce slime and are usually very polyphagous. They lay their eggs in substrate. The young snails that emerge from the eggs look very much like their parents.
On Orchids, slugs without shells and snails with more or less big shells occur. Some snails or slugs can play an important role as carrier of viruses.
The most common species of slugs is the field slug (Deroceras reticulatum Müller). This species can be 50-60 mm long in favourable conditions, the body is grey to reddish-brown with dark net-like lines and spots. The eggs with diameters of up to 2.0 mm are usually laid in groups of 4-10 eggs under pots, wood or bark. This species multiplies very quickly, in favourable climatic conditions in greenhouses, adult field slugs are able to lay eggs every 2nd or 3rd day. They mainly eat plant parts above the ground. They prefer young, soft plant tissue and are therefore often found on young plants and blossoms.
One species of snails is the orchid snail (Zonitoides arboreus Say). It is a North American species. The shell is about 2.0-2.5 mm high and 3.5-4.0 mm wide, brown-yellow to red-brown. They live on rotten and living plant material preferring the latter. This species loves bark substrates and finds ideal living conditions in it. The entire development takes about 3 months.
Snails usually occur on roots and root necks, they only cause slight direct damage, but create wounds. These wounds are then used as an entrance for harmful fungi. Snails are usually imported into the greenhouses in plant substrate, mainly in bark or coconut fibres.
Other snails and slugs in greenhouses that sometimes also eat on orchids are Opeas pumilum Pfeiffer, Oxychilus draparnaudi Beck, Discus rotundatus Müller and Lehmannia marginata Müller. Many snails are imported from tropical countries in plant material and especially in substrates accompanying that plant material.
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