This group of serious pests belongs to the phylum Mollusca, a group including the octopus and whelk and the slug's close relatives, the snails, which cause some damage to plants in greenhouses and private gardens.
Damage. The slug lacks a shell and this permits movement into the soil in search of its food source: seedlings, roots, tubers and bulbs. It feeds by means of a file-like tongue (radula), which cuts through plant tissue held by the soft mouth, and scoops out cavities in affected plants (see Figure 14.3). In moist, warm weather it may cause above-ground damage to leaves of plants such as border plants, establishing turf, lettuce and Brussels sprouts. Slugs move slowly by means of an undulating foot, the slime trails from which may indicate the slug's presence. Horticultural areas commonly support populations of 50 000 slugs per hectare.
Life cycle. Slugs are hermaphrodite (bearing in their bodies both male and female organs), mate in spring and summer, and lay clusters of up to 50 round, white eggs in rotting vegetation, the warmth from which protects this sensitive stage during cold periods. Slugs range in size from the black keeled slug (Milax) 3 cm long, to the garden slug (Arion) which reaches 10 cm in length. The mottled carnivorous slug (Testacella) is occasionally found feeding on earthworms.
Control. Private gardeners use many non-chemical forms of control, ranging from baits of grapefruit skins and stale beer to soot sprinkled around larger plants. A nematode (Phasmarhabditis hermaphrodita) is increasingly being used to limit slug numbers. The most effective methods available to both amateur and professional horticulturists at the moment are three chemicals, aluminium sulphate (an irritant), metaldehyde (which dehydrates the slug) and methiocarb (which acts as a stomach poison). The chemicals are most commonly used as small-coloured pellets (which include food attractants such as bran and sugar), but metaldehyde may also be applied as a drench. Some growers estimate the slug population using small heaps of pellets covered with a tile or flat stone (to prevent bird poisoning) before deciding on general control. Use of metaldehyde and methiocarb in gardens has recently been claimed as a major contribution to the decline of the thrush numbers. A simple device such as that seen in Figure 16.2, using a modified plastic milk carton (containing slug pellets) prevents the entry of mammals and birds.
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