The grey field slug (Deroceras reticulatum) and other Deroceras spp.
The grey field slug is the most widespread and troublesome species. It is usually light grey or brown, grows to 5 cm in length and produces milky white mucus. Populations tend to have a mixed age structure, so damage occurs whenever conditions are favourable for activity. It continues to be active in damp weather and even when temperatures are close to freezing. Breeding is generally at a peak in April and May and then again from September to October. However, in favourable conditions, it will breed throughout the year. In optimum conditions it can start to lay eggs within 16 weeks of hatching.
The garden slug (Arion hortensis and Arion distinctus)
The garden slug is usually smaller than the grey field slug, growing to 3 cm in length. The body is dark and the foot (underside) ranges from yellow to orange. It produces orange or yellow mucus. Egg hatching reaches its peak in late spring/early summer. Young slugs can develop rapidly to produce a further generation within the year. Arion species are only active at temperatures above 5°C and are less active on the soil surface than the grey field slug.
The keeled slug (Milax,Tandonia and Boettgerilla spp.)
Keeled slugs are more localised in arable crops than field or garden slugs but they can be important. They vary in size and generally produce a colourless mucus. Keeled slugs have annual life cycles, with eggs hatching from autumn to spring. All keeled species are generally subterranean but can be seen on the surface, especially during the breeding season.
The Spanish slug (Arion vulgaris)
Spanish slugs can be brown, black, fawn or mustard coloured and can grow up to 15 cm long. Unlike other slug species, the Spanish slug is omnivorous, eating dead animals, excrement and plant material. They produce twice as many eggs as native slug species.
Life Cycle
All slug species are hermaphrodite (each individual is both male and female). While some species are self-fertile, most mate before laying eggs in batches of 10 to 50 in soil cavities, between clods, under stones or at the base of plants. Up to 500 eggs per slug may be laid over several weeks. Eggs develop slowly in the winter but will hatch within a few weeks when the temperature starts to rise. The number of active slugs found at any one time and place is dictated by both the slug population density and the suitability of the weather for activity (activity-density). Rapid reproduction and growth is enhanced by mild, moist weather conditions, sufficient food supply and ample shelter. Such conditions prevail in the spring and early autumn, making crops like lettuce and Brassicas more vulnerable at these times of year. Slug movement occurs most frequently at night but they will return to their resting site by dawn if weather conditions are unfavourable. They do not travel far from where they were hatched, often taking only a circular route of a few metres in search of food.
Monitoring for Slugs
To assess the risk of crop damage, it is important to estimate the size of slug populations present. Sampling in the field is best done using refuge traps. Put slug traps out before cultivation, when the soil surface is visibly moist and the weather is mild (5–25°C). When soil conditions are dry and slugs are not actively seeking food, trapping will have little value in determining the threat to the crop. Traps consist of a cover about 25 cm across, such as a plant pot saucer, with a small heap of bait underneath. A suitable bait would be two heaped spoonfuls of chicken layers’ mash or a cereal grain-based food (NOT slug pellets).
Leave a small gap between the trap and the soil to allow slugs to enter. It may be necessary to put a weight on the trap in windy conditions. In each field, nine traps (13 in fields larger than 20 ha) should be set out in a ‘W’ pattern spread over the entire area of the field. Also, concentrate on areas known to suffer damage. In standing crops, place the traps just to the side of tramlines and mark with canes to allow them to be located. Leave traps overnight and examine early the following morning while the soil surface is still moist, counting the number of slugs present and noting any slime trails. On warm days, it is important to check the traps early while the temperature is still cool, as slugs will leave the trap as it gets warmer. If no slugs are found, continue to trap until crops have passed their vulnerable stage.
The following (Table 1) thresholds indicate a possible risk when soil and weather conditions favour slug activity.
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