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Grey field slug

1. Arion subfuscus ©Gareth Bubb.
2. Grey field on a wheat leaf.
3. Grain hollowing.

Identification

The grey field slug is a common and widespread pest which is instantly recognisable by farmers and advisers. It is the most common and significant slug pest in oilseed rape throughout the UK. Other slug species which can be found in oilseed rape are the garden slug (Arion hortensis) and the dusky slug (Arion subfuscus).

Symptoms

Given suitable weather and soil conditions slugs can be devastating to establishing crops of oilseed rape. The low-glucosinolate levels in current oilseed rape varieties have resulted in the incerased palatability of seedlings to slugs. If crops can emerge quickly, they can escape the worst period of slug damage, which is from the cotyledon to the 1-2 true leaf stage. Beyond this they can grow away from further damage as the slugs concentrate on the older leaves. It is the most common and significant slug pest in oilseed rape throughout the UK. Other slug species which can be found in oilseed rape are the garden slug (Arion hortensis) and the dusky slug (Arion subfuscus).

Life-cycle

All slug pests are hermaphrodite and after mating lay eggs in batches of 10-50 in soil cavities and under stones or other debris. Up to 500 eggs may be laid in season. After a few weeks, or longer in winter, the eggs hatch and grow steadily to maturity.

Life-span

The life span of the grey field slug has been estimated to be up to 18 months (in ideal conditions). However in a field situation life span is considerably shorter due to predation, extremes of temperatures or the effects of cultivation.

Importance

Slugs can cause significant damage to both winter and spring-sown oilseed rape by eating the cotyledons and first true leaves. If plants are killed, the resulting gaps in the crop are attractive to pigeons, and subsequent damage to the crop may occur in areas where pigeons are a problem. Slug damaged winter oilseed rape crops are more susceptible to damage from insect pests such as pollen beetle, as the vigour of the crop is reduced, and its ability to cope with further pest damage is diminished. In extreme instances whole crops can be lost.

Threshold

Insecticide seed treatments give little to no protection against slugs in oilseed rape. FOR WINTER OILSEED RAPE A catch of 4 or more slugs per trap in standing cereals, or 1 or more in cereal stubble, if other conditions were met, would also indicate possible risk of damage. Monitoring recommendations For further information on slug trapping and damage risk assessment please refer to HGCA Topic Sheets No.84 (winter wheat) and 85 (winter oilseed rape), available from the HGCA website [www.hgca.com]. To establish the need for pellet application on winter wheat or winter oilseed rape, monitor for slug activity using baited traps. Where bait traps are used, use a foodstuff attractive to slugs e.g. chicken layer’s mash which has proven to be particularly effective. DO NOT use slug pellets as bait in traps in these crops since they are a potential hazard to wildlife and pets. Put slug traps out before cultivation, when the soil is visibly moist and the weather is mild (5-25 C). Traps consist of a cover about 25 cm across, with a small heap (20 ml or 2 heaped teaspoonfuls) of chicken layer’s mash (NOT slug pellets) beneath. In each field, nine traps (13 in fields larger than 20 ha) should be set out in a ‘W’ pattern. Also concentrate on areas known to suffer damage. Leave traps overnight.

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