The grey field slug, Deroceras sp., is the most widespread and damaging slug pest in arable crops. Feeding can seriously damage plants, particularly young seedlings in which slug feeding interferes with crop establishment. Activity, survival and reproduction are dependent on moisture. Slug damage is much greater after previously cropping leafy crops such as oilseed rape. Slugs are more abundant in heavy soils with high clay or silt content. Crop residues or applications of manure, especially in the autumn, as well as weeds and volunteers, provide slugs with a source of food and shelter. Consequently, direct drilling, as well as delayed drilling, increases the risk of slug damage.
Monitor oilseed rape and cereal seedlings for slug damage. Slug traps can assist with this; they consist of a cover about 25 cm across, with a small heap (2 heaped spoonfuls) of bait (chicken layers’ mash poultry feed). A catch of 4 or more slugs per trap per night indicates a risk if soil and weather conditions favour slug activity.
Ploughing is a good way of reducing slug populations but even minimum tillage gives considerable reduction in slug damage compared to direct drilling. Shallow cultivation to incorporate crop residues reduces slug numbers. A fine, consolidated seedbed is important and will protect seeds and prevent slugs accessing seedlings before emergence. Rolling can assist in providing a fine tilth, free of clods that attract slugs. Drill at 3cm depth to deny slugs access to the seed and increase sowing depth to 4–5 cm if the seedbed is cloddy. Sow deeper and roll after sowing. Weeds should also be removed where they are sustaining slug and snail populations prior to drilling.
Parasitic nematode (‘Nemaslug’, Phasmarhabditis hermaphrodita) can be applied for biological control. Nematodes are best applied in dull weather, in the evening and before rain. They can be applied in advance of expected damage, at sowing or any time during the crop’s lifetime, however, success is dependent on wet conditions after application.
Current chemical control options currently comprise metaldehyde and ferric phosphate after methiocarb was banned in 2014 but metaldehyde will not be available for much longer. Broadcasting pellets is the best method of application and kills slugs more quickly than pellets that are drilled with seeds. Metaldehyde is a selective molluscicide and does not harm predatory ground beetles, which can help to restrict slug populations. Metaldehyde is still available but it was announced by Defra in December 2018 that a ban on the outdoor use of metaldehyde is to be introduced across Great Britain. The key deadlines for the withdrawal of metaldehyde are 31st December 2020 for sale and distribution of slug pellets and 31st December 2021 for the disposal, storage and use up of existing stock. This should hopefully give growers time to adjust to other methods of slug control. Methaldehyde has a high affinity with water. While metaldehyde is still available, care should be taken to avoid situations where it is more likely to enter drinking water - use lower doses, don't apply when heavy rain is forecast and keep away from watercourses. Soil management practices can also reduce the risk of run off. These include establishing the crop and tramlines across the slope, parallel to watercourses to minimise surface water travelling directly into the watercourse. The metaldehyde stewardship group has introduced a voluntary pilot scheme where zero metaldehyde is advocated on identified, ‘high risk’ fields; these are on heavy poorly drained soil, on a slope near watercourses. Refer to the metaldehyde stewardship guidelines at www.getpelletwise.com The antifeedant Ferric phosphate can be used as an alternative and has similar efficacy. Ferric phosphate will be the only chemical control option available after the metaldehyde ban is implemented in winter 2021.