Course: Slug Control | Last Updates: 30th August 2017
Three main types of slug attack arable crops:
- grey field (Deroceras reticulatum)
- round-backed (Arion species)
- keeled (Milax and Tandonia species).
The grey field slug (pictured) is most common and widespread, and is more active above ground than the others. It can breed at any time of year when conditions are suitable, produces many juveniles and has a short lifecycle, making it a particularly dangerous pest.
Eggs laid in spring produce adults that lay eggs in summer/early autumn. In a wet summer, grey field slugs breed quickly, each laying up to 300 eggs that can hatch in less than two weeks, producing juvenile slugs that grow rapidly.
Grey field slug
Wheat is most vulnerable at the seed stage. Provided slugs can move through air spaces in cloddy seed-beds, they will start to kill wheat seeds almost immediately after sowing. They take the embryo and often the endosperm, causing characteristic seed hollowing. Each slug can kill up to 50 wheat seeds in the first week after sowing. Slugs can also attack the shoots and roots of germinating seedlings.
After emergence, slugs graze and shred young cereal leaves. Provided seeds are in a fine, consolidated seed-bed, slugs cannot damage the growing points, which remain below ground and, after treatment with slug pellets, cereal plants will recover from slug damage to the leaves. But in cloddy seed-beds, slugs are likely to continue to kill seedlings even after emergence. Cereals are most vulnerable up to the start of tillering.
Oilseed rape seeds are not attacked, but young seedlings are very vulnerable, because they are highly palatable to slugs and also because the growing points are above ground and readily destroyed by slugs after germination, even in fine seed-beds. Seedlings are no longer at risk after the four-true-leaf stage.
Slugs can cause complete failure of establishment, necessitating re-drilling, with all the extra costs and associated loss of yield. The severity of slug damage and its impact on yield is difficult to predict because of the influence of the weather on slugs and on crop growth.
Where do they come from?
The main slug species spend their lives out in the cultivated area of arable fields and move only short distances, thriving in moist soil in mild weather (17C is optimum for development). Slugs are less able to move through seed-beds and damage wheat seeds near the edges of fields due to greater compaction, caused by turning machinery. However, large slugs living in field margins in early autumn may move into the field edges to damage oilseed rape at establishment.
Slugs survive well on heavy clay and silty soils and generally cause most damage there, because seed-beds tend to be cloddy, allowing feeding on vulnerable seeds. Populations build to high levels in preceding dense, leafy crops, especially oilseed rape. Incorporating crop residues increases slug risk.
Trapping during periods of wet weather just before cultivation and drilling can indicate slug activity and the risk of potential damage. But slug populations can be predicted for only one or two weeks at most, as slugs respond rapidly to weather changes.
Traps should be put out before cultivation, when the soil surface is visibly moist during mild weather (5-25C).
Traps consist of a cover about 25cm across, with a small heap (20ml or two heaped teaspoonfuls) of chicken layers’ mash (not slug pellets) beneath. In each field, nine traps (13 in fields larger than 20ha) should be set out in a “W” pattern, concentrating on areas known to suffer damage. Traps are left overnight and examined early next morning.
For winter wheat, a catch of four or more slugs per trap in the stubble of the previous crop indicates a potential risk, if the field is drilled during a period of generally wet weather, or if wet weather delays sowing in a prepared seed-bed or if the seed-bed is coarse and cloddy.
Trapping soon after drilling is not recommended, because slug activity is disrupted by cultivations and the trap catch is often misleadingly low. But a second risk assessment by trapping several days after drilling and before emergence is worthwhile in wet weather to update the risk. After crop emergence, risk should be assessed from seedling damage and crop growth rate.
For winter oilseed rape, a catch of one or more slugs per trap in the stubble of the previous crop indicates a potential risk. The weather may not be suitable for slug-trapping during the short period between harvesting cereals and drilling oilseed rape. It can be valuable to trap in standing cereals if it rains shortly before harvest; a catch of four or more slugs per trap represents a risk to following oilseed rape.
Cultivation soon after harvest can reduce slug numbers, especially in dry conditions. The most valuable cultural measure is the preparation of fine and consolidated seed-beds, which prevent slugs from feeding on seeds and seedlings.
Drilling cereal seeds at 3cm depth in a fine, consolidated seed-bed protects seeds. If the seed-bed is cloddy, sowing depth should be increased to 4-5cm, where the seeds will be better protected. Seed-beds should be rolled as soon as possible after drilling if soil conditions permit.
In cloddy seed-beds, slugs will start to kill wheat seeds almost immediately after sowing. Each slug can kill up to 50 seeds in the first week after sowing. Shallow-sown wheat seeds in cloddy seed-beds are readily killed by slugs, but slugs cannot reach seeds in fine, firm seed-beds. Deeper drilling of seeds in a cloddy seed-bed can prevent slugs from killing seeds, but seedlings will be exposed to slug attack as they grow to the surface, and should be protected by slug pellets broadcast on the soil surface soon after sowing.
Slug pellets are pesticides and need to be applied responsibly to reduce the risk of contaminating watercourses. The most commonly applied pellets are metaldehyde-based and levels of this pesticide have been found in water above the EU drinking standard of 0.1 ppb although not harmful to human health.
Guidelines on responsible metaldehyde use have been compiled by the industry and are available at www.pelletsarepesticides.co.uk
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