Slugs

Identification

The grey field slug is a common and widespread pest which is instantly recognisable by farmers and advisers.

Symptoms

The classic symptoms of slug damage are seed hollowing and leaf shredding or complete leaf loss. Given suitable weather and soil conditions slugs can be devastating to establishing crops of cereals and many other crops. Slugs prefer heavier soils, and where the production of a fine, firm seed bed is not possible, the risk of slug attack is much increased. If crops can emerge quickly and grow away, they can escape the worst period of slug damage.

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 a season. After a few weeks, or longer in winter, the eggs hatch and grow steadily to maturity. It has been estimated that slugs can move at speeds of up to 0.007 mph!

Life-span

The life span of the grey field slug has been estimated to be up to 18 months (in ideal conditions).

The round-backed slugs (Arion spp) are generally not as long-lived (11-13 months) as the keeled slugs (Milax, Tandonia) which can live for as long as 30-36 months. In captivity some large (non-pest) species of slug have been known to survive for 5 years.

However in a field situation life span is considerably shorter due to predation, extremes of temperatures or the effects of cultivation.

Those species which live for many months survive through the summer or winter by burrowing down into the soil or other shelter.

Importance

Slugs can cause significant damage to all winter cereals especially if crops are late sown and emerge in cold wet conditions. Where pest pressure is severe whole crops can be lost. Cereals following slug favourable broad-leaved crops, such as oilseed rape, can be at greater risk.

Modern seed treatment insecticides, such as Deter, can help to protect cereals from seed hollowing caused by slugs, but these do not give any protection to the foliage above ground.

Threshold

For winter wheat:

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, 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.