How to monitor slug populations in field crops

To target slug treatments, both across and within fields, it is important to monitor pest activity, especially during a crop’s most critical growth stages.

How to manage slugs in field crops (home page)

The use of refuge traps to detect slugs

Refuge traps are an effective way to gauge the size/activity of the local slug populations.

Traps are simply a small heap of covered non-toxic bait, such as chicken layers' mash or cereal grain-based food (not slug pellets). 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 has little value.

How to prepare a refuge trap

  1. Place two heaped spoonfuls of bait on the ground.
  2. Add a cover that is about 25 cm across (e.g. plant pot saucer, sacking, carpet or tile).
  3. Leave a small gap between cover and the soil (to allow slugs to enter).
  4. Put a weight on the cover (to secure it in windy conditions).

In each field, set out 9 traps (13 in fields larger than 20 ha) in a ‘W’ pattern, spread over the entire field. Place extra traps in areas known to suffer damage. In standing crops, place the traps just to the side of tramlines and mark locations with canes.

Leave traps overnight and examine early the following morning, while the soil surface is still moist. Count the number of slugs and note slime trails. On warm days, check traps early while the temperature is still cool, because slugs will leave as it gets warmer. Continue to trap, until crops have passed their vulnerable stage.

Slug monitoring periods and damage thresholds

Monitoring periods and damage thresholds are crop-specific. The damage-risk thresholds only apply where conditions are suitable for slug activity.

Oilseed rape

Monitor oilseed rape from sowing to the four true-leaf stage. The damage-risk threshold is one slug per trap in cereal stubble.

Broadcasting pellets, as soon as possible after drilling, is the best method of application.

Winter cereals

Monitor winter cereals from sowing to first tillering (growth stage 21). The damage-risk threshold is four slugs per trap.

Broadcasting pellets, as soon as possible after drilling, is the best method of application.

Application is most effective when slugs are active up until growth stage 14.


Admixed slug pellets (pellets mixed with seed at drilling) are ineffective in fine seedbeds, because both seeds and pellets are unavailable to slugs. However, when direct drilling or in open cloddy seedbed, pellet admixtures with wheat seeds can be effective.

It can be beneficial to repeat treatment when new feeding damage is observed, when traps indicate an increase in activity or if pellets disintegrate or go mouldy. It is important to note that pellets can be rendered ineffective after prolonged heavy rain.


There are two critical control periods:

  1. 50–75% canopy closure, usually in late June to early July, when the canopy is sufficiently open to allow pellet penetration.
  2. Early stages of tuber bulking, before slugs go underground to find developing tubers. August is the pivotal month for follow-up applications and when damage usually begins to appear.

Continue to monitor the crop until burn-down. The damage-risk threshold is one slug per trap.

Horticultural crops (vegetables)

No damage-risk thresholds established.

The sensitive phase for lettuce and celery lasts for the whole of the growing period. Pellet application before a crop is planted may be justified.

Brussels sprouts are most sensitive at the seedling stage and when harvestable buttons start to develop.

A slug trap: Covered non-toxic bait