Slugs in seedling crops

Applicable to canola, winter cereals and pulses.

Objectives Pre-sowing Germination  –  Vegetative
Insects and damage

Species of slugs

High risk:

  • High rainfall areas (>450 mm/annum)
  • Above average spring–autumn rainfall
  • Cold wet establishment conditions
  • No till stubble retained
  • Summer volunteers
  • Previous paddock history of slugs
  • Soils high in clay and organic matter

Slugs are generally not picked up when using traditional monitoring methods as they are nocturnal and shelter during dry conditions.

Damage:

  • Rasping of leaves
  • Leaves have a shredded appearance
  • Complete areas of crop may be missing

Slugs will eat all plant parts but the seedling stage is most vulnerable and this is when major economic losses can occur.

Grey and brown field slugs are mainly surface active but the black keeled slug burrows and can feed directly on germinating seed.

Monitor and record Monitor with surface refuges to provide an estimate of active density. Refuges can include terracotta paving tiles, carpet squares or similar.

Use 300×300 mm refuges when soil moisture is favourable (>20%) as slugs require moisture to travel across the soil surface. Place at least 10 refuges per 10 ha after rainfall, on damp soil before sowing. Focus on areas of previous damage. Check the refuges early in the morning, as slugs seek shelter in the soil as it gets warmer.

An alternative option is to put out metaldehyde bait strips and check the following morning for dead slugs.

Monitor for plant damage. Slug populations are not even distributed in the field and are often clumped. Where crop damage is evident – inspect the area at night.
Natural enemies Some species of carabid beetles can reduce slug populations but generally not below established economic thresholds.  Free living nematodes that carry associated bacteria that causes slug death are thought to help reduce populations under certain field conditions. Many other soil fauna, such as are protozoa, may cause high levels of slug egg mortality under moist warm conditions however biological controls alone can not be solely relied on for slug control.
Cultural control
  • Hard grazing of stubbles
  • Burning
  • Cultivation leaving a fine consolidated tilth
  • Removal of summer volunteers
  • Rolling at sowing
  • Early sowing for quick establishment of canola
Thresholds The thresholds below (per square metre) should be used as a guide only. Also take into account the field, the season, crop health and weather conditions.

  • Grey field slug:  Canola 0.5-1.5  |  Cereals 5-15  |  Pulses 1-2
  • Black keeled slug:  Canola <1  |  Cereals 1-2  |  Pulses 1-2
Chemical control Baiting slugs is the only chemical option available. Molluscidial baits containing either metaldehyde or chelated iron are IPM compatible. Baits containing methiocarb are also toxic to carabid beetles (one of the few predators of slugs). Different responses to bait can be due to species behaviour. The stability and effectiveness of baits can be impacted by cold temperatures or significant rainfall.

  • For black keeled slugs – broadcast baits when dry or place with seed at sowing.
  • For grey field slugs – broadcast baits

Calibrate bait spreaders to ensure width of spread, evenness of distribution and correct rate (25-30 baits per square metre).

Bait after/at sowing prior to crop emergence when soil is moist (i.e. >20% soil moisture). Re-apply baits to problem areas 3-4 weeks after 1st application if monitoring indicates slugs are still active. The number of baits/ha is more important than the the total weight of bait per hectare. Research indicates that 250,000 bait points per hectare is the minimum required for effective control.

Multi-pest interactions  Different species respond differently to environmental and field conditions leading to staggered emergence and need for repeated application under some conditions.
Communication
  • Discuss with your agronomist optimum times for baiting and observations regarding population activity
  • Consult industry publications for up-to-date information of pest problems

No single method will provide complete control of slugs. Consider cultural and chemical control and work on pest control year-round to achieve a reasonable level of control.

Further information

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