Snails in seedling crops

Applicable to canola, winter cereals, winter pulses, and summer crops.

Objectives Pre-sowing Seedling  – Vegetative Grain fill / Podding Harvest
Insects and damage


Species of snails

High risk:

  • Weedy fields
  • Alkaline calcareous soils
  • Retained stubble
  • Wet spring, summer, autumn
  • History of snails

All species of snails congregate at the base of summer weeds or in topsoil. Pointed snail species can also be found at the base or up in stubble as well as inside stubble stems.

Snails appear to build up most rapidly in canola, field peas and beans but can feed and multiply in all crops and pastures.

Snails are most active after rain and when conditions are cool and moist. Snails are dormant in late spring and summer.

Damage:

  • Snails consume cotyledons (may resemble crop failure)
  • Shredded leaves where populations are high
  • Chewed leaf margins
  • Irregular holes

A wide range of snail sizes are an indication of snails breeding in the area.
If most snails are the same size, snails are moving in from other areas.

Round snails favour resting places off the ground on stubble, vegetation and fence posts. Pointed snails are found on the ground in shady places.

Snails can be found up in the crop prior to harvest. Check for snails under weeds or shake mature crops unto tarps Snails are predominantly a grain contaminant
At harvest, snails move up in the crop and may shelter between grains or under leaves. They can also be found in windrows.
The small pointed snail is especially hard to screen from canola grain due to similar size.

Buyers will reject grain if more than half a dead or one live snail is found in 0.5 litre of wheat or a 200 gram pulse sample.

Monitor and record Monitor snails regularly to establish their numbers, types and activity and success of controls. Look for snails early morning or in the evening when conditions are cooler and snails are more active. Key times to monitor:

  • 3-4 weeks before harvest to assess need for harvester modifications and cleaning
  • After summer rains – check if snails are moving from resting sites
  • Summer to pre-seeding – check numbers in stubble before and after rolling/slashing/cabling

Monitoring technique:

  • Sample 30 x 30 cm quadrat at 50 locations across the paddock.
  • If two snail groups are present (round and conical), record the number of each group separately.
  • To determine the age class of round snails, place into a 7 mm (round or hexagonal) sieve box, shake gently and they will separate into two sizes: adults will stay in the sieve and juveniles will fall through. Sieve boxes can be constructed from two stackable containers e.g. sandwich boxes. Remove the bottom from one and replace by a punch hole screen.
  • Five sampling transects should be taken in each paddock: 90 degrees to each fence line whilst the fifth runs across the centre of the paddock. Take five samples (counts), 10 metres apart along each transect.
  • Record the size and number of the snails in each sample. Average the counts for each transect and multiple this figure by 10 to calculate the number of snails per square metre in that area of the paddock.
Natural enemies Free living nematodes when carrying associated bacteria that causes snail death are thought to help reduce populations under certain field conditions.
Cultural control Pre season:

  • Hard grazing of stubbles
  • Cabling and/or rolling of stubbles – when soil temperature is above 35 degrees
  • Burning – if numbers are very high and ensure hot, even burns
  • Cultivation leaving a fine consolidated tilth
  • Removal of summer weeds and volunteers

Reduce contamination at harvest:

  • Stripper fronts in medium to heavy crops
  • Raising cutting heights
  • Harvester modifications (see Snail fact sheet)
  • Seed cleaning (see Snail fact sheet)

Trials with windrowed barley resulted in reduced round snail contamination. Early windrowing when cool produces better results.

Thresholds To control snails, apply a combination of treatments throughout the year. Thresholds can be unreliable due to the interaction between weather, crop growth and snail activity. For example; high snail populations in the spring do not always relate to the number of snails harvested. Their movement into the crop canopy is dictated by weather conditions prior to harvest.

  • Suggested thresholds for round snails:  Cereals – 20/m2 |  Pulses and oilseeds – 5/m2
  • Suggested thresholds for small pointed snails:  Cereals – 40/m2 |  Oilseeds – 20/m2

Baiting before egg lay is vital.

Thresholds to warrant harvester modifications are difficult to define. Contamination depends on snail types and size in relation to grain as well the position of snails in relation to cutting height.
Chemical control Molluscidial baits containing either metaldehyde or chelated iron are IPM compatible. Apply to the bare soil surface when snails are active after autumn rain as early as March. Aim to control snails pre-season. Mature snails larger than 7 mm in length or diameter will feed on bait but this can be less effective on juveniles

  • Baiting before egg lay is vital
  • Bait when snails are moving from resting sites after summer rains
  • Stop baiting 8 weeks before harvest to avoid bait contamination in grain

Bait rates need to be at the highest label rate to achieve a greater number of bait points. The actual number is yet to be determined; hence label rates may be revised ion the future.
Note that in cool, moist conditions, snails can move 30 m/week and treated fields can be re-invaded from fence lines, vegetation and roadsides.

Rain at harvest can cause snails to crawl down from crops
Multi-pest interactions Baits containing Methiocarb are toxic to a range of other invertebrates and beneficials
Communication
  • Know paddock history and snail presence before sowing.  Discuss presence of snails in district with agronomist or neighbours
  • Consider harvester modifications if snails present at harvest
  • Consult industry publications for up-to-date information on pest problems.

Further information

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