Managing snails – latest research findings and recommendations

Take home messages

• Effective snail baiting requires applying baits at the right time and at sufficient pellet density to ensure good encounter rates.
• Therefore, bait in autumn as soon as snails become active and before they lay eggs, check spreader calibration, apply an adequate rate and select an appropriate product for the field conditions.

Background

Four introduced snail species of European-Mediterranean origin have established in southern Australia and have become major pests of grain crops. The market access threat from these snails is substantial and increasing, particularly for the acceptance of Australian wheat and barley shipments by valuable east Asian markets (e.g. China, South Korea).

The shift to minimal soil cultivation, retained stubbles and limited grazing has advantaged snail survival and reproduction in this system, and many of the harvester modifications and summer cultural controls developed and extended in the early 2000's (e.g. “Bash ‘Em, Burn ‘Em, Bait ‘Em”) have become increasingly incompatible with current farming practice.

Recent SARDI research in GRDC projects DAS00134 and YPA00002 focused on improving baiting performance by investigating the factors influencing the performance of commercial molluscicidal baits against different densities of four snail species and under different environmental conditions. This work has generated refined baiting guidelines for snail and slugs which will be made available in updated publications soon. The research findings are summarised in this paper.

Method

Field and lab-based trials were used to test a range of metaldehyde and iron-chelated snail bait products against four snail species under different environmental conditions. A bait spreader trial involved the testing of four 3-point linkage units, two ute-spreaders and four bait products of different sizes and shapes.

Results and discussion

Snail reproductive activity

In autumn, increasing moisture stimulates increasing snail activity. SARDI research in South Australia over the past three seasons has shown that the reproductive organs of snails begin to mature from late March onwards, and then the most reproductive activity occurs from late April to July.

Controlling snails with molluscicides

Snails readily become inactive when they experience sub-optimal micro-climatic (i.e. dry, low relative humidity) conditions. This ‘on-off’ behaviour makes them a difficult target for chemical control. The international literature indicates that molluscicide baits are generally more effective than sprays, and this appears to be because baits have greater persistence and hence greater likelihood of being encountered by the snails when they do become active.

Optimising bait operations

Bait encounter is random. Therefore, the effectiveness of a snail baiting program is governed by a number of factors that firstly dictate chance of encounter, then ingestion of the toxicant:

• Chance of encounter:
  • level of snail activity
  • attractiveness of bait
  • baits per unit area.
• Ingestion of lethal dose
  • palatability of bait
  • quantity of bait
  • adequate active ingredient.

Hence we recommend that baiting programs take place when snails are active and that enough bait ‘points’ are provided to ensure good rates of encounter. The number of baits on the ground is of equal, if not greater importance, than the weight of product on the ground.

Spreader calibration important to achieve even bait distribution and good results

Don’t assume your spreader is distributing bait pellets evenly. Research by the Yorke Peninsula Alkaline Soils Group and SARDI has shown spreaders calibrated for other applications (e.g. urea) may not broadcast baits as widely as expected, and ute spreaders may provide uneven distribution of bait. Also, different bait products have different hardness and ballistic properties.

Therefore:

• For your preferred bait product get your spreader professionally calibrated to evenly broadcast the target pellet density over the entire spread width
• Operators need to get off their machine and check the distribution across the width they are driving
• The single spinner ute spreaders generally perform poorly with limited spread widths and lop-sided bait distribution.

Apply baits at the right time

Baiting must occur in autumn as soon as snails become active but before they lay eggs.

There are several reasons why autumn is the best time to bait:

1. Preventing adult snails from laying eggs is critical to reducing population build-up. Juvenile snails are generally more difficult to control using baits due to reduced movement and bait encounter
2. Lab trials at SARDI have found higher efficacy when baits were applied under warmer temperatures within the range tested (10^oC-22^oC)
3. Baiting is most efficient when there is less ground cover and alternative food. The presence of stubble, weeds and crop plants at later times reduces bait encounter rates.

In autumn, even light showers or overnight dews are sufficient to stimulate movement. Ideal conditions for baiting are periods when the soil is likely to remain moist for several days. If unsure whether snails are active, bait a small area and check for dead snails after a few days. Even if snails don’t look active during the day, slime trails across dirt can be a good indicator of night activity.

Apply an adequate rate of bait

Based on recent SARDI research, a minimum of 30 bait pellets per square metre and up to 60 pellets per square metre at very high snail densities should be applied to ensure a sufficient density of bait points and chance of encounter. The higher rates may be needed in heavily infested areas, such as perimeters, fence lines or calcareous outcrops. Where current label rates do not permit this, a repeat application should be considered. Pellet densities for registered rates of commercial products are available in the SARDI Snail and slug baiting guidelines brochure (refer to Useful Resources section at end of this paper).

Monitor live snail densities and re-apply bait as necessary. A repeat application may be needed in areas with high snail densities or when rainfall has broken down bait.

Bait degradation risk

Some bait products are more stable under adverse weather conditions, such as cold temperatures and rainfall. Significant rainfall can degrade bran-based pellets and reduce efficacy, particularly for iron chelate products. SARDI trials found that UV exposure did not reduce the efficacy of baits; however, exposure to high temperatures (above 30^oC – 40^oC) degraded the active ingredient in metaldehyde baits. Avoid storing bait for long periods in places where temperatures exceed 35^oC (e.g. hot sheds).

Mice can consume snail baits. In areas where mice cannot be controlled adequately, ensure that snails are receiving a sufficient dose by checking for the presence of dead snails a few days after application, and adjust baiting rates as necessary.

Current snail research

GRDC funded project DAS00160, led by SARDI in collaboration with the University of South Australia, DPIRD in Western Australia and a number of farming systems groups, builds on the previous project and is investigating the environmental conditions that lead to feeding and reproduction. The work aims to assist growers to optimise the timing of their baiting programs.

Ten field sites have been established across Western Australia, South Australia, Victoria and Tasmania to monitor the activity and biology of key snail and slug species using fixed cameras and field sampling, along with associated climate and micro-climate variables. The research is generating insights into the reproductive patterns of snails and slugs in southern and western Australia.

Conclusion

Baiting during autumn is a key component of a year-round systems approach, which is critical for effective snail management. Baiting should be used in conjunction with cultural controls during summer and autumn to reduce snail survival, such as cabling, rolling and/or burning, or summer grazing, along with effective weed control to remove refuge habitat.

In summary, the key to successful snail control is year-round integrated management:

• Continuous vigilance
• Remove summer refuges
• Roll or cable in summer when > 35^oC
• Bait before egg laying in autumn:
  • Baiting at sowing or after is often too late
  • Baiting in winter is less effective.
• Harvester modifications
• Grain cleaning as the last resort.

Useful resources

Snail and slug baiting guidelines

Snail bait application GRDC factsheet

Snail management GRDC factsheet

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Greg Baker
SARDI Entomology, Waite Campus, Adelaide
08 8429 0933, 0427 039 544
greg.baker@sa.gov.au