Grain growers are advised to bait snails now in an effort to reduce populations of the crop pests.
Stimulated by moisture, snails are becoming active after summer dormancy, presenting the opportunity for growers to commence baiting.
Baiting is most effective when snails are actively feeding but before they commence mating and egg-laying, according to experts supported by the Grains Research and Development Corporation (GRDC).
Entomologist Helen Brodie, from the South Australian Research and Development Institute (SARDI), a division of Primary Industries and Regions SA, says snail movement has been recently observed.
“Even though conditions have been mostly dry to date in 2018, snails have been observed becoming mobile in response to light rain events and overnight dews in parts of South Australia since March 2018,” Mrs Brodie says. “Baiting programs should target snails as soon as they become active and before reproduction – therefore, now is the time to act.”
Trials conducted over the past three years by SARDI as part of a GRDC research investment have shown that the reproductive organs of snails begin to mature from late March onwards, and then most reproductive activity occurs from late April to July.
Mrs Brodie says baits are not inherently attractive to snails, therefore baiting performance relies on snail activity which determines the chance of snails encountering and consuming bait pellets. At this time of year, 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. Baiting can be carried out anytime snails are active. However, current research is supporting autumn as the best time to bait for several reasons.
“Firstly, preventing adult snails from laying eggs is critical in reducing population growth. Juvenile snails are generally more difficult to control using baits due to reduced movement and bait encounter. Secondly, SARDI laboratory trials have found higher efficacy (against both snails and slugs) when baits are applied under warmer temperatures within the range we tested (10oC-22oC). And finally, baiting is most efficient when there is less ground cover and alternative food – the presence of crop plants and weeds at later times during the year reduces bait encounter.”
To ensure snails are likely to encounter pellets, use the appropriate rate according to the label.
“The higher rates may be needed in heavily infested areas, such as perimeters, fence lines or calcareous outcrops,” Mrs Brodie says. “Where current label rates do not permit this, a repeat application should be considered.”
Mrs Brodie says calibration of the bait spreader is needed to ensure effective application: “Don’t assume your spreader is distributing bait pellets evenly. Research by the Yorke Peninsula Alkaline Soils Group and SARDI showed that spreaders calibrated for other applications (such as urea) may not broadcast baits as widely as expected, while ute spreaders often distribute the pellets unevenly.
“Different bait products have different hardness and ballistic properties. It is recommended that bait spreaders be professionally calibrated to ensure even broadcast of the target pellet density over the entire spread width. Bait spreaders can fragment pellets during application, which can reduce the number of effective baits.”
Growers are advised to monitor live snail densities and re-apply bait as necessary. Repeat application may be needed in areas with high snail densities or where rainfall has broken down baits.
Significant rainfall can degrade the physical integrity of bran-based pellets and reduce efficacy. SARDI trials have found that exposure to ultra violet radiation does not reduce the efficacy of baits, but exposure to high temperatures degrades the active ingredient in metaldehyde baits. Growers should avoid storing bait for long periods in places where temperatures exceed 35C, such as hot sheds.
Growers need to bait mice to reduce mouse numbers, before they bait for snails. Where mice (which can consume snail baits) cannot be controlled adequately, growers should ensure that snails are receiving a sufficient dose by checking for the presence of dead snails a few days after application, and adjusting baiting rates as necessary.
Ongoing research is producing a greater understanding of snail behaviour, biology and control options.
A GRDC study investment conducted by SARDI has focused on investigating the factors influencing the performance of commercial molluscicidal baits against different snail and slug species, densities, rates, and environmental conditions. This work, now completed, has generated refined baiting guidelines for snail and slugs which will soon be made available.
A current GRDC investment, led by SARDI in collaboration with the Western Australian Department of Primary Industries and Regional Development, the University of South Australia and a number of farming systems groups, builds on previous research and is investigating the environmental conditions that lead to feeding and reproductive behaviour. The work aims to assist growers to optimise the timing of their baiting programs. Ten field sites have been established across SA, Victoria, Tasmania and WA, to regularly monitor the activity and biology of key snail and slug species using fixed cameras and field sampling.
Further work is also being conducted to improve biological control of the conical snail. A new GRDC project investment is underway involving a collaboration between CSIRO and SARDI, which is aiming to introduce a strain of parasitoid fly better adapted to Australian populations of the conical snail.