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Steps to manage snails become clearer after studies into pest behaviour and control tactics

Researchers have deployed fixed cameras and remote monitoring stations to chart snail and slug behaviour in conjunction with climate and micro-climate variables at 10 field sites across South Australia, Victoria, Tasmania and Western Australia.
Photo: Evan Collis

Now is the time to be regularly checking for and treating snail activity across the southern growing region - and studies of both snail behaviour and control strategies mean the steps to take are clearer than ever.

Research into snail management has been one focus of the strategic partnership between GRDC and the South Australian Research and Development Institute (SARDI).

Working in conjunction with the University of South Australia, researchers involved in the study project investigated the seasonal conditions that appear to trigger snail feeding and reproduction.

The work builds on previous studies into baiting methods to help growers maximise their control over this significant pest.

SARDI entomologist Helen Brodie says early autumn is peak breeding season for the four introduced European-Mediterranean snail species now firmly established in southern Australia.

"Ideally farmers will get bait out before the snails start laying their eggs," Ms Brodie says.

Her research team has found baits kill snails more efficiently during and just prior to egg laying than at any other time of year, and that careful storage and application is essential to optimise bait efficacy.

"Depending on when the break in the season occurs, egg laying might start in early March - or it may start later and provide a larger window of opportunity," she says.

"It depends on when that first heavy dew or light rain event occurs."

Ideally farmers will get bait out before the snails start laying their eggs. - SARDI entomologist Helen Brodie

Aided by technology

Along with laboratory studies and field testing, the research has mobilised new technologies in an effort to better understand the behavioural patterns of snail and slug species.

Researchers have deployed fixed cameras and remote monitoring stations to chart snail and slug behaviour in conjunction with climate and micro-climate variables at 10 field sites across South Australia, Victoria, Tasmania and Western Australia.

By dissecting snails collected from these sites, the team has been able to study when the snails are in their egg-laying phase.

The researchers have found the snails' reproductive organs typically begin to mature from around late March and most reproductive activity occurs between late April and July.

"The size of the albumen gland gives us a very good indication of when the snails are likely to start laying eggs and, later in the season, whether they are still laying eggs or if they have started to shut down for the summer period," Ms Brodie says.

She says snails will continue to respond to moisture events, including summer showers.

It is important to be aware of the temperature and rainfall forecast before spreading baits. - SARDI entomologist Helen Brodie

"Because they move on their slime trails to reach food, they need to replace that moisture lost during movement," she says. "They need the moisture to move."

However, research at SARDI has established that snail mortality from bait is reduced during spring and summer, even if rainfall has made them active.

"Although the snails eat the bait throughout the year, we find that they are not actually dying from it as much during months when they are not reproducing," Ms Brodie says.

"We see much higher rates of mortality coming into autumn and winter, even though they are eating the same amount of bait.

"The April to June-July period is when they are most likely to die from eating baits. However, it is important to start baiting before the egg laying occurs.

If you are waiting until winter to start baiting, it means that a whole new generation of juvenile snails already exists and can be a lot harder to kill with baits."

These findings indicate that autumn baiting must form part of a year-round snail management strategy which includes rolling, cabling or over-grazing stubble during summer.

Fallow weed control and farrowing to turn rocks will further deny snails a refuge habitat during the hot months, so they dehydrate and die.

Taking the bait

The research has also established that bait effectiveness can be reduced by poor storage - especially exposure to hot conditions over summer.

Baits were stored in various conditions and then tested for their effectiveness on Italian white snails. The research found baits stored at high temperatures for seven days became less effective.

The active ingredient concentration in metaldehyde-based baits was found to decrease when storage temperatures passed 20°C, reducing over a seven-day period by about one gram per kilogram in Meta bait and 4g/kg in Metarex baits for every 10°C increase.

"We know metaldehyde-based products will degrade if exposed to temperatures above 30°C to 40°C," Ms Brodie says.

"Therefore, if the bait is sitting in a hot shed throughout summer, the pellets may have less active ingredient in them by the time they are applied."

Her other tip is to calibrate the spreader for the selected bait product, then make sure the output is even and accurate. She recommends assessing the application by pellets per square metre rather than weight.

"Unfortunately, snail baits in general are not attractive to snails over distance, so they will not seek the baits out," Ms Brodie says.

"Baits need to be placed at a time and density that ensures snails will encounter them before the pellets degrade and become less effective."

Autumn's reduced stubble cover and weed populations, along with the absence of emerging crops, can all help increase the likelihood of bait encounters.

"Growers should be applying a minimum of 30 baits per square metre and up to 60 baits per square metre for high-snail-density situations along fencelines and around calcareous rock outcrops.

"Where current label rates do not permit those high rates, a second application will be necessary."

Smaller may be better

Ms Brodie says smaller uniform baits may provide a better spread, but farmers must take into account the longevity of the bait format once it is exposed to environmental conditions.

"Iron chelate products are less effective if exposed to rainfall above 35 millimetres," she says.

"However, extended high rainfall will break down most bran-based pellets regardless of the active ingredient.

"It is important to be aware of the temperature and rainfall forecast before spreading baits."

Laboratory trials have shown baits to be most effective when temperatures are in the 10°C to 22°C range, with best results at the warmer end of the scale.

Baits are also most effective when soil remains moist for several days.

To maximise effectiveness, however, farmers need to store their baits in cool, dry conditions, and accurately apply them at the start of the snails' reproductive season.

The outcomes of this project were delivered via the strategic research partnership between GRDC and SARDI, in collaboration with the University of South Australia.

The GRDC-SARDI partnership has facilitated a range of projects which provide innovative research outcomes relevant to SA's cropping zones.

GRDC Research Code DAS00160

More information: Helen Brodie, 08 8429 0557, helen.brodie@sa.gov.au

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