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Slugging costly pests with potential to damage plants and contaminate grain

WA DPIRD entomologist Svetlana Micic says growers may need to trial several different slug and snail control strategies to determine the most effective option.
Photo: DPIRD

Multi-tactics best to target snails and slugs that reduce crop yields and grain quality.

Snail and slug infestations are costing Western Australian growers almost $2 million each year and, if they are not controlled, researchers have estimated that cost could blow out to almost $20 million in lost yield and grain quality (Murray et al report).

A GRDC-invested study exploring the extent of the snail and slug problem in WAs south-west and southern grain-growing regions has also ascertained the best methods of controlling these pests throughout the year, plus ways to stop them ending up in your header box.

Burning

The research by the WA Department of Primary Industries and Regional Development (DPIRD) has identified stubble burning, as an alternative to baiting, is one way of reducing snail populations.

But widescale burning may affect soil health, particularly in the south coastal regions, and at an estimated cost of $30 per hectare it is not a cheap control option.

WA DPIRD entomologist Svetlana Micic, who managed the research, says whole-paddock burning could cause soil erosion problems, particularly for those growers who have sandy soil.

Snails harbour in crop stubble, so the obvious way to attack them is to reduce the stubble load in a paddock, Ms Micic says.

Growers will have to weigh up the risk of having an exposed paddock with the impact of the snail infestation.

The research tested windrow burning and whole-paddock burning, with windrow burning showing promise in canola stubble.

Snails harbour in crop stubble, so the obvious way to attack them is to reduce the stubble load in a paddock - DPIRD entomologist Svetlana Micic

In a harvested canola paddock, 25 times more snails were found in the windrow as compared to the inter-row.

Ms Micic says this suggests snails are more likely to move into a windrow than stay in the inter-rows, allowing them to be somewhat controlled by windrow burning.

Further research in a barley paddock where stubble was windrowed showed there were still 50 surviving snails per square metre in the inter-rows after the whole paddock was burnt.

It is possible the higher numbers of snails survived because they were associated with green weeds present in the paddock, Ms Micic says.

The weeds were only found in the inter-rows and not in windrows, where the weeds or unburnt fallen stubble aids the survival of snails.

She says the message from this outcome is to ensure weeds are controlled in the inter-rows as they provide shelter and a ready food source for snails.

Population mapping and baiting

The study investigated the feasibility of using paddock imagery to detect and map small pointed snail populations, with the maps to be used for more effective baiting applications.

Ms Micic says this imaging technique, while time-consuming, would allow growers to use variable-rate technology to apply higher bait rates to patches of higher snail counts.

We need to do more work on this strategy to come up with a way to capture the images without the need for extensive manual observation, Ms Micic says.

If we can capture the images needed while the crop is being harvested, it could fast-track the baiting process and allow growers to focus on these areas pre-seeding in the following season, which would make baiting a cost-effective option.

Ms Micic says growers had asked about the cost and type of bait that provided the best control for slug and snail control, but the study determined the number of baits per square metre was much more important than the type of bait used.

If we can capture the images needed while the crop is being harvested, it could fast-track the baiting process - DPIRD entomologist Svetlana Micic

Laboratory experiments

As part of the wide-ranging study, laboratory experiments were conducted trialling molluscicide sprays, but Ms Micic says this treatment had almost no effect on either the slugs or the snails.

However, microwave treatments showed slugs and snails were sensitive to microwave radiation, suggesting this could be something to consider as a strategy in combating the pests in the future.

But the science still has a long way to go before it can be a mainstream, practical option for growers, she says.

Harvest time

If the snails are present in the crop at harvest time, Ms Micic says the only way growers can avoid contaminating the grain is to harvest when snails are low in the crop canopy and not in the grain heads.

Snails are active at night time and during cooler, overcast weather. During these times snails are more likely to climb up the stem of a plant, she says.

During the hottest part of the day, higher numbers of snails are found under stubble, so this is the best time to harvest to avoid them," she says.

"Cleaning small pointed snails from grain is often difficult, particularly if the snails are a similar size to the grain."

Ms Micic says small-scale trials have shown that drying the grain in a grain dryer will not kill snails.

If you are keeping your own seed and it has snails in it, some of the snails are likely to be alive," she says.

"Sowing the seed in snail-free paddocks can increase their spread."

While there does not yet appear to be any simple solution to snails, Ms Micic says growers may have to test various strategies to determine the most effective option on their farm.

GRDC Project Codes: DAW00251, 9176105

More Information: Svetlana Micic, 08 9892 8591

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