Economics should be one of the factors driving decisions about baiting and other control methods for slugs, seasonal influences and monitoring tools are another.

Starting in late winter and early spring 2022, invertebrate ecologist Dr Michael Nash toured key grain growing regions on behalf of GRDC to discuss with growers  local conditions and factors influencing their decision on whether or not to bait their crops.

Wet weather through 2023 in many areas and into early this year, created an ongoing challenge for producers.

Dr Nash earned his PhD studying integrated pest management of slugs, including looking at chemical and biological methods of control in the crop and on the boundaries where weeds can harbour slugs and snails (molluscs). 

He said growers need to be prepared to implement pest management strategies over a period of years, depending on the level of infestation.

Growers were reporting increased slug damage in crops in Victoria and SA in winter and early spring, after ongoing wet weather conditions for more than a year. Some growers reported they were experiencing heavy infestations of molluscs for the first time.

“Slugs are present in all major grain growing regions in Australia. They are increasingly the biggest threat to growers in the southern and western high-rainfall zones,” Dr Nash said.

“The high numbers being reported follow a run of dry years in some areas where growers aren’t used to seeing slugs as a threat. In areas like the Wimmera, growers have been caught offguard.

“Growers in south-western Victoria are concerned they won’t be able to control slugs if they lose the ability to burn stubbles. They are concerned they won’t be able to establish canola when mollusc infestations keep recurring.

“In South Australia, growers are limited in their options to drain standing water, because of government regulations. Slug pressures are sporadic in north-eastern Victoria and southern NSW, which makes it difficult to plan baiting activity.”

High-value crops such as canola are likely to yield greater economic returns from baiting but Dr Nash recommends measuring the population of molluscs before making decisions about baiting in any crop.

Growers also need to take into account supply chain issues relating to the pandemic might require them to order baiting supplies early. This is another reason why ongoing monitoring is important – to ensure baits are ordered in appropriate quantity and when required, before molluscs have a major impact on crops.

Checking numbers

Planning and implementing control methods is hampered by a lack of a distinct life cycle in slugs. They breed when moisture and temperature conditions are suitable.

“This is why mats are useful. You can put mats out overnight and extrapolate the number of slugs or snails in your crops, and their varying ages ,” Dr Nash said.

After identifying slugs on mats laid at ground level, the opportunity to subject parts of the ground to closer scrutiny could yield eggs in the soil.

Dr Nash said autumn baiting resulted in better crop results in south-western Victoria but growers need to monitor their crops in the early spring period of August, September and October, and base buying decisions on population assessment when slugs are breeding.

Given the extended wet weather since 2021 and the fact that another La Nina was declared by the Bureau of Meteorology in early spring 2022, depending on local conditions growers needed to assess whether slugs were still breeding in December.

“There’s normally a five-week quarantine  between time of baiting and harvest,” Dr Nash said. 

“A proportion of black field slugs are quick breeders. It’s these slugs that will produce a second population of juveniles, breeding in autumn and spring. In December, growers will need to assess whether slugs will still be breeding in 12 weeks.”

This requires a strategy to understand the reproductive phases of slugs, and the conditions in which they will breed - one individual slug with a good food source will produce thousands of offspring.”

Mollusc diets

Molluscs have a wide-ranging diet beyond cereal and oilseed crops. In south-eastern SA, they have been found in clover, carrots and other crops grown to yield seed. Tasmanian poppy and vegetable seed crops are also affected by molluscs.

Wheat sown into a late seasonal break exacerbates the risk of damage from molluscs, when the ground stays wet.

“Growers may have to bait three times between sowing and harvest,” Dr Nash said. “Communities of slugs change in different seasons and year to year. They may have to resow up to 25 per cent of the crop. An infestation is likely in areas where irrigation or standing water keeps the ground moist.”

Dr Nash said some growers in Victoria and SA had successfully experimented with broadcasting canola seed into a standing crop, to infill areas where serious mollusc damage was already evident.

This helped reduce crop and economic losses, but needed to be coupled with strategic application of bait when new plants were continually germinating. 

It’s always a good rule to check the precise manufacturers’ recommendations on each brand of bait.

“When crops are seedlings, they are at their most vulnerable to slugs. Slugs emerge from the ground over an extended period of time, so growers need to protect seedlings during establishment. This may require multiple applications of bait.”

Dr Nash also discussed predators of molluscs, including rogue and carabid beetles, and seed treatments to help control pests.

 “I think growers want a selection of tools in their toolbox, rather than being solely reliant on baiting.

“The use of insecticides as a seed cover was brought up as a tool to limit slug activity.

“One grower used bait mats to identify the slugs in his paddock – they identified the striped field slug, which is not a threat to cereal crops but is to horticulture. Novel molluscicides was also brought up as a method against snails. Another tool worth looking at might be ciliates.”