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Wet conditions make vigilance the key for slugs

Growers and agronomists attended workshops across the southern states to discuss slugs and snails.
Photo: Jeanette Severs

Economics should be one of the factors that drives decisions about baiting and other control methods for slugs. Seasonal influences and monitoring tools are other factors.

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 the local conditions and factors that influence their decision on whether or not to bait in their crops.

Workshops were held on farms throughout August and September in Victoria, South Australia and Tasmania. Attendance was split fairly evenly between growers and agronomists.

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

Michael NashDr Michael Nash at one of the southern region workshops. Photo: Jeanette Severs

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 off-guard.

“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 that 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 the 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 many growers needed to monitor their crops in the early spring period of August, September and October, and base buying decisions on population assessment when the 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, particularly more than once.

“One individual slug with a good food source will produce thousands of offspring.”

Mollusc diets

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

Wheat sown into a late seasonal break also 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.

slugsSlugs breed when moisture and temperature conditions are suitable. Photo: Jeanette Severs

This had 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,” he said. “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.”

More information: Slug control identification and management factsheet; Slug Back Pocket Guide; GRDC Slug Investment details.

On-the-ground stories

Liz Semmens, agronomist

“Population monitoring over the long term will get a better result. Targeted monitoring in the spring is going to help provide a better and more-controlled response during the winter cropping season. We manage slug and snails for winter crops, but maize is a high-value summer crop that we should also monitor for molluscs. Foliage and trash in the crop and along the fence lines provides a harbour for slugs and snails. Picking up the foliage on the fence lines in the early morning gives you an indication of the population that can be confirmed by using bait mats in your crop.”

Ton van Dyke, grower, Cowwarr

“The workshop is a good reminder that monitoring is very important. It’s better to be proactive; you can’t put a crop in the ground and expect it to look after itself. I knew there were slugs in my paddocks and putting out the mats identified the population. The day of the workshop showed the pressure is so high, we’ll probably have a problem in the summer. The only thing I’ll change is I’ll use bait mats earlier.”

Allister Morris, grower, Winindoo and Cowwarr

“For the best cost-effective benefit, I bait during the establishment period. The workshop reinforced that slugs are always there, so you need to try and protect your crop while it’s getting established.”

Gary Condron, agronomist

“It surprised me how many slugs – 20 and more – were under those mats in the crops. Ton van Dyke is one of those farmers who orders slug bait at the same time he orders seeds. Michael Nash reinforced that we need to monitor more to demonstrate the populations of slugs and snails. Monitor early, bait in response to monitoring and choose your baits for different circumstances. With the current supply chain challenges, growers should be ordering ahead to ensure their preferred bait options.”

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