Bait timing important for slaying slugs

Dr Michael Nash, an applied invertebrate ecologist and leader in the study of Mollusca control in Australia, recently presented at two Mackillop Farm Management Group-hosted 'Slay the Slugs' workshops in the South East, to highlight strategies to manage both slugs and snails this coming season.

One workshop was held in Furner, where black keeled slugs and small conical snails were of concern, while at Wolseley, management was focused on black keeled and grey field slugs and round snails.

"We're getting more slugs now as the snails can't afford their houses anymore," Dr Nash joked.

"It appears there has been an increase in slug and snail numbers in the SE since 2021 on all soil types.

"Peat soils aren't known for attracting snails, but in the past couple years, even those soils have had troubles with slugs.

"The growing of better, bulkier crops has contributed to increasing populations.

"Plus, the overuse of seed treatments could be affecting the natural enemies of slugs - carabid beetles."

Dr Nash stressed slugs and snails were not insects and should be managed differently, while slug compared to snail management also wasn't the same.

The workshops outlined slug and snail ecology, then focused on best baiting practices and some common misconceptions about chemical control.

"It is very different trying to control slugs and snails, compared to insects," he said.

"Molluscicides are the preferred option (over sprays) as they allow a longer window of control - slugs and snails become active over an extended period of time as adults.

"Sprays generally have to be reapplied more often in comparison with baits."

Dr Nash said anecdotally, there had been reports of a 'secondary effect' - where a fungicide has been used on bean crops less snails are observed.

"I do question some of the observations - could it just be the snails dying at that time of year anyway because of their age because they have finished breeding? Or are sprays just repelling snails but not controlling them?" he hypothesised.

Dr Nash said the continued registration of a particular fungicide for use in beans and lentils will need to be reviewed in Australia, and potentially restrict the ongoing use, as markets (namely Egypt) change minimum residue limits to zero.

"I think chemistries, like carbendazim, will have a very limited life in the Australian market, as dictated by our customers," he said.

"Farmers need to remember - anything used for limit crop threats has to have market access in mind."

Dr Nash said the use of molluscicide 'metaldehyde' could also become an ongoing concern, with it already banned in the United Kingdom due to environmental concerns.

"The majority of slug and snail baits in Australia contain metaldehyde," he said.

"With our canola going into the European market, we are increasingly being forced to use their standards, and their metaldehyde stewardship guidelines have a cap on the total amount applied in any one season or in any one application.

"While these standards may not be needed at the moment, they should be strongly considered because of market access."

The same market access concern surrounded conical snails as a biosecurity threat, as is the importance of adhering to withholding periods to avoid contamination of grain.

"There have been occurrences of baits turning up in grain samples at the sample stand, which is just a no-no as all growers are aware of," Dr Nash said.

"But not baiting before harvest makes it very difficult to control snails, which then also contaminate grain at harvest time.

"That's why good, early snail and slug control is so important. And why we are holding these workshops now - to help growers make good choices on products they might use to protect their crops from slugs to provide the best return on investment."

Dr Nash said snail monitoring generally started at this time of year.

"Obviously easy snail control is rolling or cabling, but then strategically baiting in autumn," he said.

"Some growers at these workshops indicated that they've already got a little bit of bait out along their edges to use as a monitoring tool - to see when they start to come off fence lines and start taking bait.

"Down in Furner, baiting could also be undertaken in January for small conical snails."

"But you have to monitor, to bait when the snails are ready, not when you're ready."

But management is different for slugs, Dr Nash said.

"With slugs, the timing for applying bait is after seeding to protect the seed and seedlings, and then monitoring to make sure you still have baits in the paddock to provide ongoing protection," he said.

"That's why if you've got small conical snails and black field slugs, you might actually be putting out bait two or three times - one targeting the conical snails and then one or two baits to protect the seed and seedlings from black field slugs when they emerge, say in late May.

"You might also think about how to establish crops like canola quickly before it gets eaten."

Dr Nash said slugs and snails were "adaptive strategists" that have the ability to delay their breeding.

"We have some very set knowledge about when insects breed in their lifecycle," he said.

"The trouble with slugs and snails is that they can actually delay their breeding.

"In the SE, especially close to the coast, they can lay eggs any time of the year, while closer to Naracoorte, they might be just over the wintertime.

"Some can live up to two years, some can be 12 months, depending on how quick they breed, while black keeled slugs could live up to 500 days.

"International literature has shown that we have slow and quick (slug) breeders, some take 12 months to get to lay their eggs and others are laying their eggs within three to four months."

And despite the dry outlook ahead, it is still not clear what effect that may have on populations.

"I have a hypothesis about the springtime and its effect if it's dry, so with the dry October, we wouldn't expect as many slugs this year," he said.

"But, with the rain in December, we are not sure how that will influence slug numbers.

"Again, slugs and snails are adaptive strategists, so maybe all the dry period has done is delay what they do for three months? That's a working hypothesis that may play out in a month's time.

"Once it starts raining and seed goes into the ground, we will see how big the pressures are this year."

Dr Nash recommends farmers to also monitor their crops in spring, to see if the current dry autumn was having an effect.

"It's great that the GRDC recently funded more research through the University of Adelaide into slug biology so we can capture data to answer the dry weather question, and develop decision support tools," he said.