Application timing and the use of high-quality pellets should ensure potato slug control is not compromised when potato growers switch from using metaldehyde to ferric phosphate methods.

Metaldehyde is be banned in all outdoor crops from June, 2020, so the 2019 season will be the last time the industry can use the active in the main battle against slugs.

With so much at stake in a high-value crop like potatoes there has been some nervousness about the efficacy and reliability of ferric phosphate – the only approved alternative on the market.

According to Scotland’s Rural College (SRUC) pest expert, Andy Evans, however, concern regarding the efficacy of the ferric control methods is unfounded, with two years of AHDB-funded trials in 2015 and 2016 showing no difference between the two.

So-called ‘enhanced’ metaldehyde stewardship guidelines were introduced in 2017, which included a 10m buffer zone around all field boundaries and so many Scottish growers have already moved to ferric phosphate, as it has no such restriction to hamper application logistics.

After initial scepticism, their field experience has found control with a good quality ferric phosphate pellet to be the same, if not better, than metaldehyde, pointed out Dr Evans.

“For those that haven’t tried ferric phosphate yet, this season is a good opportunity to give it a go before metaldehyde goes next year,” he said.

Dr Evans argued that one issue which is always raised when discussing the merits of ferric phosphate is the lack of evidence of slug mortality post application. Metaldehyde pellets kill slugs quickly and dead slugs can be seen on the soil surface soon after treatment, but this is not the case with ferric phosphate.

In an attempt to solve the mystery of what happens to slugs after ingesting the active, pellet manufacturer, Certis, asked researchers at Harper Adams University to conduct some controlled tests.

Project leader, Keith Walters, said his work showed that although feeding activity can occur during daylight hours, slug feeding tends to be more intense at night, when larger numbers are active in the surface layers of the soil.

During periods of inactivity, often during the daytime, they retreat into refuges, such as soil cracks or under stones. After such periods, untreated slugs eventually come back to the surface to feed.

“However, the reason you won’t find so many dead slugs after ferric phosphate treatment is because in many cases, treated slugs move to a refuge and they simply die there. Thus, the lack of visible dead slugs does not indicate the pellets have not been effective,” added Prof Walters.

With the myths around ferric phosphate efficacy largely ‘busted’, Dr Evans said that one of the key factors in ensuring optimal slug control was application timing.

The first application is the most critical and aimed to get baiting points at the base of plants just before canopy closure, when a shady, humid microclimate forms at the soil surface and brings slugs up to feed.

The second important timing is just after haulm destruction, as the pest’s above ground food source is removed and slugs retreat underground to potentially feed on tubers. Treatment between canopy closure and burndown is dictated by rainfall or irrigation and slug pressure, with increased moisture seeing faster breakdown of pellets and increased need for a top-up.

“Although there’s less control over where the pellets end up [with a full canopy], there is a good chance of getting pellets to meet the slugs and improve control,” pointed out Dr Evans.

In damper conditions, pellet formulation can be ‘tested’ and Dr Evans said experience with metaldehyde pellets showed that the more you pay, the better quality and longevity tends to be.

“In potatoes, longevity – particularly at the first timing – is important. You have three to four days after application until canopy closes and the pellets need to last as long as possible as the slugs come up to feed.”

Of the ferric phosphate pellets approved in potatoes, Sluxx HP is the only wet processed pasta pellet formed from a 100% durum wheat dough and contains an anti-mould agent that helps maintain pellet integrity after rain or irrigation.

Advising on potato crops across the England, independent potato consultant, Andy Alexander, has been relying solely on Sluxx HP for a number of years due to the environmental concerns associated with methiocarb and metaldehyde use.

He uses site history, variety and trapping to assess risk and begins pellet applications just ahead of canopy closure, then tops up every three to four weeks until a final application at haulm destruction to protect tubers ahead of harvest.

Either three or four application of Sluxx HP, within the 28kg maximum total dose limit, has not resulted in a drop off in slug control, with ferric phosphate efficacy and pellet longevity key factors, he said.

With a proportion of his clients having heavier, ‘sluggy’ land and overseeing a large area of the highly susceptible variety, Maris Piper, his strategy has been severely tested.

“Anticipation of risk and suppression through the season is key and having no buffer zone allows us to reduce damage on headlands. It is all about a safe, saleable crop that pays the bills.”

Slug control in potatoes – key points

  • 2019 the last season without metaldehyde in outdoor crops.
  • Growers urged to try ferric phosphate-based pellets this year.
  • Independent trials show ferric phosphate has equal efficacy to metaldehyde.
  • Correct application timing and pellet formulation choice key to good control.
  • Use a wet-processed pasta-based pellet for maximum longevity.