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01 May 2020
We take a behind-the-scenes look at a French company’s research into field slugs and its secret anti-leeching recipe.

HISTORICALLY, growers have been slow to take up the use of ferric phosphate as a slug control solution in the UK, and with the ban of metaldehyde now overturned, those already using products containing this compound could well be tempted to stick with what they’re used to.


Slugs respond differently to ingesting ferric phosphate and the results are less ‘visible’ owing to the fact that there are fewer dead bodies above ground. So what are the advantages offered by ferric phosphate products and how does it stand up against metaldehyde?


De Sangosse, a French company that has spent more than 40 years developing and manufacturing slug baits and biocontrol products for the UK market, recently opened the doors to one of its research and development labs, in Agen, France, to give Potato Review an insight into some of the work it carries out and why it believes ferric phosphate is a show of strength in the fight against one of the most widespread and damaging pests in potato crops – the grey field slug Deoceras sp.


This pest’s activity, survival and reproduction are dependent on moisture and they are more abundant in heavy soils with high clay or silt content. Crop residues and weeds can provide the slugs with a source of food and shelter.


Major/critical defect by the main UK supermarkets  & food processors or critical and can lead to product rejections. Typically, fresh potato packers and processors will reject grower deliveries into their factories with a slug damage threshold of 2 - 5%, according to Jon Williams, De Sangosse Technical Manager. Slug feeding damage can be deep into the tuber and therefore wouldn’t get past the three peel limit, for complete damage removal, on a standard potato peeler test at customer QC intake testing. Where higher slug damage levels are seen, the price could easily be reduced by 50%, potentially a £6K Ha loss to the grower.


In 2014, research showed that the grey field slugs caused direct losses of £53m in potatoes in the UK.

Jon said: “At the end of the day, you can’t hide slug damage. The worst consequences  to a potato  grower is that if the slug damage is over the acceptable damage level, the factory who hold the contract to take his produce, may reject the whole lorry load outright and say ‘take it away’ and will also be within their rights, to also reject anything else from that same field . However, in most situations, both parties come to an agreement and the load is accepted but with a reduced payment.”

Slugs are most damaging at the early stages of tuber bulking. They enter through small holes in the skin, causing irregular-shaped holes on the tuber surface, extending into large cavities in the tuber.

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Jon pointed out that Maris Piper, still the most widely grown in the UK, is particularly susceptible to slug damage, which can also be commonly confused with wireworm damage. Wireworms are the larva stage of the click beetle. The beetles themselves do very little damage to potatoes but the larva do, and the combined damage of the two pests can push the delivery into a ‘fail’ category when the load is checked at the factory QC intake.


“Ferric phosphate is an effective alternative to metaldehyde for reduction of slug damage to in potatoes,” Jon said. “Currently ferric phosphate and metaldehyde can still be used sequentially in programs with no loss in efficacy. The key timings for molluscicides application in potatoes are just before the crop canopy meets across the rows up to early tuber bulking but protection should be maintained right through to burn down of the crop.”

While growers sometimes put slug pellets on at planting, agronomists have claimed this is overzealous, and the most effective use is to the canopy. “You can apply up to four times, with a 7 Kg/Ha application but the important thing is persistence of that pellet," Jon said.

The dependability of ferric phosphate is nothing new, he added, referring to independent AHDB trials carried out at Carrington, Lincolnshire, and in Edinburgh, in 2015/16, which had shown at the time that three treatments of ferric phosphate applied to tubers with slug damage were giving as good, if not better, results as all other combinations, including metaldehyde.

De Sangosse's newly-launched slug pellet for potatoes, Fe-Lyn, contains 24.2g/kg ferric phosphate and we were shown how it performed in the lab and the field.


In France, where the research centre is based, the Government has dictated that by 2025 there must be a 50% reduction in pesticide use. The ferric phosphate slug pellets are classed as a biological control method, and De Sangosse has now become a global leader in the supply of slug pellets, we were told by Communication and International Affairs Manager Christophe Zugaj

"Our strategy now is clearly on bio solutions," he said.

* To continue reading this feature, and for further details, be sure to check out the May edition of Potato Review. To subscribe, visit

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