More than 20 years of research into ferric phosphate has resulted in a formulation that’s the world’s market leader. CPM talks to the company that first introduced ferric phosphate and discovers how research knowledge has been transferred into successful slug control in the field.

Research shows no increase in efficacy from including attractants in the formulation.

By Lucy de la Pasture

Ferric phosphate has been available to UK arable growers for more than a decade, but its heritage stretches back to 1997 when it was first introduced as a molluscicide.

The company behind the discovery and patented formulation of ferric (III) phosphate, is German manufacturer Neudorff, a company that specialises in natural products and is better known in the UK for their presence in the home and garden market.

The way ferric phosphate works means the only visual sign of slug control is the crop growing away.

“Although we introduced ferric phosphate in the home and garden market initially, our focus changed when we saw a demand in agriculture and horticulture for an effective and environmentally friendly molluscicide. So we set about developing a form of ferric phosphate that was appropriate for agriculture,” explains Peter Baumjohann, head of technical support at Neudorff.

A mutual pedigree in biorational products made Certis the perfect marketing partner for a professional ferric phosphate product, which was first registered in the UK in 2005 for horticultural use as Ferramol, formulated with 1% of the active ingredient.

“The 1% pellet is applied at a dose rate of up to 50kg/ha which isn’t economic or practical on broadacre crops. So we developed a 3% formulation of ferric phosphate for use in agriculture which has a dose rate of 7kg/ha and this gained UK approval in 2008 as Sluxx,” he explains.

“We set out to find a molluscicide formulation that was suitable for all temperature situations in agriculture. The pellet needed to perform over a huge range – from the high temperatures often encountered where slug control is needed in potatoes or during oilseed rape establishment, to the much colder temperatures when slug control is often necessary in cereal crops,” he comments.

Peter highlights that Neudorff now have ferric phosphate registrations in more than 20 countries and are the market leaders in Europe, the United States and Japan. Their legacy of research into ferric phosphate and a programme of ongoing studies gives the company’s product an edge when it comes to field performance, he believes.

“The formulation of Sluxx has improved over the years. The most recent formulation, Sluxx HP has improved resistance to moulding which means it remains palatable to slugs even when it’s lain on the soil surface for several weeks,” he says.

A high-quality pellet is a prerequisite for successful slug control and Peter points out that formulating a palatable pellet is no easy task. Sluxx HP contains durum wheat, which seems to be a favourite meal for slugs, and Neudorff have experimented with several different qualities of flour to find the one which they find the tastiest.

But unlike other pellets, Sluxx HP doesn’t contain a specific attractant which Peter considers isn’t necessary in their formulation, an opinion based on both field experience and scientific research.

“Studies have looked extensively at slug behaviour and have found that slugs don’t detect food over a long distance. They move randomly and test the objects they encounter by chance using their lower pair of sensory tentacles, only then do they decide whether to eat them or not,” he explains.

“In laboratory studies, there’s been no significant difference in slug preference for Sluxx HP or a competitor’s ferric phosphate product, which corresponds with our own research that shows no increase in efficacy from including attractants in the formulation.”

Because there is no plan to their movement it means that the number of baiting points is an important factor, he adds.

“The more baiting points there are, the greater the chance of a slug coming across a pellet as it moves around looking for something to eat. The 7kg/ha rate of ferric phosphate gives approx. 60 pellets/m2, which provides an additional 17 baiting points when compared with a competitive product.”

Another of the challenges in formulating a ferric phosphate pellet is to maintain its palatability while also being able to withstand periods of rainfall without losing efficacy. The fact that Neudorff’s ferric phosphate formulation is used for snail control in the paddy fields of Japan bears testimony to the fact that this balance has been successfully achieved.

Understanding what the research means in practice is the key to getting the best performance out of ferric phosphate, believes Laurence Power, technical manager for Certis.

“Research has shown that when slugs feed on ferric phosphate they go below ground and die which is why there is no visible sign of dead slugs on the soil surface,” he explains.

The slug behaviour is a direct result of the mode of action of ferric phosphate which acts on multiple organs within the slug, adds Laurence. “At first, ferric phosphate irritates the slug’s mouth and crop, causing the slug to stop feeding. Ferric phosphate leads to pathological changes on cellular level in the hepatopancreas, the central organ of the slugs, causing the slug to effectively ‘internally bleed out’.

“The slug can no longer produce the slime that it needs to keep cool, so it retreats underground in an attempt to thermo-regulate,” he adds.

That means the symptoms of successful slug control are at the opposite end of the spectrum to metaldehyde, which causes excess slime production and dead slugs are readily found on the soil surface.

“With ferric phosphate the opposite happens, with the only visual sign being that the crop is no longer being damaged so is growing away,” explains Laurence.

Over the many years of trials that have been carried out, ferric phosphate has consistently demonstrated an efficacy on a par with metaldehyde, he explains.

“But the Sluxx formulation has a key advantage over metaldehyde when temperatures begin to fall. At 20⁰C both active ingredients offer similar levels of slug control, but at 10⁰C the ferric phosphate formulation is much more effective.

“The low temperature performance is important, especially since the loss of neonicotinoid seed treatments and a shift to later planting to help control BYDV and blackgrass in crops,” says Laurence.

To get the best levels of slug control it’s important to adopt cultural approaches to slug control and only use slug pellets as the last resort in an integrated pest management strategy, he highlights.

“IPM is critical to successful slug control and growers need no reminding of the crops which offer slugs an ideal habitat and food source. OSR is a classic, being very vulnerable to seedling attack between emergence and the four true-leaf stage because the growing point is exposed.

“But OSR also offers slugs the ideal habitat for them to survive and breed throughout the year. OSR debris provides a good food source, which makes successive cereal crops more vulnerable to attack.”

Recognising the fields at increased risk is a first step and then planning a multi-pronged strategy to limit slug damage.

“Cereal seed in loose or cloddy seedbeds is particularly undesirable as the UK’s most important slug species, the grey field slug (Deroceras reticulatum), is responsible for most of the grain hollowing and is very active in the layer of soil where the seed is placed.

“A single slug can hollow out 50 seeds during the vulnerable stage, which means cultivations play an important part in slug control,” comments Laurence.

Cultivations directly impact slug populations in a number of ways, including direct mechanical damage and burying the plant debris that acts as a food source and provides shelter.

“The aim should be to create a fine tilth which inhibits slug movement and reduces the spaces where slugs can hide. My view is that setting up the machinery properly and working the ground at the right time is much more important than the type of tillage practiced.

“There’s an old adage that the most important day in a plant’s life is the day you plant it, which is also very pertinent when it comes to slug control,” he comments.

In the effort to produce a fine and firm seedbed, rolling is an important tool and is known to reduce slug damage. “There’ve even been anecdotal reports that rolling at night can help squash slugs while they are on the surface feeding,” he adds.

Even having adopted cultural controls there may still be a significant risk of slug damage in some fields. In these situations, Laurence advises monitoring the slug population using traps and then applying pellets prior to feeding damage and ahead of any predicted periods of wet weather.

“Don’t allow slug populations to build – go early and go hard,” he suggests. “When you adopt this approach with ferric phosphate, repeated applications are often not always necessary.”

Sluxx HP is manufactured using a wet process which provides the pellets with high levels of durability and rainfastness, as well as the consistency critical for good ballistics, explains Laurence.

As with all pesticides, formulation is only half the story and good application is just as critical.

“Always recalibrate the applicator when using ferric phosphate, even between different brands as the pellet size varies even if the application rate is the same. Quads will spread differently to a sprayer-mounted applicator because of the difference in battery power, which is another factor to be aware of,” he comments.

Electronic tagging reveals slug behaviour

Dr Keith Walters, professor of invertebrate biology and pest management at Harper Adams University, has been looking at how slugs behave after ingesting ferric phosphate.

“Slugs spend a large proportion of their life under the soil surface, so when you’re trying to look at how they move and disperse around a field, it’s very difficult to see what they’re doing. We came up with the idea that if we used radio-frequency identification (RFID) tags, the same kind of technology that the vet uses to microchip your cat or dog, and put them into slugs then we could track them underground using an antenna, without having to dig them up,” he explains.

To carry out the study, a large number of grey field slugs were collected from a commercial field and brought back to the laboratory where they were left to acclimate and then divided into six groups. Slugs were then released five at a time into plastic containers, filled with soil from same field as the slugs were captured, which had an airspace left above them.

“The results were remarkably clear,” says Keith. “The slugs which hadn’t eaten any ferric phosphate seemed to behave quite normally, in accordance with what we know they do in the field. They would go periodically beneath the soil surface to hide from predators and avoid dehydration but would come to the soil surface at regular intervals in order to feed.

“The group of slugs that fed on ferric phosphate behaved very differently and they eventually retreated below the soil surface and didn’t come back up again. For the first three hours we looked at them intensively and their behaviour didn’t seem to differ from the control group.

“But thereafter they began to quite rapidly move below the soil surface and, after the first day, 50-60% of the slugs were below the soil surface. That proportion increased until almost every slug was below the surface 2-3 days after they’d eaten ferric phosphate.

“At the end of the experiment, we dug out every last slug and found very high mortality in the group which had ingested the ferric phosphate pellets.”

Research Briefing

To help growers get the best out of technology used in the field, manufacturers continue to invest in R&D at every level, from the lab to extensive field trials. CPM Research Briefings provide not only the findings of recent research, but also an insight into the technology, to ensure a full understanding of how to optimise its use.

CPM would like to thank Certis for sponsoring this Research Briefing and for providing privileged access to staff and material used to help bring it together.

As the market moves from metaldehyde to ferric phosphate slug pellets the principles of good slug control remain exactly the same. Certis have been marketing ferric phosphate pellets in the UK for over 10 years and today’s high-quality products (like SluxxHP) are equally effective as metaldehyde.

Adopting an integrated pest management approach will improve overall control. Cultural controls are a key part of IPM but choosing the right quality of pellet is also important. A good slug pellet needs to be spreadable, durable, and palatable to work well.