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Longer term thinking the way forward for slug control

Even very low levels of slug damage in potatoes will result in instant rejection at packers and processors, says Prime Agriculture's potato specialist Colin Smith, advising growers in West Norfolk and the Cambridgeshire Fens. So what are his plans to keep slugs at bay this season?

Even very low levels of slug damage in potatoes will result in instant rejection at packers and processors, says Prime Agriculture's potato specialist Colin Smith, advising growers in West Norfolk and the Cambridgeshire Fens. So what are his plans to keep slugs at bay this season?


"We have to get slug control right in the potato crop or it will result in disaster, the cost of pellets is dwarfed by these potential losses. This has always been true but now is even more crucial given the current financial pressures on potato growers," he says. "Nothing quite gets the thumbs down from Quality Control departments of packers and processors so quickly as the presence of slug holes (or wireworm) in a sample. Instant rejection invariably results."


Unlike the cereal sector where metaldehyde is king, methiocarb was the market leader in potatoes, primarily because it was a good quality pellet that gave a long period of protection and had good activity on the garden and keeled slugs, largely held as being most damaging to potatoes. Although some growers still have supplies of methiocarb to use up, its withdrawal means a wholesale change in tactics for slug control in potatoes.


"Most potato farmers and agronomists tended to use methiocarb products as the mainstay of their slug control programs, I know I always have," says Colin Smith. "Metaldehyde was mainly used to supplement methiocarb to keep within label doses, but I had tended to use ferric phosphate to do this as well because it has none of the restrictions of metaldehyde. We all want to keep metaldehyde, so using less of it makes sense."


"The loss of methiocarb was a major blow but, with ferric phosphate and metadehyde still available, this is a manageable problem. After all, we have had to learn to deal with the loss of major actives and may well have to face a lot worse yet."


Independent potato specialist John Sarup of Yorkshire based Spud Agronomy agrees. “I'm encouraging my growers to try Sluxx (ferric phosphate) because it's really the only viable alternative to methiocarb being a quality pellet with comparable efficacy and rainfastness characteristics, together with the added advantage of no restrictions in use  (up to 28kg/ha/crop).


"We don't want to start using more metaldehyde in potatoes and end up being the nail in its coffin. Now's the time to learn more about ferric phosphate because, realistically, it may be all we have in a year or two.”


An increase in usage is a potential problem to the water industry. Although most of the metaldehyde 'spikes' in raw water supplies have occurred over the winter period, there have been summer peaks which have correlated with metaldehyde application to potato and maize crops.


Catchment management studies carried out by the water companies have shown pretty conclusively that the best way to keep metaldehyde below threshold levels in drinking water is to manage its use upstream of their drinking water abstraction points, and this may prove to be the way forward to improve water quality and solve the problems currently posed by metaldehyde, believes Dr Katherine Cherry, Catchment Management Planner for Severn Trent Water.


"Trials were launched back in October 2013 and results have been very encouraging," she says. “We have seen significant improvements in water quality.” Describing one of the projects in Warwickshire, Farmers as Producers of Clean Water, Dr Cherry explains, “This approach encourages growers to work together in the catchment and make their own management decisions, rather than follow an approach prescribed by Severn Trent Water.”

The scheme allows farmers to choose the mitigation options to reduce metaldehyde that are best suited to their farm business. Mitigation options such as producing fine, consolidated seedbeds, implementing shallow cultivation, use of trapping to assess slug thresholds, applying soil management practices, making field risk assessments and using alternative controls such as ferric phosphate are all examples of alternative ways to better manage metaldehyde application.


“There was a 90% reduction in metaldehyde peaks compared to a neighbouring catchment. The results promoted us to expand the trial to another catchment in Shropshire with equally impressive water quality improvements,” explains Dr Cherry. In another Severn Trent Catchment trial on the Derbyshire / Leicestershire border, an alternative approach has been employed. Farmers are encouraged to use the active ingredient ferric phosphate (Sluxx) for their slug control, resulting in a ‘metaldehyde free’ catchment.


“Facilitating participation through one to-one visits and evening workshops, sixteen farmers in the catchment have been involved in the trial and the support of agronomists has been key to its success,” says Dr Cherry. “Metaldehyde levels have remained consistently below the drinking water standard at the water treatment works for two years now,” says Dr Cherry, adding that metaldehyde exceedances were seen at other water treatment works not involved in the trial.


Independent agronomist Graham Partington, looks after many of the crops in the Staunton Harold ‘metaldehyde free’ catchment. “Severn Trent Water has taken a very sensible and proactive view. They want to help and work together with farmers to solve the issue we share with metaldehyde.”


With no previous experience of using Sluxx, switching to a relatively new active was a step into the unknown, but according to Graham, slug control has been ‘exceptional’.


There's no doubt that experience with ferric phosphate in potatoes is relatively limited, says Colin Smith. "Where I have used ferric phosphate I have to say that I have had satisfactory results.  I have grown organic crops of potatoes where you could only use ferric phosphate and it seemed to do a decent job."


Longer term strategies to reduce slug populations throughout the rotation is something potato growers should be thinking about, believes Colin Smith. "In arable crops, cultural control practices are well established such as drilling depth, rolling, trash removal etc.  I will use what cultural controls I can in my potato crops, but with many fields rented this is more difficult and you are dependent on what your landlord’s practices are," he says.


"Cultivations do reduce slug populations - pre-planting and pelleting well before planting on milder days in early spring or autumn can be very effective as you only have to kill them once."


One of the major risk factors for slugs is the inclusion of oilseed rape in the rotation. "Unfortunately on many of the more bodied soils where slugs are likely to thrive, oilseed rape is a crucial part of the rotation so no joy there in reducing risk, and then there’s the three crop rule as well…"


Where slugs are active, the timing of pellets will be critical and getting them on at the right time is probably as important as the active ingredient used, says Colin Smith. "On my sluggiest land, applying little and often is my favoured approach because with blight sprays going on every 7 days you can easily do this and I will continue this approach with ferric phosphate."


John Sarup also suggests growers pay close attention to timing their applications of slug pellets. "Getting pellets on just before the canopy closes is the key timing. At this timing, pellets can still reach the soil and slugs are encouraged on to the surface by the shady and moist conditions the canopy provides."


Recognising that grower experience is limited, Severn Trent Water has also undertaken ferric phosphate trials so growers can gain understanding and confidence in the active, which kills slugs in a different way to metaldehyde.


A key point is that the active ingredient stays in the pellet after rain or irrigation, says Certis’ Morley Benson. "You won't see dead slugs on the soil surface and slime trails, which are the hallmarks of metaldehyde treatment. It’s a key difference that’s important to understand.


“After slugs have eaten a ferric phosphate pellet it causes pathological changes to its digestive system which means they stop feeding immediately but have time to retreat underground before they die."