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Keeping one step ahead of slugs is key

Use all the tools in the toolkit to stay ahead of slugs this autumn, advises Dr David Ellerton, technical director at Hutchinsons. Even though this year’s crops are still to be harvested, now is the time to turn thoughts to assessing slug risk prior to drilling crops.

Use all the tools in the toolkit to stay ahead of slugs this autumn, advises Dr David Ellerton, technical director at Hutchinsons. Even though this year’s crops are still to be harvested, now is the time to turn thoughts to assessing slug risk prior to drilling crops.

“Identifying risk is an essential part of protecting susceptible crops against slug damage. Although weather conditions over the next few months will play an important role, there's no substitute for putting traps out in fields before any cultivations and monitor for slug activity to get an idea of what's happening on a field by field basis," says Dr Ellerton.

Slug traps, containing a handful of layers mash to attract slugs, can be put out in either the standing crop or in stubbles after harvest. Although the official recommendation is to place 9 traps in a W-formation across the field, Dr Ellerton believes that it's practical and just as useful to target traps at heavier, wetter areas of the field or areas that have had historical slug problems.

"It's important to get some idea of slug activity before any cultivations have been made. Threshold numbers that will trigger a treatment in the growing crop are 1 slug per trap prior to oilseed rape and 4 slugs per trap for cereal crops," he says. "Cultivations will disrupt slugs and some will die, but once cultivations are underway it's difficult to get a true picture of likely slug populations in the next crop."

Each cultivation technique has its good and bad points, points out Dr Ellerton. "It's not as simple as it may first seem to say one method reduces the risk of slug damage more than another. Ploughing probably kills more slugs but is often associated with cloddy seed beds, something to be avoided in slug prone situations.

"Even when direct drilling, where you may expect minimal slug damage because the soil hasn't been disturbed, slugs can move along the drill slots and attack seeds and plants," he says. "Min-till can be a good alternative, as long as the seedbed is properly consolidated and seed is drilled deep enough to mitigate any problems with trash, which can be attractive to slugs."

Hollowing of seed can be a big problem, especially when soils are heavy, wet and cloddy. "As well as doing all you can culturally, treating seed with insecticidal seed dressings containing clothianidin can help reduce grain hollowing," suggests Dr Ellerton.

Signal seed treatment (cypermethrin) has also recently been observed as having a useful effect on small slugs in trials work.

Dr Ellerton doesn't believe that treating fields with slug pellets, before crops are planted, is particularly effective, preferring to apply as soon as possible after drilling. "It's important to treat crops only where the risk is high enough," he says.

"Regularly monitoring fields by having a look for slugs under clods of soil, in addition to trapping, can give an idea of slug pressure. You may also be able to see clusters of translucent slug eggs when lifting clods or under surface trash, indicating slugs are breeding."

Traps placed in fields need to be monitored during early establishment until the risk of economic damage from slugs has passed. In oilseed rape this is when the crop has reached the four leaf stage and in cereals, early tillering.

The differences in modes of action

Understanding that the way Sluxx (ferric phosphate) works is very different to metaldehyde is a crucial thing to appreciate, explains Alan Horgan, technical officer at Certis, manufacturer of both active ingredients.

"When slugs eat a pellet containing metaldehyde, they produce excessive amounts of mucus and dehydrate," explains Alan. "The kill is very visual, with slime trails and dead slugs on the soil surface."

Ferric phosphate does not leave such obvious evidence to prove it's working, which can be disconcerting for new users, he explains. "When a slug eats a ferric phosphate pellet, iron displaces calcium in the gut, paralysing the system which stops the slug from feeding on the crop instantly. The slug basically starves to death.

"The slug is unable to feed right from the moment it eats a Sluxx pellet, so crop damage ceases regardless of weather conditions. The experience we have gathered in trials work over the last few years, shows that slug death usually occurs within a day, even in cold conditions, which is faster than initially thought," says Alan.

With ferric phosphate, there is no visible kill of slugs on the soil surface after application. "Slugs move away to die and tend to migrate underground, possibly because they are under stress so will tend to move away from light into dark, damp crevices in the soil," explains Alan.


"There's no doubt that ferric phosphate is just as effective as metaldehyde. We know it works, but in a very different way to metaldehyde," says Dr Ellerton. "If there is any risk to water, growers should automatically be using ferric phosphate.

"You can assess your own risk by looking at the Environment agency's 'What's In Your Back Yard' website ( and by considering the three S's - soil type, slope and proximity to a stream in each field. If you can get away with applying less than 210g/ha metaldehyde then use the 160g rate," he recommends.

"We have to use metaldehyde responsibly to minimise peaks in drinking water catchments. We can’t afford to lose yet another molluscicide and the bottom line is that, if everyone doesn't stick to using metaldehyde within the guidelines, it will go."

Grower case study

Farming in the Cambridgeshire Fens, Simon Wilcox has soil types ranging from gravels to black organic peat. With a rotation of oilseed rape, peas, sugar beet and winter wheat, slugs are becoming an increasing problem on the farm.

"We aim to drill rape in the first week of September and wheat in the second week of October and have gone away from ploughing routinely, preferring shallow cultivations but with the result that we have more trash.

"We monitor for slugs and if required, apply 7kg/ha ferric phosphate as soon as the crop has been drilled," explains Mr Wilcox. "We've moved away from metaldehyde. We're in the Fens so we have dykes everywhere."

But water wasn't the only reason for a switch to ferric phosphate, Mr Wilcox believes it's important to move with technology and Sluxx is a quality pellet that spreads well. "Ferric phosphate is from a natural source and we have a lot of walkers with dogs and 4 m ELS field margins. We need to use pellets responsibly and ferric phosphate takes away any risk with these factors in mind."

Using ferric phosphate requires a different mindset to using metaldehyde, believes Mr Wilcox. "You' don't expect to see dead slugs, it's all about baiting points. If the pellets have disappeared, it's because the slugs have been active and have gone underground to die." This effect will be more visual when a brighter dye is added to a new formulation, available soon as Sluxx HP, he adds.

In a normal season, Mr Wilcox would expect a full dose of ferric phosphate to pretty much do the job. "If more pellets are required, I may follow with a lower dose 10 days later and this then covers the crucial 3-4 week window after drilling.”