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Slug solutions – CPM – David Howard

Ahead of the ban on metaldehyde next spring, growers have been looking at different ways to control slugs in both cereals and potatoes ...

CPM explored some of the key challenges and options in a recent survey with De Sangosse. By Charlotte Cunningham

Slugs are reputed to be cereal and potato growers’ biggest in-field foes, with a long trail of destruction often following an infestation. What’s more, control is challenging. And with the imminent removal of metaldehyde, growers are having to be more creative with their strategies.

But what exactly is the best way of getting on top of slugs?

The secret to winning a battle is knowing your enemy, and in a recent survey carried out by CPM/De Sangosse, 89% of growers correctly identified the grey field slug as the most common species in arable land.

However, it’s important not to overlook the potential impact of less common species, says Phil Carpenter, commercial manager at De Sangosse.

“Grey field slugs are clearly the main concern here, but it’s important to remember that we do see a range of species – just not in the same numbers.

“Species like the Budapest slug or keeled slug (Tandonia spp) can be more of a problem in potatoes, sugar beet and high value horticultural crops, so think about your cropping and what your threat level is likely to be as a result.”

Threshold limits

With many farms operating mixed cropping programmes, using traps to assess the risk of slug damage can cause confusion with regards to the threshold limits. This was reflected in the survey, with 40% of growers saying they believed the limit in potatoes for triggering an application of slug pellets is four slugs/trap, when in fact, it’s much lower than this says Phil.

“Four slugs/trap is the threshold for cereal crops. However, in potatoes, the damage can be so severe and even the slightest mark could cause a rejection, so it’s important to stick to a one slug/trap threshold for triggering an application.”

Interestingly, 17% of growers said they didn’t know what the appropriate limit was. “The most important advice here is to be aware of the situation and which crop you’re targeting – different crops have different thresholds, and this is based predominantly on the damage that slugs can cause,” he adds.

“In terms of the traps themselves, I recommend checking as close to dawn as possible as this is likely to give the most accurate indication of your threat level.”

David Howard, head of integrated crop management at Hutchinsons, says that having good traps in each field helps growers to be ahead of the game when it comes to slug control.

“Effective trapping is a really important element of slug control -particularly in high value crops. When thinking about risk and what might trigger a pellet application, I also recommend considering previous cropping to give you an idea on likely hosting.”

If traps show in-field variations in numbers, Andrew Rhodes, Harlow Agricultural Merchants, says that it should be presumed that the highest number of slugs is representative of the severity of the overall burden.

“Good coverage is crucial for slug control and while individual traps may vary, if you’re looking for the warning signs to trigger an application, this should be based on the highest number found in a single trap. For example, if you have a potato field where some traps show one slug and others show none, an application is still necessary to protect the crop.”

Though ferric phosphate pellets have filled the gap following the phasing out of metaldehyde-based products, the way in which they work can be quite different.

Evaluating the effectiveness of a ferric phosphate application

When it comes to evaluating the effectiveness of a ferric phosphate application:

  • 82% of growers said they look for healthy plants with reduced feeding damage;
  • A small percentage (5%) revealed they look for dead slugs on the surface;
  • A further 4% noted that they look for mucus trails to check for signs of activity.

“The key thing here is that growers are starting to glean a good understanding of how ferric phosphate works,” says Phil. “However, dead slugs and visible mucus trails are a bit of a hangover effect from metaldehyde, and they’re not signs we’re looking for so much with ferric phosphate pellets.”

David agrees: “Ferric phosphate works slightly differently. Effectively, it makes slugs ill, so they go back into the soil, meaning there won’t always be bodies on the surface to see. For me, the best way of evaluating whether your ferric phosphate pellets have been effective is to observe the crop and see what’s actually changed since your application. Monitoring new leaves is a good way of telling whether there’s recent damage or not.”

Regular crop walking will also help this assessment, adds Andrew. “If you can aim to walk a field a week, you’ll soon start to see a difference in healthy and non-healthy crops.”

Application methods affects efficacy

And it’s not just product choice that affects efficacy, but also application methods. The survey showed that:

  • 83% of grower use a quad/ATV mounted applicator;
  • While 26% use sprayer mounted;
  • And 17% opt for tractor mounted applicators.

“All of these methods are relatively effective, so a lot of it depends on the circumstances at the time of application,” says David.

“With a quad/ATV, it’s easier to travel at difficult times and leaves less of a footprint on the ground. On the other hand, if an application is needed around the same time as you’re going through with a tractor/sprayer, then it makes sense to use a joint application approach.”

Phil says that whatever method growers choose, the key thing is to ensure the applicator is calibrated.

Application width also important

With this in mind, application width is also important. The survey showed that:

  • 36% of growers spread pellets at 12m;
  • And 37% at 24m;
  • While 6% push this to 36m

“Coverage is key, and the wider we go, the more difficult this becomes,” says David.

“It all comes down to how your applicator is set up and the product you choose. When you get to bigger widths, the ballistics of pellets vary and this itself will be different depending on the individual pellet. Good application and coverage are fundamental to an effective application, so it’s important to keep this in mind.”

Andrew agrees and says, personally, he doesn’t advocate going above 12m applications. “In my view, 36m is just too far for the majority of pellets. If you had a sudden gust of wind at this width, then your whole application could just be for nothing.”

An integrated approach

Integrated pest management has been an important part of day-to-day practice for some time now and Phil says the benefits are clear when it comes to preventing against slug burdens and infestations. “In my view, its good practice to think about and optimise other cultural methods of control before just jumping in with a chemical solution.”

Maintaining a fine, consolidated seedbed and using a stubble rake to disturb crop debris were flagged as the most well-known cultural controls. Over half of growers (53%) also noted introducing beetle banks to support predatory beetle populations.

This approach isn’t commonly practiced, but could potentially have some mileage, reckons David. “It’s something that’s becoming an increasingly important part of IPM. Beetles are pretty good predators of slugs, and if we can find a way to support them then that’s only going to be a good thing.

“That said, the benefits are yet to be proven. However, if you can ensure your practices aren’t doing anything that reduce beetle levels at least, then you could get some additional support on the control front.”

But Andrew warns that this approach may have its limitations. “It’s important to remember that roving beetles will only come out a few meters from margins and beetle banks, so when you’re in the middle of the field it might not be enough.

“Also, it’s mainly just the juveniles that get eaten. So while beetle banks may have their benefits, it’s important to be realistic about just how influential they may be.”

To apply, or not to apply?

The survey also presented growers with a scenario and asked them what course of action they’d be likely to take in this situation:

“Your crop is vulnerable and the treatment threshold has been reached. An application is necessary and there’s moisture in the air and there’s a threat of wet weather on the horizon-which of the following is your preferred course of action?”

Over half of growers (51%) said they’d use a wet-processed durum wheat flour-based pellet, while 42% said they wouldn’t make an application at all. So what’s the best approach?

“While traditionally the metaldehyde stewardship programme would have advised not to apply in wet weather, if the treatment threshold has been reached and the crop is at risk then it’s important to make an application with ferric phosphate,” says Phil.

The product of choice in this situation will affect the effectiveness of the pellet, he adds.” If there’s a threat of wet weather, then a wet-processed pellet will withstand pressure much better.”

David says that making an application ahead of bad weather can put the pellet at risk of degrading, so it’s understandable why some growers would choose not to.

However, Andrew says it’s really important that crops are protected, regardless of the weather, if the threshold has been reached. “If it’s wet, or there’s wet weather ahead, then that’s the perfect environment for slugs, so it’s crucial to make an application to ensure the protection of your crops.”

Like Phil, David stresses that the type of pellet will play an influential role in the longevity of a pellet if it’s applied in less ideal conditions.

“Wet-processed pellets last longer and don’t break down as easily, so if you do need to apply pellets ahead of bad weather, then this may be a better choice.

“Sometimes, decisions are dictated by what is most cost-effective, but perhaps the more important consideration is how effective they are against slugs.”

The survey also asked growers which were the most important attributes of a slug pellet, but there was a fairly even split between the four options: spreadability, palatability, attractiveness, and persistence.

Attractiveness is something De Sangosse specialise in, says Phil, with an in-house recipe featuring a blend of two brassica extracts to entice slugs. “We do believe this is really important-offering slugs something that they enjoy no doubt makes the pellets more attractive.”

With regards to persistence, this will also vary largely on the pellet itself, adds Phil. “The way in which a product is made will have a huge impact on persistence. Comparing wet durum flour and dry wheat flour, for example, if you think of the wet durum type pellets, they’re comparable to how pasta behaves in water-it keeps its shape and size despite the water, which is basically what we’re trying to achieve with an effective slug pellet application.”

David says attractiveness is a relatively new element, so where this isn’t especially accounted for, it’s essential to make sure there’s a good, even spread of pellets to optimise control.

“Slug control is a bit horses for courses, but as long as you’re prepared, using good trapping and crop history to inform decision making-teamed with cultural controls and an effective pellet – you should be able to get a good level of control.”

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