Slug solutions – CPM – David Howard

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.

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.”

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.”

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.

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.”

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.”