AHDB: Following the Slug Trail

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AHDB’s Crop Protection Scientist, Charlotte Rowley, assesses new methods of control against slugs.

Following the announcement from Defra on the withdrawal of metaldehyde in 2020, thoughts are again turning towards slug management. The good news is that there is an effective alternative: ferric phosphate has been shown to be as effective as metaldehyde in controlling slugs.

Unlike metaldehyde, slugs die underground rather than on the surface, meaning it may not be as obvious when an application has been effective. The bigger issue, apart from differences in product cost, is that relying on only one active ingredient leaves the industry in a vulnerable position when it comes to slug management. Now more than ever it is important to use every tool in the armoury to ensure that molluscicide applications are kept to a minimum. Using cultivation as a control Cultivation is generally regarded as one of the more effective forms of cultural control, therefore no-till farmers may be more concerned than most. It’s important to remember though that there is no silver bullet for slug management, and a whole system approach is needed.

So while cultivation is bad news for slugs, it can also be bad news for predatory ground beetles that help keep slug populations in check. Ground beetles, as the name suggests, forage and hunt for their food on the soil surface and have soil dwelling larvae, and therefore tend to be vulnerable to soil disturbance. Keeping pesticide use to a bare minimum and providing habitat in the form of diverse field margins will also help maintain numbers of these beetles and other natural enemies. Cover crops have a role to play Cover crops can provide much needed habitat for beneficial insects, however they have a bad reputation when it comes to slugs.

It is often reported that cover crops make slug damage worse, creating a sheltered, moist, food-rich habitat that slugs love. However, research suggests that the type of cover crop can make a huge difference when it comes to slug pressure, with some crops such as ryegrass thought to be less attractive to slugs due to the high silicone content of the leaves. There are other insects too that are benefitted by increasing plant diversity, and while they may not feed on slugs directly, they provide food to support populations of slug-eating birds and carabids. What can the farmer do to help? However, as helpful as they are ground beetles cannot do all of the hard work.

This is where farmer intervention is needed to get the crop off to a good start. A fine, consolidated seedbed (e.g. through rolling) means that the seed is more inaccessible to slugs and helps get the crop established and able to grow away from damage. Anecdotally, some farmers have had success protecting their wheat from seed hollowing by sowing it directly into the stubble of the previous oilseed rape crop, allowing the volunteers to act as a trap crop until the wheat is no longer vulnerable to attack. Other farmers report that raking of straw and crop debris can reduce the risks of slug damage in min- or no-till environments. This might work by exposing slugs and their eggs to the sunlight, drying them out and making them more accessible to predators, as well as reducing the number of damp hiding places on the soil surface.

What we know and what happens next? As for oilseed rape crops, we know from work on cabbage stem flea beetle that the plants can be extremely resilient to damage and establishment is key. A DEFRA-funded desk study done by ADAS in 2014 suggested that for OSR sown at 60 seeds/m2 a single dose of metaldehyde only became cost effective at moderate slug pressures (where 3050% of plants were expected to be lost). Where pellets are more expensive (as with ferric phosphate) or slug pressure is lower, it might make more economic sense to instead increase seed rate to mitigate against slug damage.

While this doesn’t take into account further losses from e.g. pigeon damage, it demonstrates the ability of the crop to compensate for losses. Unfortunately for farmers, slugs tend to congregate in patches which can lead to an uneven distribution of damage, making crop compensation less effective. AHDB PhD student Emily Forbes, at Harper Adams University, has been modelling these patches in the hope that this will lead to the ability to target control options more effectively. “My PhD project has identified and confirmed the potential for patch treatment of slugs” said Emily, “further work is required to refine the methods for locating the patches without the requirement for trapping in order to make the findings commercially viable”.

Her report, due this spring, will shed light on some of the slug behaviours leading to patch formation. Other research to look out for this year will be from AHDB’s Nuffield scholar Jenna Ross who has been travelling the world learning about slug and snail management in other countries, in a range of agricultural and horticultural systems. Both of these projects will help push towards improving or increasing the current range of control strategies which will be essential if we are to have effective and sustainable slug management in the future.

For further information, please visit: cereals.ahdb.org.uk/crop-management/pest-management.aspx

Or to download our slug control factsheet: ahdb.org.uk/knowledgelibrary/integrated-slug-control

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You can read the full Direct Driller magazine online here: https://issuu.com/directdriller/docs/direct_driller_magazine_issue_5
 

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