Crop health update: Post-metaldehyde slug management

20 February 2019

Defra recently announced a ban on metaldehyde use for outdoor use from spring 2020, as a consequence of long standing concerns over it getting into drinking water. At first sight this announcement looks like yet another loss from the pesticide tool chest, and it comes on top of the loss of chlothianidin as a seed treatment which gave some reduction in slug grazing of cereal seed.  This leaves us with just a single option for slug control in the form of ferric phosphate. It is never great to be down to just one tool, but the  better news is that trials by SRUC and others comparing ferric phosphate efficacy to that of metaldehyde show that, in the main, ferric phosphate tends to be as effective as metaldehyde.

Ferric phosphate is also broadly equivalent in terms of cost so short term, the impact in terms of cost of production is pretty neutral. There is a tendency for slug pellets to be used at reduced rates, and this is particularly true of metaldehyde and less true of ferric phosphate but this has more to do with users conforming to the Metaldehyde Stewardship Guidelines that limit the max dose that can be applied and also the total amount in a calendar year (and from Aug-end of the year) than it does with efficacy. Some of the perceptions about the efficacy of metaldehyde may come from the fact that with its mode of action you get a very visible confirmation it is working because lots of dead slugs are visible on the soil surface. Ferric phosphate works differently so causes ‘invisible’ deaths which is potentially less reassuring to a user.  Ferric phosphate molluscicides affect the slugs’ digestive system by disrupting calcium metabolism, quickly causing a cessation of feeding, and slugs bury themselves in the soil where they subsequently die. This may be one reason why ferric phosphate use by growers lags behind that of metaldehyde at the moment.

Longer term there is some risk should resistance arise to this single site mode of action active, and ferric phosphate (although of lower mammalian toxicity to metaldehyde) has some environmental impacts of its own so integrated and targeted management measures should still be used, to minimise reliance on chemistry.

The use of integrated approaches to slug management such as the use of bait traps, minimum tillage, soil management, coupled with judicious use of molluscicides is sensible. Such methods will not give adequate or consistently reliable control of slugs so will not directly replace the use of chemical slug pellets however they could go some way towards reducing application rates and application numbers.  Slug trapping can be used to assess the risk from slugs in a particular field prior to sowing, particularly for winter crops. Traps should be placed at multiple points in a field and checked in the morning 1-2 days later.

2019 will therefor be our last season of metaldehyde use and care should be taken in its use. Best practice guidelines were developed by the Metaldehyde Stewardship Group (MSG) to minimise environmental impacts when using metaldehyde slug pellets and, in particular, protect water (www.getpelletwise.co.uk). These guidelines advise that no metaldehyde pellets should be allowed to fall within a minimum of 10 metres of any field boundary or watercourse. For growers this may well lead to a direct switch from metaldehyde to ferric phosphate use, as it is more practical to apply ferric phosphate to the whole crop than to treat the crop edges with ferric phosphate and the rest with metaldehyde.

Fiona Burnett and Andy Evans, SRUC for the Farm Advisory Service

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