Slugs are a big threat to outdoor crops in the damp UK climate and growers there will soon be losing their most widely used chemical control, metaldehyde. We get some expert opinion on how they might cope after its withdrawal
A mix of integrated pest management (IPM) principles and well-timed applications of ferric phosphate pellets will be key in maintaining good slug control as the UK phases out key molluscicide metaldehyde by June 2020.
In December 2018, the UK’s Government’s Department for Agriculture and Rural Affairs (DEFRA) announced it would be withdrawing all outdoor uses of metaldehyde slug pellets due to its “unacceptable” risk to birds and mammals
The news caused alarm amongst crop producers, as the loss of the active ingredient leaves farmers with just ferric phosphate-based pellets as a chemical intervention.
However, Newcastle University slug control expert Gordon Port, who has been studying the pest for many years, believes growers should not worry unduly about the impending ban.
He sees a basic understanding of slug ecology to inform cultural methods and ferric phosphate applications as enough to maintain the slug control currently achieved with metaldehyde in the mix.
Firstly, Dr Port says landscape or rotational changes can help reduce pest risk, with beetle banks and field margins helping to encourage natural predators that are proven to reduce slug populations.
Furthermore, risk is much higher after crops such oilseed rape, so reducing its frequency in the rotation and avoiding planting slug-susceptible crops immediately after will reduce damage.
Cultivations are also an effective means of limiting the slug’s impact, either physically killing or exposing them to predation on the soil’s surface. Good seed-bed consolidation post-drilling will pest’s movement and speed up crop establishment, reducing the risk period.
“Some growers have taken to rolling at night when slugs are active on the surface and achieved some effective suppression of crop damage,” explains Dr Port.
Once cultural controls have been exhausted, slug populations should be monitored using bait traps and a chemical treatment applied once economic thresholds are exceeded and slugs are active.
Beyond June 2020, growers will have to use ferric phosphate and as it has a different mode of action to metaldehyde, it will require a different mindset from growers used to metaldehyde.
When slugs ingest metaldehyde, they quickly die and evidence can be seen on soil surface, but with ferric phosphate the active ingredient stops them feeding and they retreat to shelter beneath the soil surface before dying, leaving little evidence of mortality.
This has led to some scepticism about the efficacy of ferric phosphate, but Dr Port says the two actives are equally as effective and the more important factor in effective chemical control is formulation quality.
Extensive behaviour monitoring has shown that slugs never veer off towards a food source, countering manufacturer claims that “attractants” in a formulation draw them to the bait.
Instead, slugs bump into pellets when active on the soil surface and if the food is palatable, they will continue to feed until their crop is full.
“So, the trick with molluscicide baits is to have something that the slug enjoys eating.”
Premium ferric phosphate products like Sluxx HP are made using patented Ferric Field Technology, incorporating pasta dough made from 100% durum wheat flour, food grade active ingredient, a food grade anti-mould agent and patented stabiliser.
At the opposite end of the spectrum, some pellets are made from a whole feed wheat grist, leading to uneven size, breakage and poor spread pattern during application. They also break down and go mouldy much faster under wet conditions.
Certis technical manager Lawrence Power says if a slug comes across a mouldy slug pellet, it will reject it and your crop is then at risk.
“It is really important to make sure the pellet is high quality, robust and stays mould free for a period of time, increasing the chances of a slug bumping into the bait and consuming it all.”
Successful control is also about having the right number of baiting points/m2, with a greater number of pellets improving the chances of a slug bumping into the molluscicide.
Mr Power points out that some cheaper formulations will give fewer baiting points at the recommended label rate and should be considered ahead of ordering pellets.
“Sluxx HP delivers 60 bait points/m2 at 7kg/ha, compared to just 43 when using a leading competitor,” he adds.
View from the field – Ed Brown, H L Hutchinson
With tighter stewardship guidelines placed on metaldehyde, agronomist Ed Brown saw it as a “no-brainer” to switch exclusively to ferric phosphate across his clients’ farms in 2017.
A buffer zone designed to protect field margins and watercourses was increased from 6m to 10m by the Metaldehyde Stewardship Group in the autumn of that year.
Using a ferric phosphate – which has no buffer zone restrictions – around field edges and metaldehyde in the centre of fields had been viable for Mr Brown before the increase.
However, once the 10m buffer zone was advised, it was much easier to drop metaldehyde altogether and avoid over-complicating what should be a logistically straightforward operation.
“We have found Sluxx HP to be a very good product that spreads well, and we haven’t seen any difference in the level of slug control since switching.
“The only thing we need to do as advisers is make growers aware that they won’t see any dead slugs on the surface after application,” explains Mr Brown, who advises on combinable crops in western counties Shropshire and Staffordshire.
He adds that with no buffer restrictions, the whole field can be treated and it is possible to keep baiting points up during slug risk periods with Sluxx HP’s 28kg/ha maximum total dose.
Conversely, metaldehyde stewardship guidelines recommend tight active ingredient limits, particularly in the autumn crop establishment period when slug damage can be most acute.
“With metaldehyde you are always watching those limits, but ferric phosphate gives you the flexibility to maintain baiting points in a high-pressure year when you need to top up protection,” he explains.