Given fresh restrictions on metaldehyde, Andrew Blake examines this season’s options for slug control in potatoes

Last autumn it seemed potato growers might have to live without slug pellets containing metaldehyde after new EU maximum residue level proposals led to its temporary withdrawal.

In 2006-2007 the chemical accounted for about half the slug control market in potatoes, about 80% of the area being treated with it.

Fortunately approval for metaldehyde’s continued use on the crop, albeit at reduced rates to avoid exceeding the new MRL, ensures the option remains available, says the BPC’s Rob Clayton.

However, the new limitations including a harvest interval could see growers exploring alternatives more closely, Mr Clayton believes. “It’s an ongoing challenge.”

So what else is in the armoury?

All stocks of thiodicarb-based slug killers had to be used up by last November, leaving just two further chemical options – methiocarb and ferric phosphate – and one significantly more expensive biological approach.

All metaldehyde products with renewed approval now have lower maximum individual and total doses, and may no longer be applied within 21 days of harvest.

“Good practice with metaldehyde is essential if we are to avoid losing it completely,” stresses the SAC’s Andy Evans. “So following the Voluntary Initiative and using reduced rates in potatoes is in everyone’s best interests.”

SAC trials show metaldehyde and methiocarb are equally effective at reducing slug damage in single treatments. But at least two applications, and preferably three, are needed for significant results, says Dr Evans.

Timing is key as the pests must be active on the soil surface, says Bayer CropScience’s Richard Meredith.

“Just before canopy closure, say 50-75% ground cover, can be highly effective.

“Of course in a hot, dry June/July, as in 2006, no self-respecting slug is within a foot of the surface, so any baits applied will do nothing.”

In that year treatment just before August rain gave the best result in Cambridge University Farms trials, says Dr Meredith.

Single full doses of metaldehyde and methiocarb respectively cost £10-15/ha and £20-25/ha, but focusing on cost per hectare can be misleading, he warns.

“Independent work has shown that two or three well timed, full rate treatments with methiocarb, for example Draza Forte, are at least as effective as multiple weekly treatments of metaldehyde mini-pellets at reduced rates.”

Very early treatments can be wasteful and later ones should be on the basis of monitoring to show the pests are active at the surface, he adds.

Ferric phosphate, available for example in Omex’s Sluggo and newly approved Sluxx from Certis, is the other chemical control option.

In trials for PSD approval the recommended 6kg/ha of Sluggo pellets, containing 1% of active ingredient, were just as effective at reducing tuber damage as 5kg/ha of Lupus (methiocarb) or 10kg/ha of Escar-Go (metaldehyde), says Omex’s Gidon Bahiri.

At £3.50/kg each treatment costs much the same as one of methiocarb, he adds.

Sluxx is novel in that it is a more concentrated formulation containing 3% ferric phosphate making it a “premium product”, says Certis’s Robert Lidstone. “Independent trials show it’s an economically viable alternative to other controls.”

Bayer also markets ferric phosphate but not for agricultural use, notes Dr Meredith.

Also moving into the market is Branston’s Nemaslug Xtra.

A specially formulated version of a parasitic control product for high value horticultural crops, it has been used commercially on potatoes for two seasons over several hundred hectares, says the firm’s Andy Barker.

“Some growers think it’s the bee’s knees hitting the parts that pellets don’t reach, and we hope to cover 1000ha this season.”

Intended to be integrated with pellet use and applied through sprayers or irrigation equipment, Nemaslug Xtra is particularly useful against keel slugs which tend to stay buried to damage tubers in the run up to harvest, explains Mr Barker.

However, it is relatively expensive – £110/ha per treatment – and needs rain or irrigation to wash its parasitic nematodes into the soil. “We’re developing a new raingun which will greatly increase the flexibility of application timing.”

Control methods

Don’t rely solely on pellets for slug control, urges Dr Meredith.

“Field planning, cultural techniques, varietal choice and early lifting can all help reduce damage,” he says.

Pentland Dell and Lady Rosetta are much less vulnerable to slug attack than Maris Piper, Cara and Maris Peer, with Desiree, King Edward and Nadine somewhere in between, says Dr Evans.

“The longer you leave tubers in the ground, the greater the risk of damage.”

In one trial Piper harvested on 8 August suffered only 10% slug damage. Delaying lifting until early November led to 45%.