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Potatoes: critical period for slug control – CPM – Amie Hunter

Potatoes are a tricky crop to keep free from slug damage, with control relying on management across the whole rotation as much as in the potato crop ...

Even with cultural controls growers are tied into some practices without, in many cases, much opportunity to change, suggests Amie Hunter, an agronomist for Hutchinsons based in Cornwall.

“It’s difficult – technically you can switch varieties into less susceptible ones but, in reality, growers are tied into growing what the end market requires.”

Similarly with rotation it can be difficult to make changes to reduce risk, even more so on rented ground. “Brassica crops in the rotation can build slug numbers ahead of potato crops, but it’s usually a case of being aware of it rather than something potato growers can change,” she says.

Cultivations help disrupt slug life cycles, which is one positive attribute of the relatively heavy cultivations made ahead of potato planting. But once the crop is in the ground, the key to in-crop control is an initial slug pellet application just before canopy closure, with repeat applications usually after three weeks and again around desiccation when the canopy opens up again, she says.

The initial applications help ensure pellets reach the soil surface before it becomes too difficult to penetrate the crop canopy-usually in late June to early July, when the canopy is sufficiently open to allow pellet penetration (0-75% canopy closure).

A second critical period for slug control occurs at the early stages of tuber bulking, before slugs go underground to find developing tubers. August is the pivotal month for follow-up applications and when damage usually begins to appear, she explains, with continued monitoring necessary until burn-down.

“We always use a trapping system in each field to monitor populations before deciding on treatment. The damage-risk threshold is one slug per trap-understanding and monitoring thresholds is a vital management tool for slug populations.” says Amie.

 

Look for persistence

“The key characteristic potato growers are looking for in pellets is persistence – it’s no good having something that disintegrates quickly in the moist environment under the canopy or with a bit of rain – and spreadability.

“You usually need them to spread 24m. It’s important for any crop that you can do that accurately, but especially in a potato crop – you don’t want to leave a whole row of potatoes unprotected, which could have a huge impact on the financial viability of the crop.”

As well as its ability to spread, pellet integrity is critical in the potato crop. Amie has found the De Sangosse pellet X-Ecute has shown that when spread at 24m it can withstand being thrown over this distance.

“Manufacturers put a lot of work into how well they spread in terms of the hardness and size of the pellet, and also its uniformity. If pellets are variable in size or weight they spread differently and less effectively.”

Those factors favour the use of a high-quality ferric phosphate-based pellet, she says. “It spreads well and is very durable in terms of weatherability. It’s a slightly bigger pellet than others which I think helps with durability.”

The 2.7mm wet process durum wheat-based pellet also contains unique attractants based on oilseed rape, which De Sangosse claim helps attract slugs to the pellet. That could be helpful in a potato crop, where targeting the slugs is challenging and relies on them coming to the surface to feed on pellets.

“In the field it’s difficult to say whether that actually helps – but they are definitely palatable to slugs,” says Amie. With metaldehyde in its final year of use up, many potato growers have already switched into ferric phosphate alternatives, she notes. “There hasn’t been any loss of efficacy, but you won’t see the slime trails and dead slugs on the surface that growers used to see with metaldehyde as it has a different mode of action. It’s a difference you have to get used to.”

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