Spring slug warning

Crops will need closer monitoring for slugs this spring to avoid a repeat of the damage seen in 2016, FAR’s new environmental research manager Abie Horrocks says.

While weather ultimately drives slug damage, reduced tillage is increasingly a factor.

“Don’t wait until you can see slug damage in the crop,” she says. “By then the slug population will likely be so high that you’ll be on the back foot with control.”

Monitoring in-crop population, especially at establishment, is crucial, she says.

“It’s so easy to get complacent, and then you’ll get taken by surprise when the slug population erupts.”

Monitoring is particularly important in susceptible crops such as clover, seed grasses and brassicas, but all crops, including cereals, can suffer damage. Slow early growth, particularly in generally wet weather, should ring alarm bells.

Slug control strategies should be built into an overall cropping plan including cultural controls.

“For example, pay attention to seedbed quality. Aim for good seed and soil contact and coverage, which prevents slugs from moving through crevices in the soil where they can readily access the seed.”

‘Pay attention to seedbed quality. Aim for good seed and soil contact and coverage, which prevents slugs from moving through crevices in the soil where they can readily access the seed.’

Natural predators are worth encouraging. Carabid beetles – of which New Zealand has three native species commonly found in paddocks – make a valuable contribution, particularly on immature slugs, reducing the need for, and frequency of, pellet applications.

“But you’ll need to be careful with insecticide use to encourage [beetle] populations… Carbamate-based products, for example, are not ‘beetle-friendly’.”

Four species of slugs are agronomically significant, a FAR and Plant and Food Research Sustainable Farming Fund project found, the two most common being the grey field slug (Deroceras reticulatum) and the brown field slugs (Deroceras panormitanum).

Horrocks says the grey field slug “is the one to look out for” because lab trials have shown it responds most quickly to changes in moisture and causes up to four times more feeding damage.

FAR’s recommendation on slug monitoring has been welcomed by Zelam, whose parent company Lonza has been supporting European farmers with slug-control strategies for several years.

“There’s been a big change in Europe over the last 10 years with much greater emphasis on awareness and pre-emptive control strategies that involve population monitoring to stay ahead of the game, and a move away from the ‘one size fits all’ approach to slug control,” Zelam’s Mike Swift says.

“There’s a growing realisation that slug control should be part of the same, modern precision-led approach to crop protection that we adopt with products such as fungicides where several factors contribute to an effective decision-support system.”

Subject to registration, Zelam hopes to have a new European molluscicide available in New Zealand for spring crops this year. Its support package will include a smartphone app to ensure accurate application calibration in the field and a guide to slug species, crop damage symptoms and incorporating slug control into an integrated pest management strategy.

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