Clover weevil 'will never be eradicated'

BY MARK HOTTON

The early introduction of a biological control in Southland might restrict numbers of a new pest but the destructive clover root weevil will never be eradicated, a scientist warns.

The weevil, Sitona lepidus, destroys clover by feeding on roots and nitrogen-fixing nodules, and could cause millions of dollars in damage to the Southland economy. It was found in four sites in Gore and is likely to have been in the province for at least a year.

AgResearch scientist Colin Ferguson said the fact four populations were found indicated populations were more widespread.

Introducing a biological control agent, the Irish wasp, in the North Island appeared to keep down weevil numbers. The wasp lays eggs in the weevil, which are made infertile by the venom. The wasp will not eradicate the weevils but can keep numbers low.

"We think if we can get the biocontrol agent into Southland nice and early, maybe the farmers won't be impacted to the same extent (as in the North Island)."

The weevil had the potential to have a serious impact on farming in Southland.

If left unchecked, it has the potential to damage clover production by 30 per cent and possibly affect farm production by 15 per cent.

Carterton farmers Selwyn and Gordon Taylor found the weevil in 2006 on their 80ha farm and in 2007 estimated it cost them more than $160,000 in lost milk production.

Mr Ferguson said Southland farmers could lose about 1000kg of dry matter of clover per hectare each year.

"It does have a big impact – you're losing your clover, you're losing your production, your pasture life is shortened so you're having to renew your pasture (more often)."

Clover fixes nitrogen from the atmosphere so it can be released to grasses growing in the pasture. The nitrogen is converted by the clover to high-protein feed, so animals return it to pasture through faeces and urine.

Meat&Wool southern South Island director Leon Black said the organisation had been funding research into weevil control because of the impact it would have on production.

"But when the wool levy was knocked off so was that funding, so it's another of those casualties of the decision that farmers made in the last (levy) referendum," Mr Black said.

"In the next 12 to 24 months we'll know where the populations are and what the sort of likely damage and that will dictate the management and releases (of the wasp).

"It's another challenge we really don't need."

The impact would be felt regionally so it was important farmers maintained vigilance for the pest, he said. "Farmers also need to establish good strong vibrant clovers in those early years because generally your old and more established pastures will deal with it better than younger ones."

A submission is to be made to the Environment Southland annual plan process for funding to release the wasps.

CLOVER MENACE

• Notching on clover leaves, about the size of a match head is the signature of the weevil.
• No other insect damages clover in the same way.
• If notching is found in large quantities, it's likely a population has been established.
• If you find it, contact AgResearch to let them know how far it has spread.

 

WEEVIL CONTROL

AgResearch hopes to introduce a biological control, the Irish wasp, to populations in Gore in spring. While the wasp will not eradicate the weevil, it will keep numbers in check.

- © Fairfax NZ News