Pasture weevil spreads to south

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A destructive pasture pest has been found in Southland for the first time.

The economic impact it could have on the province has yet to be determined.

AgResearch scientist Colin Ferguson yesterday told the Environment Southland regional services committee that clover root weevils had been found at four sites near Gore.

They were discovered last month at the Gore showgrounds, Gore racecourse and two dairy farms between Gore and Mataura.

The weevil fed almost exclusively on white clover but was also known to eat other varieties, he said.

Infected pastures were more susceptible to drought, disease and poor management, Mr Ferguson said.

Farmers in the infected areas could see signs of damage by next year, Mr Ferguson said.

Estimates of how much damage the weevil could do once established across the country were about $1 billion, he said.

Southland had been one of the last remaining bastions holding out against the pest and their arrival had been inevitable, Mr Ferguson said.

It was not known how the weevils arrived but they were good hitchhikers and strong fliers.

Insecticide control was ineffective and there were no known natural diseases or competition for the weevil in New Zealand, he said.

However, there is a form of biological control, the imported Irish wasp, which infects the weevil by implanting an egg.

Weevils were made infertile by wasp venom and its larvae gradually devoured the insides of the weevil, he said.

"But you shouldn't think it's a silver bullet; the pest is here to stay," he told the committee.

It would cost $10,000 for a release in Southland and the ideal time would be spring, Mr Ferguson said.

The councillors supported a biocontrol release but funding would have to be applied for through the annual plan process.

Westpac Southland-Otago agribusiness area manager Peter Moynihan said it was impossible to know yet how much financial damage the weevil could do.

WEEVIL WORRIES

Clover root weevils reduce pasture growth and productivity and the ability for clover roots to fix nitrogen. wAdult weevils feed on clover leaves while larval stages feed on clover roots. wSymptoms of infestation include chewed leaves and yellowing plants. wThe weevils were first identified in New Zealand in 1996 and have been in the South Island since 2006. wThey are establised across the Taieri Plains in Otago and have been found near Balclutha. Fighting the weevil wThe imported Irish wasp infects the weevil by implanting an egg. wWeevils are made infertile by wasp venom and its larvae gradually devours the insides of the weevil, until it bursts through its skin. wAgResearch has carried out controlled releases of the wasp across the country with positive results. The wasp does not wipe the weevil out but restricts its impact.

- © Fairfax NZ News