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Weevil biocontrol expanding

Weevil biocontrol expanding

14 December 2006

Buoyed by the successful establishment and persistence of the biocontrol agent for clover root weevil at experimental release sites, the AgResearch Biocontrol & Biosecurity team is planning to roll it out into other regions. Releases are planned for Taranaki and Northland on the 19th and 20th of December.

The tiny parasitic wasp from Ireland, known as Microctonus aethiopoides, was released last summer at experimental release sites in Waikato, Hawke's Bay and Manawatu, and a winter release was carried out near Nelson.

The wasp, which is totally harmless to humans, kills the clover root weevil by injecting it with an egg that makes the female weevil sterile. This breaks the weevil life-cycle. Wasp larvae grow inside their host, finally killing the weevil when they break out of its body. The larva then turns into a pupa which in turn becomes an adult wasp.

White clover is crucial to New Zealand farms and is estimated to contribute more than $3 billion annually to the economy through nitrogen fixation and feed value for stock.

The adult stage of clover root weevil feeds on clover leaves and the larval stages feed on clover roots. Young larvae tend to feed in clover root nodules (which capture atmospheric nitrogen), while bigger larvae will feed anywhere on the root system.

When clover quality is compromised, extra nitrogen fertiliser is required, and this can have negative economic and environmental impacts.

Dr Pip Gerard, leader of the Dairy Insight and Meat & Wool New Zealand-funded release programme, says she was delighted to find that the wasp had made it through the winter successfully at all North Island sites and was busy attacking the new generation of adult weevils that started emerging last month.

"However, it will be a few more months before the team will know how well the wasp can spread across surrounding farmland," she says.

The Northland and Taranaki releases, each supported financially by their respective Regional Councils, are being used to establish nursery sites for the wasp.

"Mass rearing in the laboratory is an extremely labour intensive operation," says Dr Gerard. "Now we know the wasp can build up population levels rapidly within a release paddock, it makes sense to establish nursery sites where local people, who know the area and weevil hotspots, can collect parasitised weevils and spread them round.

"It is great to have the Northland and Taranaki Regional Councils involved. Working with them will help us with nursery sites in other regions in 2007."

Since clover root weevil was discovered it has spread throughout the North Island and was confirmed in the Nelson region early this year. Conservative estimates put the potential cost of clover root weevil damage to white clover at more than $300 million a year if a 'do nothing' approach was adopted, says Dr Gerard.

"While our entomologist team is confident the Irish wasp will reduce weevil numbers, we also know levels of control may vary across the country, especially in response to climate and insecticide use."

The team has other projects aimed at helping farmers maintain quality pastures in spite of chronic infestations of the weevil. These include a project with the New Zealand clover root weevil action group on grazing and fertiliser management, and developing a biopesticide with Ballance Agri-Nutrients that can provide added protection in high-value pasture.

Dr Gerard says that along with the release of the wasp, these measures are part of a package aimed at minimising the impact of the clover root weevil. "The wasp will have an important role in helping manage weevil's impact but in itself it is not a silver bullet."


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