The parasitoid bio-control for the clover root weevil has rapidly gained a toe-hold in Nelson says AgResearch clover root weevil programme leader for the South Island Dr Craig Phillips.

In July 2006 hundreds of adult clover root weevils, each impregnated with the egg of Microctonus aethiopoides, a tiny parasitic wasp that preys on the clover root weevil were released at Richmond, and the Rai Valley near Nelson. Follow up releases of parasitised weevils were made in December 2006 and January 2007.

In early May 2007 AgResearch scientists Scott Hardwick and John Proffitt collected clover root weevil adults from the Richmond and Rai Valley release sites and found between four and 13 per cent of the weevils were parasitised.

“This clearly shows that released populations of the biocontrol agent are growing very rapidly. The results strongly suggest that the biocontrol agent will survive the winter and become permanently established. This is great news for South Island farmers since there is now a chance their pastures will be spared from the most severe effects of clover root weevil,” says Dr Phillips.

A collaborative project involving AgResearch, Nelson Federated Farmers and the NZ Landcare Trust, the programme is funded by New Zealand’s dairy farmers through Dairy InSight and Meat & Wool NZ.

The programme aims to reduce the economic impact of clover root weevil in the Nelson region, help reduce the rate of spread of clover root weevil to other South Island locations, increase the opportunity for the biocontrol agent to spread along with its host to other South Island locations and reduce the number of additional biocontrol releases that may eventually be needed in the South Island.  

First discovered in the Waikato in 1996, clover root weevil causes significant damage to clover pastures across New Zealand. It is estimated that it has the potential to cost the pastoral sector up to $300 million per year.

“White clover is crucial to New Zealand farms and is estimated to contribute more than $3 billion to the economy through nitrogen fixation and feed value for stock,” says Dr Phillips.

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

Dr Phillips says the tell-tale sign of clover root weevil presence are match-head sized, crescent-shaped, notches eaten out of the edges of clover leaves. The notching arises from feeding by the adult stage of clover root weevil, but larvae do the most damage to white clover by feeding on its roots and root nodules.

The parasitic wasp Microctonus aethiopoides, 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.

Dr Phillips and his AgResearch colleagues searched the globe for a suitable clover root weevil bio-control and eventually AgResearch scientist Mark McNeill discovered the wasp in Galway, Ireland. The Environmental Risk Management Authority of New Zealand deemed the release of the wasp as environmentally safe and gave permission for it to be released in late 2005.