Evil weevil comes to a grisly end

The basis of our pasture production system is the clover plant, that fixes free nitrogen into the soils to make our grass pastures perform.

This cheap source of nitrogen has for many years driven NZ pasture production, and enabled us to grow low cost grass.

 The arrival of the clover root weevil  has decimated much of the  clover  in the North Island and also been found establishing in the south.

A natural pathogen which attacks this weevil was introduced by AgResearch to control this pest and  it appears it has been doing its job and clover plants are now recovering back into pasture.

The weevil is Sitona lepidus, the clover root weevil, which, since it was first discovered in 1996, has steadily advanced through NZ, flying, crawling and hitchhiking on vehicles. Its sole diet is the leaves and roots of white clover, especially the nodules that fix nitrogen in the soil. The value to the economy of this natural nitrogen-fixing capability has been estimated at more than $3 billion a year.

The weevil's grisly nemesis is an Irish wasp, Microctonus aethiopoides, whose sole diet is the clover root weevil. Selected after a worldwide search by AgResearch scientists, it was first released in New Zealand in 2006 and is now chasing down weevils at an impressive rate. The tiny wasp is the poster boy for AgResearch's biocontrol programme. The principle of biocontrol is simple: go to where the pest came from, find a natural enemy and bring it back for release.

AgResearch entomologist Pip Gerard expects this will prevent the South Island from suffering the devastating population explosions of the north. After 22 widespread releases and 2000 mini-releases in the North Island, she is confident no more will be needed there. So far, farmer bodies DairyNZ and Beef + Lamb NZ have invested $3.5m in the fight against the weevil.