The population of the root weevil preying wasp has been bred up at AgResearch Invermay, assisted by a $30,000 grant from Environment Southland, and is seen as the best tool to combat the clover killing insect.
The weevil was discovered in the North Island in 1996. In 2006 it was found in the South Island and then in February last year on a Clinton sheep farm. It has since been found at the Gore A and P showgrounds and on farms between Pukerau and Brydone.
The parasitic wasp, a natural predator of the weevil, has been imported from Ireland. Within weeks of its release on a Mataura farm, the wasps were expected to hatch, start spreading to other farms and begin killing their hosts.
The weevil dramatically reduces clover growth and compromises its nitrogen-fixing ability, with some paddocks that were once 30% clover now down to 5%. In others, the clover foliage has a sickly yellow hue instead of its normal bright green.
On some infected North Island farms dry matter production has fallen by a third and farmers have had to increase feed supplements and use nitrogen fertiliser to plug the widening gap between stock feed requirements and pasture supply.
Environment Southland biosecurity manager Richard Bowman has urged farmers to start looking for telltale signs of the weevil's presence, which included notches in clover leaves the size of a match head, and declining health in clover crops.
Once established, the pest could build to densities of up to 1400 larvae per sq m. Clover damage started occurring at 300 larvae per sq m, which was a more typical infestation.
The weevil could result in loss of available plant nitrogen of up to 300kg per hectare per year.
The Otago Regional Council has not funded work to breed up the wasp, with a spokesman saying it was seen as a national problem which was being addressed nationally through central funding of AgResearch.
It has also not been involved in releasing the weevil.