Scientists are deploying a tiny species of wasp to combat the spread of a clover-munching pest through the South Island.
Clover root weevil, discovered in New Zealand in 1996, was thought to have travelled here via a shipping container, before being carried to a Waikato farm on farming equipment.
In the North Island, damage by the weevil had been particularly significant in dairy pastures, and dairy farmers had reported substantial loss of productivity due to the weevil.
Increasing clover root weevils populations are today being observed on the west coast of the South Island, but the spread is being closely tracked by AgResearch entomologists Dr Scott Hardwick and Mark McNeill.
The scientists are zeroing in where to release the Irish wasp, a tiny parasitic wasp first released in 2006 and considered a highly effective biocontrol agent for the serious pest.
The tiny wasp, which feeds on nectar and doesn't sting humans, laid its eggs in the adult clover root weevil where they hatched and grew before emerging from their host.
Sampling last winter and early spring for the DairyNZ-funded biocontrol project revealed that the weevil is now present through much of the northern parts of the West Coast.
AgResearch was now asking southern West Coast farmers who suspect they may have the weevil to get in touch, so they could be sure the wasp kept apace of the problem.
Dr Hardwick said there were potentially damaging populations of the weevil from Greymouth north through to Karamea, but in spite of extensive sampling south of Greymouth, they only discovered a single infested site in Waitahi.
"The good news is that clover root weevil has brought its own destruction with it."
The Irish wasp has been confirmed at many localities including Little Wanganui, the outskirts of Westport, Cronadun, and Greymouth.
"However, we're concerned that the weevil may be getting a jump start on the wasp further south on the West Coast.
"In wetter areas, clover root weevil may not fly as readily as it does in summer dry areas such as Canterbury."
This both limited the dispersal power of the Irish wasp, as it needed to be carried into new areas as eggs inside parasitised weevils, and led to isolated "hot spots"of clover root weevil, he said.
"We're now considering carrying out releases of the Irish wasp south of Greymouth when new populations of the weevil are found, rather than relying on it making its own way."