Irish wasp may need help moving west

Increasing clover root weevil populations are being seen on the West Coast, but the AgResearch-introduced biocontrol is hot on its tail.

AgResearch entomologists Dr Scott Hardwick and Mark McNeill, based at the Lincoln Campus in Canterbury, have been tracking the spread of clover root weevil (CRW) in the South Island, so that they know if and where to release the Irish wasp, a very effective biocontrol agent for this serious pest of white clover.

Sampling last winter and early spring for the DairyNZ-funded biocontrol project has revealed that the weevil is now present through much of the northern parts of the West Coast. AgResearch is now asking southern West Coast farmers who suspect they may have the weevil to get in touch, so they can be sure the wasp keeps apace of the problem.

In 2006, AgResearch scientists, supported by DairyNZ, Beef + Lamb NZ and AGMARDT, made a breakthrough in CRW control by releasing a potential biocontrol agent, a tiny parasitic wasp from Ireland.

The first trial releases were made in Waikato, Hawke’s Bay and Manawatu, and within just 18 months the wasps’ performance had exceeded the expectations of even the most optimistic scientists.

Dr Hardwick says they found 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," he says.

"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 limits the dispersal power of the Irish wasp, as it needs to be carried into new areas as eggs inside parasitised weevils, and leads to isolated ‘hot spots’ of CRW, which are often started by CRW hitching a ride on vehicles," says Dr Hardwick.

"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."

West Coast farmers who suspect they may have CRW - it can be identified by the distinctive U-shaped notches on clover leaves made by the adults - should contact AgResearch.

This will help them confirm the need and locations for Irish wasp releases this year.

More information about the weevil and how to recognise it can be found on Pestweb.co.nz the recently re-launched directory of New Zealand’s most damaging pests and weeds.