Wasp puts North back in clover

Biological control experts are hailing the success of a parasitic wasp which is proving highly effective in battling a clover-decimating pest in Northland.

White clover, a plant that underpins the agricultural economy, has been under threat from the clover root weevil since its arrival in Northland in 2006.

The weevil's larvae feed on white clover roots, destroying an important source of stock feed and reducing the nitrogen essential to pasture growth.

To combat the weevil, the Northland Regional Council has helped accelerate the spread of a parasitic wasp from Ireland.

Introduced to New Zealand by AgResearch following extensive testing, the wasp (Microctonus aethiopoides) lays eggs within the weevil, which sterilise and eventually kill the weevil as they grow into larvae. Those larvae then grow into adult wasps and the cycle repeats.

"We're seeing a huge decline in numbers of this destructive pest in Northland where the parasitic wasp has been successfully established," says Cable Bay-based entomologist Dr Jenny Dymock, who is facilitating the project for the regional council.

"This type of wasp is very fussy about what it parasitises, which makes it ideal for biocontrol. As the weevil's population declines, so will the wasp's."

The wasp's release is already paying dividends for Northland farmers, including Taupo Bay couple Erica and Lindsay Whyte, whose property had such serious weevil damage its clover stocks had almost been wiped out.

"The wasps have spread more than 20 kilometres since they were released on their farm in 2007 and have been a highly successful tool," Dr Dymock said.

"By May last year 88 per cent of the weevils there had been infected by the wasp, and now I'm unable to find any of the damaging weevils on the property. It has dropped below the radar and the clover is doing well."

Following regional council-funded releases in Okaihau in 2006 and Taupo Bay the next year, the wasp is spreading across northeast Northland.

The regional council is helping spread the wasp through western areas, and the insect is spreading through southern Northland from releases in the greater Auckland region.

"The current ideal clover growing conditions coupled with the presence of this tiny Irish wasp means Northland farmers should soon be 'back in clover'," Dr Dymock said.