The Environmental Risk Management Agency, ERMA New Zealand, has now received an application from AgResearch for approval to release a parasitic wasp to control the clover root weevil.

Since it was first discovered in 1996, the weevil has spread throughout the North Island and AgResearch scientist, Dr Pip Gerard, says it is now recognised as one of New Zealand’s most serious pasture pests. 

It attacks clover all year round causing marked declines in clover content and quality in pastures.

“The worst affected farmers have to apply very high levels of nitrogen fertiliser at costs of around $300 a hectare to maintain soil fertility and farm profitability.”

AgResearch wants to release a new Irish strain of a small (2.7-3.0mm) parasitic wasp Microctonus aethiopoides to control the weevil.

The wasp has multiple generations per year and parasitized weevils quickly become sterile, which in turn reduces the number of larvae attacking plant roots.

Dr Gerard says the advantage of biological control is that it will spread to all New Zealand farmers in clover root weevil-infested regions, providing an option of maintaining sustainable and highly productive grass/clover pastures without the use of high levels of nitrogen fertiliser.

The search for a biocontrol agent began seven years ago and has been well supported by the pastoral industry, especially Dairy Insight and Meat and Wool New Zealand.

Because little was known then about the weevil’s natural enemies overseas, Dr Gerard says the Biocontrol and Biosecurity team at AgResearch had to start from scratch.

“Searches were mounted in North America and Europe and all initial screening and identification work was undertaken in laboratories in France and England.”

M. aethiopoides ultimately proved the most effective natural enemy, and strains of this parasitic wasp were imported into quarantine at Lincoln in late 2000 where in-depth research on strain efficacy, behaviour and host range began once sufficient numbers were available.

A Moroccan strain of this wasp species, which is ineffective against clover root weevil, is already widespread in New Zealand. Introduced in 1982 to control the lucerne weevil, it has successfully prevented significant losses in lucerne production.

It has also, however, been subsequently found to also attack a number of native broad-nosed weevils that live in our tussock grasslands.

Dr Gerard says, in light of the finding, researchers wanted to ensure any potential risk to these native weevils by the Irish strain was also assessed prior to an application for release being made.

“We needed to also make sure that the two strains do not mate with each other and produce hybrids.” (The Irish strain reproduces asexually and all are female).

Dr Gerard says the seasonal availability of both weevils and wasps has meant this work has of necessity taken several years.

But, she says, the wealth of information available on the Morrocan wasp has enabled the team to make comparisons between the two strains and conclude “the Irish strain is likely to cause less impact on the native weevils”.

As the species was already present in New Zealand, Dr Gerard says the Irish strain could have been released immediately after importation.

“AgResearch entomologists believe”, however, “its introduction should be fully scrutinised and risks and benefits properly weighed by both outside agencies and the community.”

In March this year M. aethiopoides became a “risk species” under HSNO legislation and ERMA New Zealand became responsible for deciding whether or not the new Irish strain could be released.  

Submissions from interested organisations and individuals can assist this decision-making process, help raise issues, provide information and contribute different perspectives.

Anyone can make comments.

An application summary, that should be read beforehand, is available from ERMA New Zealand at PO Box 131, Wellington, by email  (submissions@ermanz.govt.nz) or through the website (http://www.ermanz.govt.nz/consultations/consult-apps.cfm).

The public submission period closes on 13 July 2005.