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Clover Root Weevil bio-control gathers steam

Clover Root Weevil bio-control roll-out gathers steam

18 December 2007

Following excellent results from experimental releases at selected sites around the North Island, AgResearch entomologists are busy rearing thousands of parasitised weevils for large scale distribution to farmers, thanks to support from DairyNZ and Meat & Wool New Zealand.

The first batches of parasitised weevils were given away at a farmer field day at Heritage Valley Dairy farm, Henderson’s Road, near Hamilton last week.

AgResearch Clover Root Weevil programme leader for the North Island Dr Pip Gerard, reported on the progress of the biocontrol agent as well as latest research on how to get better clover establishment and persistence.

“The advantages and economic benefits of using a break crop to reduce weed and pest burdens prior to establishment of new pasture are significant,” says Dr Gerard.

“We’ve compared pastures sown in autumn 2004 under different establishment regimes and three years later the pasture quality is significantly superior in the ex-crop pastures compared to the grass to grass pasture. In addition, the trial results on this farm indicate further economic advantages would be gained by using clover-friendly tetraploid ryegrasses in dairy systems.

“Management options for pasture in the presence of Clover Root Weevil were also discussed.”

This clover research was undertaken for the NZ Clover Root Weevil Action Group by AgResearch with funding from the MAF Sustainable Farming Fund and AGMARDT.

The biocontrol agent from Ireland, nicknamed the Irish wasp but scientifically known as Microctonus aethiopoides, has been introduced to control the Clover Root Weevil which is a severe pest of white clover.

“The weevil larvae attack clover underground, resulting in reduced nitrogen fixation and clover growth, especially in spring and early summer. Farmers have observed that livestock have slower weight gains and lower milk yields on infested pastures unless fertiliser levels are increased to compensate, says Meat & Wool New Zealand Portfolio Manager – R&D Dr Andy Bray.

The tiny Irish wasp strikes at the adult weevil, injecting one or more eggs into the abdomen. This makes female weevils sterile and breaks the weevil life cycle.

The wasp larvae grow inside the weevil and kill it during the last larval stage as it bursts out of the weevil’s body. The larva then pupates in the pasture litter, before emerging as a wasp to start the next generation.

The wasp can spread around 2 km/year by itself, but the dispersal strategies being initiated this summer will greatly speed up establishment of the wasp wherever the weevil is a problem, says Dr Gerrard.

“The search and subsequent research to ensure the wasp will be effective and won’t harm any of our native or beneficial species has taken nine years to reach this stage. It has been supported throughout by the Foundation for Research, Science and technology, Meat & Wool New Zealand and Dairy Insight [now DairyNZ following the merger of Dairy InSight and Dexcel],” she says

It is in their role of enabling research and encouraging adoption on-farm that both DairyNZ and Meat & Wool New Zealand are behind a wasp giveaway scheme.

This involves AgResearch entomologists rearing and packing batches of 10 parasitised weevils for distribution through industry extension networks that start this summer and will continue until wasps have gone to every weevil-infested district.

As it will take several years for wasps from a giveaway to multiply and spread throughout a farm, other strategies are being used where faster control is desired.

“Nursery sites have and are being established in locations where good numbers of parasitised weevils can be collected from field populations and distributed by trained local people.

“In the South Island where the clover root weevil has only been found in localised areas, the wasp has already been released so that it can help slow the spread of the weevil, and prevent it reaching damaging levels,” says Dr Gerard.


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