Irish wasps set to take on root weevils

AgResearch scientist Scott Hardwick vacuums clover root weevil from a paddock at Mt Peel Station. Supplied photo
AgResearch scientist Scott Hardwick vacuums clover root weevil from a paddock at Mt Peel Station. Supplied photo

The clover root weevil ravaging pasture throughout the South Island is meeting its match.

Last year's mild winter resulted in a proliferation of the weevils that leave a distinctive U-shaped notch on clover leaves. However, a parasite decimating the weevil population will soon take effect. AgResearch scientist Scott Hardwick told Courier Country parts of South Canterbury and North Otago had suffered ''quite significant damage'' from clover root weevil. The area from Timaru south was the worst affected.

AgResearch pest specialist Colin Ferguson said mild conditions had allowed a greater number of weevil eggs to hatch and more of the larvae survived through the winter.''

"Tremendous pressure is now being placed on pasture from the increased numbers of larvae that fed on the clover roots and their nodules last winter", Mr Ferguson said.

That damage was followed by the large population of adult clover root weevil emerging in early summer and feeding on the clover leaf. Normal spring and summer grazing of clover has added to the stress on already struggling plants.''

Affected farmers are either seeing clover that disappears very quickly once grazed or a complete absence of clover plants in their pasture. Without clover, farmers can't fatten lambs and those that are looking to overwinter dairy grazers can't do that. It is having a huge impact on farm returns right now.

Federated Farmers South Canterbury meat and fibre chairman Miles Anderson said clover had vanished from a lot of local pasture this year. Farmers were not sure if it was because of dry weather or clover root weevil, but probably both.

He knew of farmers having difficulty fattening lambs because of a lack of clover.

It's becoming a bigger issue in South Canterbury.

Dr Hardwick had also fielded calls from concerned farmers and was visiting farms to take samples and release batches of weevils infected with the parasite that would help.

The tiny parasitic Irish wasp was proving highly effective, knocking back clover root weevil populations by up to 90%. It had been used in the North Island for about 10 years and there were no signs of the weevils developing immunity, Dr Hardwick said.

"We're watching it closely".

AgResearch staff were using blower-vacs to collect weevils from Canterbury pasture.

While most of these collected weevils are already parasitised, we boost the parasitism levels by adding a few Irish wasps to the packs of 100 we make up for farm release,'' Dr Hardwick said.

Packs were being delivered to Southland, where the weevils were making a major nuisance of themselves.''

The result will significantly accelerate spread of the bio-control and the process of clover recovery will begin,'' Dr Hardwick said.''

Farmers can expect clover content to return to normal levels two to four years after the wasps' arrival on their farms.''

The wasps had already been released in Mid and South Canterbury and North Otago, he said. Their numbers had not built up sufficiently to wipe out the weevils yet, but it was happening. The adult parasitoids were now emerging.

At Mt Peel Station, they were released at the eastern and western ends of the property in January and February last year. Samples were being taken from neighbouring paddocks to gauge their spread.''

"The parasitoid is already all up the river valley", Dr Hardwick said.

It was also released around Timaru, Waimate, the Hakataramea Valley, Danseys Pass, Maheno, and Island Stream.

A monitoring trip revealed there were scattered populations of the parasitoid in the Waitaki River valley from Rugged Ridges Station to the coast, he said. The wasps were also present from the Hakataramea Valley down to Morven and through the Waimate Gorge.

They had spread about 30km a year, he said.

North Canterbury weevil populations had ''really declined'' after the parasitoid was released there a number of years ago, Dr Hardwick said.

''It's really knocked the stuffing out of them.''

The parasitoid was ''performing beyond expectations'', especially with the ''rapidity of effect'', he said.

In the next 18 months, the parasitoid population would grow and the weevil population would drop back.

''Don't count on clover production for the next 18 months,'' he told farmers.

''It will be difficult this spring".'

The clover would eventually come away again because there would be enough of a seed bank in the soil, Dr Hardwick said.

He recommended farmers considering resowing their clover to try for autumn establishment.

''The weevil flies during the summer. With spring sowing, there is a risk it will affect it".

''The best strategy for establishing new clover is a full cultivation after a summer crop - autumn cultivation then drill it in and get going over winter. Baby it along".

Farmers wanting further advice should contact Beef + Lamb New Zealand and DairyNZ officers. DairyNZ also has a ''Farmfact'' on managing dairy pastures with clover root weevil, including how to support clover-depleted pasture.

- by Sally Brooker