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
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
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
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
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