Research scientist Scott Hardwick collects clover root
weevils from South Canterbury pasture using a blower-vac.
Photo by AgResearch.
An industry-wide effort is under way in Southland to
combat the damaging clover root weevil, whose economic damage
has been measured in hundreds of millions of dollars
Clover root weevil (CRW), identified by distinctive U-shaped
notches on clover leaves, was discovered in the Waikato and
Auckland in 1996 and has now spread as far as Southland.
A project, involving AgResearch, Beef and Lamb New Zealand,
DairyNZ and Environment Southland, which has been releasing
parasitised clover root weevils on Southland farms, is being
The aim is to collect up to one million weevils infected with
the Irish wasp parasite and release them on up to 1000
Southland farms before winter.
Last year's mild winter meant the weevil had taken its
foothold on Southland farms to a widespread infestation that
had a ''huge'' impact on the production ability of the
AgResearch pest specialist and scientist Colin Ferguson, who
is based at Invermay, said CRW had been in Southland since
2010 but large numbers were present in only a few locations,
until last year.
The mild conditions allowed a greater number of CRW eggs to
hatch, more of the larvae survived the winter and the
population ''just exploded''.
Affected farmers were either seeing clover disappear quickly
once grazed, or a complete absence of clover plants in their
''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,'' he said.
In 2006, a parasitoid wasp was released in New Zealand to
help suppress CRW populations.
It laid an egg inside the adult CRW which immediately stopped
the weevil from being reproductive.
AgResearch scientists had seen reductions of more than 90% of
the CRW population in monitored North Island farms where the
wasp had been released.
Last year, AgResearch said the wasp was returning benefits of
about $57 per hectare per year, or a total of $353 million
That was based on reductions in CRW populations caused by the
wasp and associated increases in clover nitrogen fixation and
clover dry matter production.
AgResearch scientist Scott Hardwick, who leads the
Lincoln-based CRW collection team, said weevils from
Canterbury farmland where the wasp was already at work were
being vacuumed up.
''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.
''These packs are then delivered to Southland for release.
The result will significantly accelerate spread of the
bio-control and the process of clover recovery will begin,''
Dr Hardwick said.
Southland farmers could expect clover content to return to
normal levels two to four years after the wasps' arrival on
their farms, he said.
Mr Ferguson advised farmers not to panic, saying it would
pass over time.
''Although things are going to be tough for the next few
years, farmers will get through this with the help of the
''North Island farmers have already gone through this process
and clover root weevil is not much of an issue now for any of
them,'' he said.
Beef and Lamb New Zealand, in conjunction with AgResearch, is
holding four CRW field days in the South this week - today at
Waikoikoi at 10am and Glenham at 3pm, and tomorrow at Winton
at 10am and Mossburn at 3pm.