AgResearch scientist Scott Hardwick (left) hands Ikawai
farmer Mark Caldwell phials of parasitised clover root
weevils to be released in his paddocks. Photos by Sally
The evils of weevils were explained at a workshop in
Waimate last week.
AgResearch, DairyNZ and Beef and Lamb New Zealand combined to
help local farmers combat clover root weevil, then handed out
phials containing their best form of defence - weevils
infested by a tiny parasitic Irish wasp.
The workshop was one in a series aimed at spreading the word
- and the parasite. It was already in most parts of the South
Island suffering from weevil damage, but the phials would
boost its population.
AgResearch scientist Scott Hardwick said although adult
weevils ate clover foliage, most damage was caused by larvae
feeding on roots and nodules.
Weevils went through winter as larvae, then the adults either
reproduced or flew up to 30km at a stretch.
They also hitchhiked with humans or on vehicles.
The weevil was first noticed in the North Island in 1996 and
10 years later in the South Island.
It could cost up to $1.2 billion a year if unchecked - about
$40,000 for the average-sized farm.
A ''moderate larval population'' equated to 30kg to 60kg per
hectare loss of nitrogen, costing $150 a hectare.
There were a number of measures farmers could take, Dr
Insecticides could produce a short-term kill of the adults,
but the larvae were ''very difficult to get to''.
Extra nitrogen could be applied to combat the losses, but
that was a cost to the farm.
A chillybin of phials containing parasitised clover root
weevil at a workshop in Waimate last Wednesday.
Other plants could be used instead of white clover, with
chicory, plantain and lucerne not affected by the weevil.
When dairy pasture was being renewed, a break crop like
brassica or short-term ryegrass could give a weevil-free seed
However, Dr Hardwick said the best option was the parasite.
It was discovered by New Zealand scientists in Ireland almost
by accident, after exhaustive searches in England.
''It's sweet little thing, about the size of a sandfly.
''We're lucky - they don't interfere with anything else.''
The wasp lays its egg in the adult weevil, a process that
also sterilises the weevil.
Each wasp can produce about 60 eggs a year and weevils would
fly with wasp larvae inside.
Dr Hardwick warned farmers to expect weevil pasture damage
until late next year, before the wasp became dominant.
With 100 weevils in each phial, he guaranteed 80 would be
Farmers were told to release them into a paddock with clover,
but not one that would be fed out on or dug up in the next
The wasps were likely to spread a kilometre in a few months.
In the meantime, agronomist Chris Sanders advised farmers not
to oversow weevil-affected clover with more clover, as
weevils preferred seedlings.
White clover could be included in new pasture sown after full
A break crop should be used for at least four months to get
rid of the weevil, he said.
Seasonal strategies were avoiding pasture shading in spring;
avoiding excess sun exposure in summer; and avoiding
overgrazing and pugging in autumn and winter.