Workshop spreading the word and weevil parasite

 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 Brooker.
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 Brooker.
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 Hardwick said.

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.
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 bed.

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 parasitised.

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 two years.

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 cultivation.

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.