Clover root weevil invades Wairarapa

The clover root weevil ravaging the North Island's farm pastures has spread to southern Wairarapa.
Clover weevil was first found in Waikato in 1996, and has been spreading through the North Island since then ? New Zealand's temperate climate and a lack of predators provide ideal conditions for the weevil.
In 2003, it was found in Taranaki, and last year Agresearch plant breeder Chris Mercer found clover root weevil in Manawatu, near Marton, and at a site in Palmerston North.
Now adult weevils have been found by chance during routine sampling of pasture south of Featherston, as Agresearch staff were investigating the benefits of biological control of another introduced pest, Argentine stem weevil.
Agresearch entomologist Mark McNeill said the clover root weevil appeared to be restricted in a relatively small area of Wairarapa, south of Featherston ? a brief survey on farms near Masterton, Martinborough and Greytown did not find specimens.
He said the limited distribution suggests the weevil, which is very mobile, might have been carried from either Hawke's Bay or Manawatu on trucks, cars or agricultural equipment.
The pest was found on Gwavas Station in Central Hawke's Bay at the end of 2003.
Severe weevil infestations can eradicate from pastures the white clover on which farmers depend for high quality pasture and the "free" fixation of atmospheric nitrogen to boost plant growth.
Farmers in regions where the pest is newly established often face a drop of up to 30 per cent in pasture production as the weevil feeds on white clover and destroys its ability to fix nitrogen to maintain soil fertility.
In the North Island, clover content in pastures typically has declined from an average 20 per cent to 10 per cent because of the weevil.
The pest is eventually expected to reach the South Island.
According to scientist Pip Gerard the weevil could be transported across Cook Strait in cars, on hay bales, on sheep, or even by flying.
The weevils are no more than 6mm long and have a brown head and creamy-white body. An adult female can lay hundreds of eggs as she grazes and when these hatch, the larvae burrow down to attack the root nodules.
Agresearch scientists have said conservative estimates suggest it could cost around $300 million a year in lost productivity and profitability across New Zealand.
But that estimate does not take into account environmental damage from increased run-off into waterways of higher levels of nitrogen fertiliser applied by farmers in infested areas to counter the loss of "natural" nitrogen the clover would normally fix from the atmosphere and put into the soil.
Agresearch researchers have identified a biological control agent to control the weevil and will this month file an application with the Environmental Risk Management Authority for the release of an Irish parasitoid wasp that attacks the adult root weevil. ? NZPA