Clover Root Weevil spreads further South

The pasture pest known as clover root weevil has spread into Otago. An AgResearch entomologist based at the Institute’s Invermay Campus, Colin Ferguson, a week ago, discovered a low density population on a sheep farm near Clinton in South Otago. This is the furthest south it has been found since it was first detected in Waikato in 1996.

He says although it was just a matter of time for the weevil to spread further down south, it’s nevertheless disappointing to see it has finally arrived in Otago. Mr Ferguson says although the newly discovered population is low density, it is bound to be more widespread than just one farm with possible larger populations elsewhere in the area. “The discovery was made on a sheep farm, but there are many dairy farms in the area and dairy farms tend to better support CRW populations,” he says.

He says the investigation into this new discovery is still in early stages and no decision has been made on how to control it. “There is a possibility of releasing the CRW parasitoid at some stage, but first we have to identify a suitable population to act as a carrier.” He says farmers in the area are naturally concerned but keen to work together on finding ways to control the pest.

He says it is important to also find out where else in South Otago the weevil may be present. Farmers in South Otago and even Southland (Clinton is close to the Southland border) are being urged to inspect their clover for any signs of weevils. If they find any weevils or unusual damage to clover leaves, they are urged to get into contact with Colin Ferguson on colin.ferguson@agresearch.co.nz. Information to help identify the weevil and a web page to help report new infestations is also available at www.agresearch.co.nz/crw/

Clover root weevil knocked clover production as it spread through pastures following its discovery in the Waikato in 1996. By 2005 it has spread throughout the North Island. But, it was only detected in the South Island in 2006 in Christchurch as well as near Nelson in Richmond and Rai Valley.

AgResearch entomologist, Dr Pip Gerard, says the impact of the weevil varies from farm to farm. Good pasture management helps maintain healthy and productive clover, in particular keeping pasture well grazed in spring and ensuring sufficient cover in summer to protect the stolons from sunburn. A pamphlet on clover root weevil management, which has been compiled with input from North Island farmers affected by the problem, could be helpful to farmers in the south, she says. This pamphlet is also available through Colin Ferguson at Invermay.

A biological control programme was initiated in 1998 and a parasitoid wasp from Ireland has been released in many areas of the North Island as well as at Richmond and Rai Valley. This biocontrol programme has been strongly supported by Dairy NZ, Meat & Wool NZ, and the Foundation for Research Science & Technology ever since it began. The CRW parasitoid has readily established at nearly all of its release sites, including those around Nelson, and is proving to be helpful in most areas in keeping the CRW populations under control.

Meanwhile the leader of AgResearch’s Biosecurity Group at Lincoln, Dr Craig Phillips, says additional CRW have recently been detected by his group in new locations elsewhere in the South Island including Takaka, Blenheim and Ashburton. It has also been found in a variety of new sites around Christchurch.

With a few exceptions, such as at Richmond and Rai Valley, CRW populations in the South Island have so far only reached low densities and have not yet caused severe pasture damage, but entomologists warn that the populations may still be building up.

Craig Phillips says given the propensity of CRW adults to fly in warm, dry conditions the current localised and patchy distributions of the pest are likely to spread at increasing rates, particularly in Marlborough and Canterbury.

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