Control of clover root weevil

Clover root weevil (CRW) is difficult to control in pasture partly because the larvae live in the soil which makes them difficult to kill with insecticides. Also, the adults are very mobile and can fly, so treated pastures can be rapidly reinvaded from surrounding untreated areas. However, the establishment of clover seedlings in areas infested by clover root weevil could be assisted by the application of a systemic insecticide at sowing.

The use of biological control agents in tandem with careful management of clover will play an important role in reducing the impacts of CRW.

A parasitoid wasp for biological control

AgResearch first released the CRW parasitoid (a biological control agent) in Waikato, Hawke's Bay and Manawatu in January 2006 where it rapidly became established. It has since been released throughout the North Island and it appears to be effective in most places.

The agent was subsequently released in the South Island at Richmond and Rai Valley in winter 2006, with funding from Dairy New Zealand and Meat & Wool New Zealand, and logistical support from Nelson Federated Farmers, the NZ Landcare Trust, and local farmers. The CRW parasitoid has become well established at those places and plans are underway to release it in Takaka Valley, Blenheim and Canterbury during autumn-winter 2009.

The biological control agent is a tiny parasitoid wasp called Microctonus aethiopoides which AgResearch collected and imported from Europe. The Environmental Risk Management Authority of New Zealand accepted the results of AgResearch's testing which showed this biological control agent would be environmentally safe and gave permission for it to be released in late 2005. The CRW parasitoid is a different strain of the same species which was imported to control lucerne weevil (Sitona discoidues) in 1982.

The lifecycles of CRW and the parasitoid are described in detail in an information sheet produced by AgResearch.

Beauveria for biological control

Beauveria is a fungal pathogen which is specific to insects, and is safe for plants, livestock and humans. It infects and kills weevil larvae and pupae in the soil, and adults can also pick up the disease as they emerge from pupae and move out of the soil. AgResearch is testing the fungus as a biological insecticide, applying a granular formulation to the soil with a seed drill to try to protect clover roots from feeding damage. AgResearch is currently evaluating if Beauveria needs to be re-applied every year, or if will it establish in the soil to provide long-term suppression.

Clover management

• Look for the presence of CRW damage on leaves, and dig under clover plants looking for healthy pink nodules and for CRW larvae.
• Do not assist CRW dispersal by moving hay from infested areas to those that are not.
• Observe the survival and growth of clover plants in new and old pastures, to assess CRW impacts
• Review your nitrogen fertiliser policy in light of CRW, consistent with feed demand and feed supply, and environmental guidelines.
• Review total fertiliser and lime policy in light of production, current soil fertility, reduced nitrogen fixation, and increased dependence on fertiliser nitrogen.
• Do not attempt to re-establish clover into CRW infested pastures by drilling or over-sowing clover seed, because adult CRW prefer clover seedlings.
• Include white clover in the seed mix sown after full cultivation. New clover plants develop a taproot, which may be an advantage for survival in year one.
• Apply nitrogen fertiliser for the clover as well as for the grass to support clover growth and survival, especially in new pasture.
• Additional phosphate based fertiliser, lime and any other soil-additives, will not rejuvenate clover in presence of CRW. Re-grassing after cultivation does not have a lasting effect on CRW, as adults re-populate these areas.
• Over 80% of New Zealand dairy farmers currently apply nitrogen fertiliser from 25 to 200 kg N/ha annually, at typical rates of 25 to 50 kg N/ha per application. While these application rates are primarily targeting a grass response they are the first and most important step to minimise the impact of CRW on the farm business.
• Farmers with CRW infested pastures report improved clover growth and plant survival from small but frequent applications of nitrogen fertiliser applied year round.

More tips for managing clover in the presence of clover root weevil can be found in the pamphlets produced by Waikato Clover Management Group, Meat and Wool New Zealand, and Homelea Ltd.

There is a MAF Sustainable Farming Fund project currently researching the on-farm management of the economic and environmental impacts of clover root weevil.