Biocontrol agent helps keep clover root weevil in check

Biocontrol agent helps keep clover root weevil in check

The introduction of a biocontrol agent for clover root weevil in 2006 has done much to suppress the pest's impacts, but vigilance is still critical to detect new infestations and ensure that the biocontrol agent quickly reaches the areas where it is needed.

When clover root weevil (CRW) was first found in Waikato, New Zealand in 1996, almost nothing was known about its impact on pasture or about how to manage it. Effective economical management options remained elusive, as over the next decade, CRW slowly spread throughout the entire North Island.

In 2006, AgResearch scientists, supported by DairyNZ, Beef + Lamb NZ and AGMARDT, made a breakthrough in CRW control by releasing a potential biocontrol agent, a tiny parasitic wasp from Ireland. The first trial releases were made in Waikato, Hawke's Bay and Manawatu, and within just 18 months the wasps' performance had exceeded the expectations of even the most optimistic scientists.

Although the biocontrol agent can never entirely eliminate CRW, it can strongly suppress the pest within just a year or two of arriving in a new area, alleviating some of the pressure both on pastures and on farmers' pockets.

AgResearch distributed the biocontrol agent throughout the North Island as quickly and comprehensively as possible, in part by giving it away through industry networks and at field days so farmers could make releases on their own properties. Hundreds of these mini-releases combined with mass releases at fewer key sites have led to the spread of the biocontrol agent throughout the North Island. CRW is now in decline at all sites AgResearch has monitored and only in the Far North are there concerns that the weevil could still retain the upper hand.

Meanwhile, on the South Island CRW made its first unwelcome appearance near Christchurch and Nelson in 2006. Fortunately for South Island farmers, the biocontrol agent for CRW, which had just been liberated in the North Island, was available for immediate release in southern pastures. AgResearch quickly introduced the biocontrol agent to large populations of the weevil at Richmond and the Rai Valley in 2006, and then again in Golden Bay in 2009 in the hope that the wasps would spread along with CRW and thereby minimise the pest's damage in the South Island.

Again, the biocontrol agent exceeded expectations by catching up with the outer limits of CRW populations in Golden Bay, Nelson and Marlborough, and then staying hot on CRW's heels as it expanded southwards. Sampling in 2011 showed that the wasp has been carried by the weevil south as far as Springs Junction and Kaikoura. It has also reached Lincoln, probably from North Canterbury or Rakaia Island where it had previously been released.

Unfortunately, it is still likely that CRW will spread throughout the South Island, because it is adept at spreading about 20 kilometres every year by making flights during warm summer days. In addition, CRW can also cover longer distances by moving with vehicles and freight, which has resulted in a series of outbreaks in locations distant from previously established populations.

The first such outbreak was detected near Ashburton in 2008, followed in 2009 by Clinton in Otago, Rotherham in North Canterbury, Mosgiel in Otago and Mataura in Southland, then in 2011 at Woodlaw in Southland. Just in the last 6 weeks it has been found at Catlins in Southland, Ophir in Central Otago, and near Duntroon in North Otago. The timing and location of these isolated outbreaks is difficult to predict, and thus AgResearch, with support from DairyNZ and Environment Southland, has developed a broad-scale monitoring programme with careful prioritisation of sampling localities.

In addition, over the last 18 months scientists have made further releases of the biocontrol agent in isolated CRW infestations at Rotherham, Mosgiel and the Taieri Plains, Mataura, Gore, Woodlaw, Telford and Rakaia Island, with establishment already confirmed at Rotherham, Mosgiel, Mataura and Rakaia Island.

Experience shows CRW is very difficult to detect during the first few years of its arrival at a new location, partly because it is initially present in very low numbers and also because it spends much of its time hidden underground as a larva or pupa.

The build up of CRW in a new location is insidious, and sometimes farmers only realise the pest is present once their clover has all but disappeared. Notching of white clover leaves and feeding damage on root nodules are symptoms of CRW, but careful observation is required to detect these symptoms before the pest has reached high numbers.

This means AgResearch's CRW monitoring is imperfect and some infestations could remain undetected for years unless farmers assist by being vigilant and reporting their observations. Advising AgResearch that CRW is suspected in a new location will help scientists to ensure the biocontrol agent quickly reaches the areas where it is needed.

South Island farmers who suspect that they have a CRW infestation can access the website www.agresearch.co.nz/crw contains all the information farmers might require to assist in identifying CRW.

Research on clover root weevil will feature at the AgResearch exhibit at the National Agricultural Fieldays at Mystery Creek on 15-18 June 2011.

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