Pasture-damaging clover root weevils have spread further in Otago after first being found in the province a year ago.
AgResearch said today that it had discovered clover root weevils (CRW) on the Taieri Plain and its scientists were sampling local farms to see how well established the pest was in the area, before deciding how to address the infestation.
The CRW reduces the contribution clover makes to soil fertility by helping "fix" nitrogen from the atmosphere in soil to help pastures grow.
The nitrogen-fixing role of clover is also under threat from the loss of "feral" honeybees to pollinate clover for free, as varroa mites kill off bees outside managed hives.
The CRW was first discovered in 1996 in Waikato and Auckland, and by 2004 it had spread throughout the North Island. In 2006, AgResearch found a large population established in dairy pasture at Richmond, near Nelson, and a year ago it was detected at Clinton in south Otago.
AgResearch scientist Dr Barbara Barratt said the weevils were small and hard to see, but farmers could assist by telling AgResearch if they saw the typical weevil feeding damage on their white clover.
"The adults feed on white clover leaves producing a characteristic 'notching' of the leaf margin. Up to 1500 weevil larvae per square metre were counted in the soils underneath Waikato pastures in 1997, and this dramatically reduced clover content," she said.
AgResearch has released a tiny wasp from Ireland as a biological control agent for the weevil.
"The wasp has been used successfully in the North Island to control this pest. The wasp lays its eggs in the weevil, thereby making it sterile," Dr Barratt said. "However, it is important more is understood about the distribution of CRW on the Taieri before we consider how we tackle the pest in the area.
"The arrival of the weevil on the Taieri Plain is disappointing but inevitable."