Landcare Research has imported and raised tiny weevils that feed on the seeds of Darwin's barberry, a pest plant with the potential to overrun native plants and farmland.
Last year, 70 weevils were released in the Glenure Scenic Reserve, Northern Southland, and 200 more were released on a private property on Bushy Park Rd, near Waimumu, last week.
There have also been trial releases in the Wellington and Manawatu regions. Their effect will be monitored by Environment Southland (ES) staff.
ES senior biosecurity officer Randall Milne said Darwin's barberry was brought into New Zealand as a hedge or ornamental garden plant and had thrived in the wild in Southland and Otago and made its way north as far as Waikato.
It was now a banned plant and could not be cultivated, bought or sold.
The Chilean weevils - about the size of a house fly - fed on new growth on the plants, but it was the larvae that are expected to have the biggest effect, by feeding on the seeds, he said.
This week's release was the first true trial in Southland because of the small number released last year, he said.
‘‘It was decided [last year] that rather than over-winter the weevils in containment, it would be better to release them into the wild and see how they went.''
Using weevils as a biocontrol agent was still very much in a trial phase, Mr Milne said.
‘‘It is still unknown how they establish themselves in the field and what level of impact they will have, but they are a natural pest of Darwin's barberry in Chile.''
Extensive tests were carried out on weevils to ensure they did not damage any other plant species, he said.
A further release of 200 more weevils was planned elsewhere in Southland if they had a good breeding season.