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ONE OF 70: A trial group of barberry seed weevils have been released near Invercargill, with several thousand more to come.

Tiny Chilean beetles have been released in Southland to try to head off a threat from an invasive weed that scientists fear could become as big a problem as gorse.

The weed threat is from Darwin's barberry, which comes from Chile and became established in this country in 1946. It is common from the central North Island to Stewart Island.

It grows on grazing land and can displace native plants in bush remnants and aggressively invade forest margins.

The 5-metre tall spiny shrub, has deep orange-yellow flowers, followed by abundant purplish-black berries. Birds feed on the fruit then drop the seeds elsewhere.

The beetle that researchers hope will control the plant is the barberry seed weevil, which Landcare Research has studied for years in collaboration with Chilean scientists.

Landcare provided 70 of the weevils to Environment Southland for a pilot release just north of Invercargill, with several thousand more to be released early next year.

Extensive tests were carried out on weevil adults and larvae to ensure they could not damage other plant species.

Weevil larvae fed on barberry seeds and were expected to stop the spread of the plant, lead researcher Lindsay Smith said.

Landcare Research scientist Simon Fowler said other biocontrol agents would be needed to destroy existing Darwin's barberry infestations.

After the seed weevil was established, researchers would decide whether another weevil which attacked the flowers was needed, he said.

Pathogens, which could potentially damage the plants, were also being assessed.

- Photo has been corrected

 - Stuff