And officials in charge of a pilot programme in the South say the green thistle beetles - public enemy No 1 of the Californian thistle - are back this summer, after being launched in late 2007.
The Californian Thistle Action Group and Landcare Research, which arranged the pilot scheme through the Maf sustainable farming fund, said the reappearance of the beetles was encouraging news.
Action group projects manager Malcolm Deverson said there were promising signs for the biocontrol programme launched with much fanfare and media coverage at a Waitahuna farm in November 2007.
"Thistle beetles released last summer and autumn have been sighted again this summer," he said last week.
The programme's launch was a major milestone in a 10-year push by the group of South Otago farmers looking for a biological control for the weed, which smothers large areas of pasture between October and April.
The beetles spend winter in the leaf litter, and begin a new life cycle when the thistles emerge again in the summer.
Mr Deverson said the beetles and evidence of their feeding has been seen at most of last year's seven release sites in South Otago and Southland.
A North Island site had also reported sightings of the beetles.
The beetles create transparent "windows" in the leaves that are easy to distinguish from other leaf damage, Mr Deverson said.
"It's really exciting and unexpected to see so much evidence of the insects' establishment so early.
"Often, these biocontrol agents that get released are difficult to find. Only 50 or 100 are released at a time and it can be four or five years or more before they are seen again."
Mr Deverson stressed it was a long-term programme, and farmers were unlikely to find huge patches of damaged thistles until numbers built up.
Breeding of the beetles was continuing at Landcare Research's Lincoln facility in Canterbury, and more beetles would be released later this month.
Farming officials believe Californian thistles cost Otago and Southland farmers $30 million a year in chemicals, mechanical topping and lost production.
Nationally, that figure was closer to $200 million.