Scientists are assessing the effectiveness of a tiny green beetle released in 2007 to help control one of the country's worst pasture weeds.
The green thistle beetle has become well established in areas where it has been released and its larvae appear to be doing a good job of munching their way through Californian thistle, and other types of thistle.
But AgResearch weed scientist Mike Cripps said there were still things to learn about the beetle's effectiveness as a biocontrol and that was the subject of an on-going Sustainable Farming Fund project.
"There are a lot of reports that it's having quite a significant impact. At certain sites we're seeing devastating defoliation of the thistles, so that's certainly promising and indicative of a good biocontol agent, but we are still lacking some quantitative data," he said.
The thistle was a perennial weed, so it regrew every year from a perennial root mass.
"We believe we need to defoliate the thistle to reduce that population, so what we don't really know is how much does that defoliation in one season affect the growth of the thistle the next year," Dr Cripps said.
Mike Cripps is running field days for farmers at hill country sites where the thistle beetles were released last year.
The first is at Scargill in North Canterbury today and near Taihape in the Rangitikei district in a week.