Farmers will be able to observe first-hand the impressive impact of a biocontrol agent that acts against one of the country’s worst agricultural weeds at an upcoming field day hosted by AgResearch and Beef + Lamb NZ.

The green thistle beetle (Cassida rubiginosa) was released in 2007 to help fight Californian thistle, a plant recently estimated to cost almost $700 million each year in lost productivity. The beetle is now established in several parts of New Zealand including North Canterbury, Manawatu, and the site of the upcoming field day, near Wanaganui, is the site of the latest release.

In other areas where the beetle occurs, reports of severe and extensive feeding on Californian thistle are encouraging, and suggest great potential for this biocontrol agent. AgResearch is hosting the field day so farmers can see for themselves the impact of the beetle, discuss wider issues of thistle management and learn more about the role of biocontrol where one live organism (usually insects or fungi) is used to control another.

The adult green thistle beetles emerge in early October and quickly begin laying egg masses. Within a few weeks the first larvae emerge, and feed on the thistle leaves. By mid-November feeding damage from the young larvae is obvious but the most extensive damage is apparent by late December. The larvae tend to move steadily up the growing thistle shoots, consuming all the green leaf tissue and in some cases, all that remains are dead, skeletonised shoots. An additional advantage is that it also feeds on Scotch and Nodding thistles.

“The damage observed on Californian thistle at our trial site at Lincoln in Canterbury is impressive, and greater than anything I saw while working with this beetle in its native range of Europe,” says Dr Mike Cripps from AgResearch.

He suspects this is due to “enemy-free space” experienced by the beetle here, allowing for the maintenance of higher beetle densities and a longer duration of sustained feeding.

“In Europe, I recorded approximately 50% mortality one week after a field release of hundreds of green thistle beetle larvae. In contrast, at Lincoln I noted constant densities of larvae for a month on the same shoots,” he says.

This ongoing research programme is funded by Beef + Lamb New Zealand and the Ministry for Primary Industries through the Sustainable Farming Fund. The most recent component is to better understand the value of this biocontrol agent, particularly in hill-country pasture where conventional control techniques are not practical or cost-effective. Releases of the beetle were carried out in spring 2014 in hill-country pasture in Canterbury and Manawatu and will be evaluated over the next two years.

Dr Cripps warns that while initial progress has been impressive any significant and prolonged impact won’t be seen for many years.

“That’s the nature of biocontrol. It takes many years, or even decades, for the biocontrol agent to spread and become common and be able to achieve damaging levels.

“And, while they do not eliminate weeds, biocontrol attack is likely to result in smaller, weaker plants that are less likely to spread and can be more easily outcompeted by other plants or controlled by traditional means. Infestations may be reduced to a level that we can live with, or eliminate effectively and economically by other means. Biocontrol has the greatest impact when used in conjunction with wider good land management practices. “

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