Beetles Decimate Californian Thistles at Lincoln
Green thistle beetle (Cassida rubiginosa).
The green thistle beetle (Cassida rubiginosa) is attracting a lot of attention, not surprisingly, since it can cause impressive levels of damage on Californian thistle (Cirsium arvense), one of the worst agricultural weeds in New Zealand.
The beetle was first released here 5 years ago and is now well-established. Where the beetle occurs, anecdotal reports of severe and extensive feeding on Californian thistle are encouraging, and suggest great potential for this biocontrol agent. Although it is still early years for this beetle in New Zealand, it appears to be the most successful biocontrol agent released for control of Californian thistle to date.
At Lincoln, near Landcare Research a small population of green thistle beetles caused some striking damage to Californian thistles this past spring/summer. The adult beetles emerged in early October and quickly began laying egg masses. Within a few weeks the first instar larvae had emerged, and were greedily feeding on the thistle leaves. By mid-November feeding damage was obvious, but this was only from the first and second larval instars. The most extensive damage was apparent by late December, caused by the larger third to fifth larval instars. The larvae tend to move steadily up the growing thistle shoots, consuming all the green leaf tissue. In some cases, all that remained were dead, skeletonised shoots. Californian thistle is the primary target of this biocontrol agent, but an additional advantage is that it also feeds on other thistle species. At the Lincoln site, damage to Scotch thistle (Cirsium vulgare) was observed, but not nearly to the same degree as on Californian thistle.
“The damage observed on Californian thistle at Lincoln was impressive, and greater than anything I saw while working with this beetle in its native range of Europe,” said Mike Cripps of AgResearch. The greater damage in New Zealand compared to Europe is intriguing, and Mike suspects it’s 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,” explained Mike.
The degree of damage observed at Lincoln will likely cause population declines in Californian thistle. However, quantitative data from controlled field experiments under realistic pasture management systems in New Zealand are still lacking. Now that this biocontrol agent is established there are many new research questions to be answered. What level of impact does it have on Californian thistle, and other thistles here, and what limits the beetles’ population numbers in New Zealand? To properly evaluate the effectiveness of this biocontrol agent substantial financial investment will be required to support on-farm experiments carried out at several sites over multiple years. “Given that thistles are among the worst pastoral weeds in New Zealand, causing tremendous productivity losses, the importance of understanding this biocontrol agent, its effectiveness, and how to best utilise it for thistle management cannot be overemphasised,” concluded Mike.
Mike Cripps
mike.cripps@agresearch.co.nz