Moth wroth unleashed

The invasive weed ragwort will feel the wroth of moths after ragwort plume moths were released into the Grebe valley, Fiordland National Park, as a biocontrol measure.

The project, undertaken by the Department of Conservation (DOC) in partnership with Transpower New Zealand Ltd aims to protect threatened native plant species in the Grebe valley.

DOC Senior Ranger Biodiversity Colin Bishop says ragwort is a severe threat to the biosecurity of some of New Zealand’s most exceptional wetlands.

“Highly invasive weeds, such as ragwort, Russell lupines and heath rush have been detected in the Grebe River and along the Borland Road. If nothing is done, they will continue to spread, threatening not only the wetlands but the wider Fiordland National Park.”

Transpower Stakeholder Engagement Manager Geoff Wishart says Transpower is delighted to be working with DOC on this project.

“Transpower has supported weed control in the Borland/Grebe area since 2015 because our high voltage transmission lines run through the area, and we respect the conservation initiatives that DOC manages. Introducing a biocontrol agent like the ragwort plume moth, which is already used elsewhere in New Zealand, is a great way of thinking outside the herbicide box.”

“We are delighted to work with DOC to ensure the Borland and Grebe area remains a haven for New Zealand’s native species.”

Colin Bishop says the plume moth, which is native to Europe, is well suited to wetter areas like the West Coast of the South Island.

“Plume moth releases began in New Zealand in 2006 and in this time have been effective in reducing ragwort. In Australia, where the moth is also used as a biocontrol agent, ragwort density has been reduced by 60–80 percent at some sites after only 1–2 years. However, it may take up to 5 years before the impact of the moth becomes fully noticeable.”

The moth works by laying larvae on the plants. The resulting caterpillars then burrow into the crown, stems and roots of ragwort plants. If the plant is not killed, then the damage caused means it produces fewer flowers and seeds, Colin Bishop says.

It is extremely unlikely that the moth will damage plants other than ragwort and marsh ragwort, also a weed in New Zealand, and hybrids of these two species, Colin Bishop says.

“Photopoints will be taken to document its effectiveness in the Grebe valley.”

The moths, caterpillars and larvae, which were sourced from the West Coast and Southland, were released last week.

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