Sawflies released to suppress weed

European sawflies have been released in parts of Marlborough in an attempt to combat the growth and spread of another European import – old man’s beard.

Old man’s beard is a highly invasive alien pest plant and is a serious problem in the Marlborough region.

Introduced to New Zealand in the early 1900s, the weed can grow extremely quickly and extremely high – up to 20m tall, smothering and killing off all other vegetation and native flora.

As a result, the Marlborough District Council has released the sawflies to work as a biocontrol agent – a process where living organisms are used to interfere with the productivity of other living organisms.

This was part of a regionwide effort between top of the south/Te Tauihu councils and the Kotahitanga mō te Taiao Alliance (KMTT) to suppress the spread of old man’s beard.

Hudson Dodd is a project manager for KMTT’s Restoring and Protecting Flora project, which has been doing ground control of old man’s beard across the region.

Dodd said that biocontrol was promising and aligned well with the alliance’s vision of a future in which native species thrived, ecosystems were restored, and ecological connections and resilience were protected.

“The Restoring and Protecting Flora project has been controlling old man’s beard at strategic locations across the region using traditional ground control methods,” he said.

“Places like the Hinepango wetland were chosen because of the high biodiversity values threatened by this weed.

“We’ve collaborated with the Marlborough

District Council, the Department of Conservation and the Hinepango Wetland Conservation Group here to help do the heavy lifting and open the wetland up for planting and ongoing maintenance weed control.”

Dodd said it was exciting to see councils investing in new science and technology to control old man’s beard.

A “landscape-scale approach” was needed to protect native ecosystems before it was too late, he said.

The sawfly stocks were sourced from Serbia in 2017 and their release began in Marlborough in February this year.

The larval stage of the sawfly damages the plants by chewing their way through leaves, which ultimately affects the vine’s vitality and vigour.

In other attempts to combat the spread of the invasive weed, the district council’s biosecurity team carried out five releases of the old man’s beard gall mite in the region between 2021 and 2023.

Old man’s beard plants were purposely infested with the microscopic mites, and then planted at various sites in the hope that they would colonise the pre-existing weeds.

Rob Simons, the council’s senior biosecurity officer, said some invasive weeds were not included in the council’s pest management plan as controlling them was beyond the scope of ratepayer funding.

“The introduction of biocontrol agents is the best option for suppressing this weed in roadside reserves and less significant ecological sites,” he said.

“Hopefully they will target old man’s beard and reduce seed sources with minimal impact on the wider ecosystem.”

TARANAKI DAILY NEWS Sawflies, pictured in larval and adult form, have been released as biocontrol agents to help control the spread of old man’s beard in Marlborough. (File photo)