Horizons Regional Council releases mite to curb invasive weed old man's beard

Old man's beard produces dense masses of foliage that smothers and kills all plants up to the highest canopy. Photo / Supplied

Horizons Regional Council is to introduce a leaf-galling mite in a New Zealand-first bid to control the highly invasive pest plant known as old man's beard.

On August 19 a small number of plants, shipped from Manaaki Whenua - Landcare Research's Lincoln campus, will be planted in the Taihape area. They have been infected with the gall mite Aceria vitalbae.

Old man's beard (OMB or Clematis vitalba) is a serious problem across much of our region, says Horizons environmental programme co-ordinator Craig Davey.

"It's a fast-growing competitive vine which establishes rapidly and spreads easily. OMB smothers all but the tallest trees, replacing indigenous species and forming dense carpets in the understorey.

"Implementing an effective biocontrol agent for old man's beard will help to slow the spread and impact of this highly invasive and difficult to control species."

Horizons applied to the Environmental Protection Authority to introduce the gall mite on behalf of the National Biocontrol Collective, comprising of 14 regional councils and the Department of Conservation.

The application was approved in 2018 and shortly after OMB plants infected with mites were sourced from Serbia by Landcare Research.

The Aceria vitalbae mite is only 1mm long and has no method of dispersal. It works by forming galls in the "host" plant, forcing the host to redirect resources to the galls, which in turn reduces the host's growth rate and can cause shoots to die back.

Horizons spends more than $500,000 annually trying to eradicate OMB. Both ground control and targeted helicopter spraying of the most suitable herbicide can cost up to $1500 per hectare.

"Old man's beard requires intensive, manual effort to control and requires follow-up maintenance control. While herbicides can be effective, this method is limited because herbicides can also harm the host vegetation.

"The use of a biological control like the gall mite is not only more cost effective, it allows nature to promote change. We like to think of it restoring some sense of balance," Davey said.

Horizons has engaged with local iwi to introduce this biological control into the Taihape area. Once it is established, Horizons is keen to involve tāngata whenua in monitoring and care of the sites.

"This is early days, and we need to allow for the mites to mature through their life cycle, but this introduction of the gall mite gives us immense hope in the fight against a pest like old man's beard," Davey said.