Mite introduced to control one of region's worst weeds

Eradicating old man's beard in Whanganui in 2014 were (from left) Horizons pest plant officer Neil Gallagher, Horizons councillor Rod Pearce and volunteer Ann Handley. Photo / Stuart Munro

One of the country's most noxious weeds may have just met its match.

Horizons Regional Council, on behalf of the National Biocontrol Collective, has been successful in its bid to be the first organisation in the world to use a mite for biological control against the invasive weed old man's beard.

The gall mite Aceria vitalbae has been cleared by the Environmental Protection Authority (EPA) to be imported and distributed within New Zealand. The import will mark a world first for a country attempting to control old man's beard by introducing an insect from its northern hemisphere home range.

Horizons environmental programme co-ordinator Craig Davey says the council has invested heavily in the fight to date.

"Old man's beard cloaks vegetation, ultimately killing other plant species such as our native trees and plants that make up our natural biodiversity," he said.

"Every year Horizons spends more than $500,000 controlling the pest by spraying, cutting and supporting community efforts to do the same. To date this has stopped the spread of old man's beard, however we are always looking at what more can be done."

The council has attempted other forms of biocontrol including a sawfly, leaf miner and fungus, but they haven't been very successful. Davey is excited by prospect of using the gall mites, which have been successful with other pest plants.

Importing the gall mite is the culmination of 10 years of hard work organising funding, rigorous testing and going through a thorough EPA application process to ensure the agent only affects the intended host, Davey said.

"Biological control is a technique used worldwide to restore balance between a weed and the environment by recruiting some of its key natural enemies. Pest plants that have been introduced to New Zealand are often not considered a weed in their home country because insects or diseases keep them in check."

The introduced mite will form galls on the host plant, which the plant will redirect resources to, reducing its capacity to flower, produce leaves and photosynthesise.

Extensive testing has been completed as part of the EPA application to ensure the gall mite will not pose a danger to other plant types.

"During the process we were also considering a bark boring beetle that would have been very damaging to old man's beard, however its taste for our native clematis meant this was not a suitable option," Davey said.

The gall mite is expected to be imported into the country in autumn 2019, for release in approximately spring 2019.

"We are planning to make the first introduction of the gall mite to the Taihape area. Many unique and wonderful habitats have been ravaged by old man's beard, and as we have a really engaged community in the Rangitīkei, we'd like to locate these mites in the places too challenging or risky for chemical control."

Horizons applied to the EPA to introduce the gall mite on behalf of the National Biocontrol Collective, comprised of 14 regional councils and the Department of Conservation.

Biocontrol has been undertaken by Horizons in a number of other parts of the region to address pest plants such as field horsetail, broom, wandering willy and ragwort.