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Weevils imported to take on a pest plant in Whanganui and Rangitīkei are establishing themselves, four years after being released.

Horizons Regional Council environmental manager Craig Davey said an adult field horsetail weevil had been discovered alongside larvae, a sign they had survived the winter season.

“Field horsetail is a simple plant and has been around unchanged since before the dinosaurs, so its mechanism of resilience is quite renowned,” he said.

“It has a little escape capsule in its root system, so if you even get herbicide in there, it won’t go into the storage organs that allow regrowth to happen.”

The plant could “have kilometres of roots under the ground”, he said.

“Weevils bore into and feed inside the stem, which kills everything above them.

“The adults then lay eggs, and the larvae, once hatched, eat down the stem through to the roots, which helps to effectively kill the plant.”

He said they were imported from the UK and only ate field horsetail.

For the project, the council partnered with the Rangitīkei Horsetail Group, established in 2012 and facilitated by the Landcare Trust.

Davey said the weevils were released in the Whanganui and Rangitīkei districts around four years ago.

“Collaborating with the group has led to this wonderful outcome.

“They helped research all the options to manage field horsetail, which is how we landed on the weevils.

“A special mention to Manaaki Whenua–Landcare Research, who have also helped us on this journey."

Four locations with intergenerational increases in weevils had been discovered this year, he said.

“We just have to have patience. Over time, they will build up and help to keep it in balance with other vegetation.”

The weevils only eat field horsetail plants, Horizons environmental manager Craig Davey says.

He said the plant was found in Hawke’s Bay, Taranaki, on the West Coast and particularly in the Horizons region.

It was a threat to the agricultural sector, especially since 2004, when pasture in lower Rangitīkei was badly flooded, he said.

“There was no horsetail, then there were 10 hectares of horsetail.

“You were unable to establish good pasture without doubling your costs.

“We don’t want that to happen elsewhere.”

Davey said the plant was brought into the country because of its homoeopathic, herbal and practical uses.

“We actually got contacted by a German sauna house wanting us to supply them with out-of-season, in the Northern Hemisphere, horsetail.

“They wanted to use it for flagellation. We said, ‘Yeah, we’re not going to do that’.”

Earlier this year, tradescantia leaf beetles were released into Rangitīkei’s Tutaenui Reservoir to eat tradescantia, a weed originating from South America that grows quickly and creates thick mats of vegetation.

Davey said releasing insects to tackle invasive plants was “a well recognised tool”.

“Our intent is to have less herbicides and less cost.

“We did the numbers with Manaaki Whenua – Landcare Research, and for every dollar invested in ragwort biocontrol over 30 years, there is at least a $14 payback.

“That’s a good investment.”

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily Whanganui District Council.