Horizons Regional Council releases beetles to tackle invasive tradescantia plant

Horizons Regional Council pest plant biosecurity officer Rory Johnson says environmental technology is improving dramatically.

A special beetle has been released into Rangitīkei’s Tutaenui Reservoir to combat a plant causing “a very big problem”.

Tradescantia leaf beetles will go after tradescantia, a weed originating from South America that grows quickly and creates thick mats of vegetation.

Horizons Regional Council pest plant biosecurity officer Rory Johnson said the plant was widespread throughout the region and the rest of New Zealand, and the beetles were a good control option for “something so large”.

“You get it in waterways and, like most things, it travels through human movement,” he said.

“It ends up at parks, private land and native forest, and it densely covers forest floors. That completely stops native regrowth coming up.

“Tradescantia is a very big problem.”

The beetles came from Manaaki Whenua-Landcare Research, which did a lot of testing to make sure they did not spread from the target plant, Johnson said.

“They [Manaaki Whenua] pick a bunch of species similar to tradescantia and test them all with the bug to see if they will transfer and start eating them.

“It’s a really rigorous process.”

Johnson said tradescantia was brought into the country as an ornamental.

“There are some genus of it that actually look quite good.

“Unfortunately, New Zealand’s climate is perfect for a lot of these pest plants.”

Manaaki Whenua was constantly testing insect species overseas to see if they would be appropriate for control in New Zealand, he said.

The old man’s beard gall mite and sawfly were released in the Horizons region last year.

The beetle was first imported from Brazil in 2007.

“Environmental technology – biocontrol – is improving dramatically,” Johnson said.

“People are branching out and trying to find different ways to be effective.

“I think providing a somewhat natural way to control invasive species is quite cool.”

According to Manaaki Whenua, the tradescantia leaf beetle was first imported from Brazil into containment for testing in 2007.

However, releases did not begin until 2011 because of the need to clear the beetle of a gut parasite.

Johnson said the beetles had cleared more than 180ha of St John’s weed in Marlborough in about four years, saving an estimated $15.5 million a year.

In Canterbury, beetles and fungus “completely destroyed” a large area of tradescantia in two years.

“In the right climate and undisturbed, they can really do well,” he said.

“Naturally, they’ll spread out, find a new patch and breed.”

Yellow spot fungus was also deployed to tackle the plant at Tutaenui.

“A combination often works even better – you can get one eating the leaf and one that damages the stem or the roots,” he said.

People could call Horizons to get beetles for their property, but there was a waiting list.

“January to February is the perfect time to distribute them.

“It might take a little while to get beetles, but I have really good populations of fungus, so I can organise a day and do the rounds.”

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.