Bioscientists aim to bring in wasps to control spread of invasive plants on Whanganui and New Zealand coasts

Horizons Regional Council wants to bring in a species of wasp to control thickets of acacia plants sprouting up on Whanganui coastlines. Photo / Bevan Conley

Bioscientists from Horizons Regional Council want to introduce wasps to control the spread of an invasive plant on Whanganui coasts.

The Environmental Protection Authority (EPA) is calling for public submissions on the plan to release the bud-galling wasp as a biological control agent for the spread of Sydney golden wattle (Acacia longifolia).

Sydney golden wattle is a native plant to Australia, a fast-growing shrub or small tree which was introduced to New Zealand as an ornamental and became naturalised before 1897.

Horizons applied for the approval and release of the insect on behalf of the National Biocontrol Collective, a group of regional and district councils from across New Zealand and the Department of Conservation (DoC).

Horizons' biodiversity and biosecurity manager Craig Davey said biocontrol scientists were excited to potentially bring the wasp in as there was evidence of its success in stopping the plant's spread.

He said in the early 1980s South Africa used the wasp to stop wattle from spreading and it was extremely effective while not damaging other plants.

"So we've got experience from scientific literature, reports, analysis and monitoring since the early '80s ... showing the effectiveness of the wasp in South Africa."

The wasp lays its eggs in flower buds, inducing abnormal growths (galls) that prevent flowers from forming and seed production.

Davey said in South Africa, three years after the introduction of the wasps, the spread of the plant was limited to around 20km.

"So we're really excited to limit the growth of the plant and also limit the spread of the plant with this one little wasp, so that's a really good result," he said.

In Whanganui the acacia was originally planted with the purpose of stabilising Castlecliff's sand dunes, Davey said.

"You can go back through photos of Castlecliff, go back to the black and white days, and see lines of acacia put out there for sand stabilisation."

He said since that planting, and due to the high winds the coast often received, the plant's flowers and seeds were blown across the river, resulting in heavy thickets of acacia along the west coast of the lower North Island.

Davey said the thickets now appeared from Seafront Rd extension in Castlecliff, and in sporadic but often thick amounts down South Beach to the Turakina and Whangaehu river mouths, before continuing down the coast into Manawatū.

He said Horizons' main concern about the plant was the rate at which it was spreading, and how it had overtaken areas previously inhabited by native plants.

"That's our concern, that areas where it wasn't existing, were on the fringe, are now either thickening up or have new plants in them, and it's like, holy crap, this is still spreading,"
he said.

The measures against the acacia needed to be taken to protect the coasts, both for the people who used them and for New Zealand's biodiversity, Davey said.

"Acacia takes away from that, it's a real stealer of value because it creates a monoculture of impenetrable thickets."

He said the wasps don't sting or bite people, live for only three days and are 3mm in length.

If the wasps were brought in, the plants would be removed from some areas, such as tracks or picnic areas, once the seeds had stopped spreading.

Horizons would prefer to mostly allow native species to naturally retake the areas to restore a balance to the coastline.

"The hope is to let the wasp do its thing and you get this gradual change on the acacia-dominated vegetation to a matrix where natives will be brought in by birds or wind and start to fill the gaps where the acacia is weakened," he said.

Public submissions on the introduction of the wasps close at midnight on September 30. More information is available on the EPA website epa.govt.nz