The largest population of the invasive weed, purple loosestrife, in Manawatū-Whanganui is at Lake Horowhenua and the surrounding area. Photo / Janine Baalbergen
The largest population of the invasive weed, purple loosestrife, in Manawatū-Whanganui is at Lake Horowhenua and the surrounding area. Photo / Janine Baalbergen
Four species of insects are set to be imported and released in the Horizons Regional Council area after the Environmental Protection Authority approved their use to control an invasive wetland weed.
Horizons applied to introduce the insects in the Manawatū-Whanganui region as biocontrol agents to target different parts of the purple loosestrife plant, Lythrum salicaria.
“These insects have been used as biocontrol agents for more than 30 years in the United States and Canada, where they reduced purple loosestrife infestations by up to 90 per cent,” the authority’s hazardous substances and new organisms general manager, Dr Chris Hill, said.
“We hope the introduction of these insects will help slow the spread of this aggressive weed in our precious wetland ecosystems, which are home to many amazing native species, as well as capturing and storing carbon.”
The insects include two beetles that eat the leaves of the purple loosestrife: Neogalerucella calmariensis (commonly known as the black-margined loosestrife beetle) and Neogalerucella pusilla (golden loosestrife beetle). The others are a root-feeding weevil, Hylobius transversovittatus (loosestrife root weevil) and a weevil that eats purple loosestrife flowers, Nanophyes marmoratus (loosestrife flower weevil).
Purple loosestrife is a bushy plant that forms high, impenetrable stands which overwhelm other plants, threatening native biodiversity and significantly impacting wetland ecosystems.
It is listed in the top 100 invasive species worldwide by the International Union for Conservation of Nature and has been rated by the Department of Conservation as similar to old man’s beard and Japanese honeysuckle in terms of its impact on conservation land.
It is native to Europe, parts of Asia, and Australia, and was introduced to New Zealand as an ornamental herb before naturalising in the wild in the 1950s.
An established plant can create more than 2.5 million seeds.
“Using chemicals or manually removing this invasive plant is often challenging due to the sensitive and remote nature of wetlands,” Hill said.
“These insects are an environmentally friendlier method of controlling these plants, rather than using chemicals.
“The testing results provided to us showed these insects are highly unlikely to harm native plants or animals. There is also no risk to people.”
The Neogalerucella calmariensis, commonly known as the black-margined loosestrife beetle.
Purple loosestrife was well-established in some regions, such as Canterbury, the West Coast, Wellington, and Manawatū-Whanganui with the largest populations at Lake Horowhenua, Hill said. However, it was yet to spread more widely through New Zealand and the release of the insects may help to stop, or dramatically slow, the infestation.
“Our panel of independent experts approved these insects for import and release following a rigorous, evidenced-based investigative process which included the consideration of public submissions and international best practice, and engagement with mana whenua.”
Horizons biodiversity and biosecurity manager Craig Davey said he was pleased approval had been granted, as using chemicals or manual removal was not an option in lakes and wetlands.
“These insects are an environmentally friendlier method of controlling these plants, rather than using herbicides over water and native vegetation.
“The introduction of these insects will stop the negative transformation of our precious wetland ecosystems. We have to do all we can to protect the tiny amount of these unique habitats which are home to many amazing native plant and animal species.
“Before the release of the insects, the next step is to continue our conversations with mana whenua and the wider community about this approach to habitat protection.”
