Biological allies: Rust fungus and weevil to combat invasive Darwin’s barberry

In the ongoing battle to protect New Zealand’s unique ecosystems from invasive species, two unlikely heroes have emerged - a rust fungus and a small South American weevil. The Environmental Protection Authority (EPA) has granted approval for these biological control agents to be introduced to combat Darwin’s barberry, a noxious weed that has taken root across the country.

Darwin’s barberry is a spiny, invasive shrub threatening native ecosystems across New Zealand.

Native to Chile and Argentina, Darwin’s barberry (Berberis darwinii) was introduced to New Zealand in the 1940s as a garden ornamental. With its dense, spiny branches and bright yellow flowers it was prized for its hardiness and visual appeal. But the very traits that made it desirable in gardens have become a problem in the wild. The plant thrives in disturbed forests, pastures, shrublands and tussock grasslands crowding out native flora and reducing biodiversity. Birds feast on its blue berries and disperse seeds far from the original plant, allowing it to spread rapidly across the landscape.

Now with infestations particularly severe in Canterbury, Otago and Wellington, two South American organisms are being enlisted to help curb the spread. One is a rust fungus, Puccinia berberidis-darwinii and the other is the Darwin’s barberry flower weevil, Anthonomus kuscheli. Both target the barberry in specific and destructive ways. The rust fungus infects the plant’s leaves and fruits, weakening it and reducing seed production. The flower weevil lays its eggs in the flower buds, damaging the plant from within and further cutting back on viable seed formation. Together, the two agents are expected to significantly reduce the vigour and reproductive capacity of the weed.

“The effects of the agents are expected to combine and result in weaker plants and fewer viable seeds,” an EPA spokesperson explained. That could translate to a significant reduction in the spread of Darwin’s barberry over time - without the ongoing cost and labour of mechanical or chemical control.

Environment Canterbury submitted the application for release on behalf of the National Biocontrol Collective, a consortium of regional councils and the Department of Conservation that coordinates the development and implementation of weed biocontrol programmes in New Zealand. Standard control methods such as herbicides or physical removal are often impractical for Darwin’s barberry due to its remote locations, tough physical structure and extensive root systems. They are also expensive and can pose risks to surrounding native vegetation and waterways.

Biological control on the other hand, offers a targeted self-sustaining solution. “Once the organism is released into the wild, there’s no need for more human intervention,” said Dr Chris Hill, General Manager of Hazardous Substances and New Organisms at the EPA. “New Zealand has a strong track record of using biological control agents to manage invasive weeds with minimal impact on native ecosystems.”

That success is grounded in caution. Every potential biocontrol agent must undergo a thorough, science-based assessment before it can be released. “It’s important to know that using a biocontrol agent is one tool in our toolbox,” said a member of the EPA’s assessment team. “But we need a combination of all the tools we have to help us mitigate pest populations in New Zealand.”

The EPA’s process weighs each agent’s environmental, economic and cultural impacts. “We ensure that our biocontrol agent is safe to be released by doing a risk assessment on how it will impact our environment, but also our economy, our public health, the community and Māori and their relationship with the environment,” the EPA spokesperson said. “We also look at the latest research that is done on the agent to help us assess this impact. Once we finalise our assessment, if the benefits outweigh the risk then the biocontrol agent can be released into the environment.”

In this case, both agents passed that test. The flower weevil which is just a few millimetres long, does not bite or sting and poses no threat to people. It also does not attack any plant other than Darwin’s barberry. The same is true of the rust fungus - it is highly specific to Darwin’s barberry and poses no threat to other plant species or crops.

That specificity is crucial. New Zealand’s experience with past biological control efforts - such as those targeting purple loosestrife, old man’s beard, Sydney golden wattle and moth plant has demonstrated that when carefully selected and properly assessed, biocontrol agents can help restore native habitats without collateral damage.

For mana whenua and many communities around the country, that careful stewardship matters. The EPA’s process included engagement with Māori to ensure the proposal aligned with cultural and environmental values. The ultimate goal is to protect New Zealand’s native biodiversity, which includes not only unique plant and animal species but also the ecosystems that support traditional practices, food gathering and spiritual connections to the land.

The rust fungus and flower weevil are not silver bullets, but they mark an important step forward. “Assessing biocontrol agents is an important role of the EPA,” the spokesperson said. “It helps us to continue to protect the environment and our unique biodiversity.” Over time these two agents are expected to reduce the density and reproductive success of Darwin’s barberry populations, slowing its spread and allowing native vegetation to recover. The success of the programme will be monitored, and further releases may be planned in areas where the weed is particularly aggressive.

The EPA’s decision reflects international best practice in weed management, drawing on lessons from decades of experience in other parts of the world. By using nature to fight nature, and doing so with care and precision New Zealand continues to lead the way in innovative and sustainable environmental protection.

In a country where introduced pests have often outcompeted native species with devastating effect, the rust fungus and the flower weevil may not look like heroes. But in the quiet corners of tussock country and the shaded edges of native bush, their work could mean the difference between losing and regaining the delicate balance of our natural world.