Hurunui Biodiversity Trust’s first event this year drew a great turnout of people keen to hear experts share their perspectives and experiences about environmental weeds in our district, and what to do about them.

John Clemens, biosecurity adviser for special projects with Environment Canterbury explained that there are now 25,000-plus exotic plants in New Zealand, outnumbering native plants by about 10:1. Around 2600 exotics have become pest plants and their cumulative impact harms biodiversity, agriculture, recreation, infrastructure, and the economy. Many of these plants are not included on official biosecurity registers of unwanted species.

John monitors nurseries, garden centres and informal marketplaces where plants are traded. He described how he goes about this work, and where he finds most problems. The best way to get people onside is to give them clear information about any problem plants and explain why preventing their spread helps care for the environment generally. He suggested practical ways that community weedbusters can communicate about garden escapees and encourage action. This includes providing locally-specific information resources, and urging local bodies to prioritise removing weeds like spur valerian and pig’s ear from public places where they are easily visible and accessible.

Our next speaker was Noel Crump, ECan Biosecurity Officer based in Cheviot. Noel emphasised the importance of thorough planning before embarking on a targeted weed-control programme, and ongoing follow up once the initial control is done. Keeping records to measure progress, being careful not to spread plant material from the work site and educating local communities are also on his must-do list.
The main focus of Noel’s work involves agricultural pest plants and currently he’s going hard on Chilean needlegrass. He also deals with environmental weeds such as Old Man’s Beard, banana passionfruit and boneseed; this last is a garden escapee affecting the coastal environment in Hurunui and Banks Peninsula.
As a case study, Noel elaborated on his long campaign to rid Gore Bay of dense boneseed crowding out native vegetation around the cliffs, which started in 2003 and involved persistent and arduous work involving abseilers. The weed is still hanging on in a few places but is successfully contained. Noel and his crew are also controlling boneseed on Banks Peninsula, using drone spraying along with manual methods.

Our third speaker was Arnaud Cartier, entomologist at the Bioeconomy Science Institute (incorporating Manaaki Whenua Landcare Research). His topic was biocontrol, a technique of controlling unwanted organisms by reuniting them with some of their natural enemies. Arnaud explained how rigorous testing is conducted to ensure that agents attack only the target weed and won’t damage like species.
New Zealand is a pioneer in biocontrol and over the decades 70 agents have been released here, with mixed results. Biocontrol is most effective when a number of variables align. An effective biocontrol agent reduces the target plant’s health, hence its rate of reproduction and spread. Notable successes include tradescantia, broom and St John Wort. A big disappointment is gorse, which has beaten off six biocontrol agents. Notwithstanding some failures, the three most economically successful weed biocontrol programmes (ragwort, Saint John Wort and nodding thistle) yielded an annual saving of $84.7 million in 2022.
Four agents have been introduced to combat OMB. One that showed promise was the OMB leaf miner but it has fallen prey to a parasite. Focus is now on the OMB sawfly, which has been carefully bred up in the lab and released in several sites in Waipara under Arnaud’s programme. Following afternoon tea, Arnaud took workshop participants to visit one of these sites to view sawflies in action. There is abundant OMB in Hurunui for the sawfly larvae to feast on, and although it’s early days yet, slowing the spread of this notorious vine is a realistic goal.

All three presenters expressed concern that biosecurity investment in environmental weed control lags well behind that committed to agricultural weeds. They would like to see a better balance and overall increase in resources to fight our worst weeds and stop new ones emerging – a desire shared by Hurunui Biodiversity Trust!