Broom was introduced into New Zealand by settlers in the 1800s to plant as hedges. What was intended to be a decorative and functional plant thrived in our climate and soon became a pest plant as it invaded the landscape.

ORC Director Environmental Monitoring and Operations Scott MacLean said his team has been testing the effectiveness of broom gall mite, which is a tiny bug that can stunt the growth of broom, and eventually kill it.

“We released broom gall mites at two sites in Central Otago in late 2012, and while progress started out slowly, it took off last summer,” he said.

“We had an exciting surprise at one of our release sites, where the mite has spread out over 400 meters from the original release site, which is further than expected.

“Thankfully the mite only likes broom, so it won’t damage any other plants,” Mr MacLean said.

The mites form galls (deformed lumps on the bush) and feed in these during spring and summer. They move to new stems as the weather cools and the galls start to wither.

The mites spread on the wind, so infested plants need to be harvested to move the broom gall mite to new sites. Harvesting involves tying a branch with at least 50 galls on it to a healthy broom bush, then waiting for the mites to move across to the plant and start feeding on it.

“It’s a long-term approach and requires patience, but what we’ve seen so far looks promising,” Mr MacLean said.

“Once we have enough galls at our original release site, we want to harvest them and see how well the mite copes in wetter areas so we can eventually spread the mite to coastal areas of Otago.

“Biosecurity is not always glamourous or fast-paced, but it’s an essential part of our work at ORC to keep pest plants such as broom from affecting biodiversity and taking over our beautiful Otago landscape,” Mr MacLean said.

Broom is a pest plant because it grows almost anywhere and can take over native tussocks. The seeds can be viable for up to 100 years, which is why it’s important to keep on top of it.

Over half of Otago (16,784 square kilometres) has been designated as broom-free. These areas are Queenstown, Wanaka, Cromwell, and Alexandra, and land occupiers need to destroy all broom (and gorse) on their land.

ORC is reviewing its Pest Management Strategy in the next year and aims to extend the broom-free area as part of this review.

For more information contact:

Scott MacLean
Director Environmental Monitoring and Operations
ORC
Ph 0800 474 082 or 027 411 9459

Communications contact
Mark Peart
Team Leader Communications – Channels
Ph 0800 474082 or 027 5312620
Email: mark.peart@orc.govt.nz