Broom Gall Mite a Decade on
It has been 10 years since the broom gall mite (Aceria genistae) was brought to New Zealand to try to reduce the menace caused by Scotch broom (Cytisus scoparius). Although the mites are too small to see with the naked eye, the damage they are doing to broom is now becoming obvious.
The gall mites spend the winter hiding away in the buds on broom stems, and in the spring feed on the new growth, causing galls to form that can be up to 30 mm in size. As well as damaging the plant, the galls offer a refuge for successive generations of the tiny gall mites and protect them from predation. “We are excited to be finally making progress on this plant, which causes extensive damage to both conservation and productive landscapes,” said Quentin Paynter, who has devoted several decades now to studying broom and how best to beat it.
Hugh Gourlay has been checking on the mites’ progress at Leslie Hills in North Canterbury where trials have been set up to measure the impact of broom biocontrol agents. He has been surprised at how quickly the mites are getting around. “The mites are being more easily dispersed by the wind than we thought they would,” said Hugh. Adults initiate a stand-up posture when they want to be carried by wind, but a number of factors affect this, including wind speed, temperature and humidity,” said Landcare Research’s mite expert (acarologist) Zhi-Qiang Zhang.
While the mite is proving slow to establish in some regions it is booming in others, such as Canterbury, where it is now killing whole plants in the field. “I marked 144 plants at Leslie Hills to follow and 3 years later half of them were dead and the rest were galled so might still follow suit. Also, at Lincoln 70 out of 72 plants planted out for trial work have died, most likely due to gall mite attack,” said Hugh.
Environment Canterbury (ECAN) has been the first agency to see the benefits of the gall mite. Steve Palmer from ECAN was involved in redistributing the mites last year using a helicopter. “Infested stems were dropped from a height of 3 m onto broom plants in a 30-km stretch of the Clarence River bed,” said Steve. "We delivered 1382 stems using this method. When the galls dry out, the mites exit them and crawl into developing shoot buds where they feed and initiate new gall formation,” said Steve. “Weeds such as broom are a big problem in river beds because they reduce the amount of habitat available to nesting birds such as terns, dotterels and gulls,” Steve explained. “We had to gain ministerial approval to introduce the mites onto Department of Conservation estate in December, but when it came time to release them we were surprised to find them already there,” he said. “Either we missed seeing them because they were less obvious on our first visit or they had dispersed there since,” said Steve. “In any case, there were broom plants that were heavily loaded with galls and others that were completely dead. It was really impressive to see and very encouraging.” ECAN Biosecurity Officer, Terry Charles has also reported significant damage to broom plants in the Waiau River, North Canterbury, 3 km downstream of the original 2009 release site made by the Canterbury Broom Group (which formed to tackle the serious broom situation in North Canterbury). “The broom is heavily galled and this, in combination with the dry conditions, has killed large areas of broom. ECAN biosecurity officers have been making strategic releases of this agent in North Canterbury and Kaikoura since 2012. We are increasingly confident that this is going to be a successful agent,” said Terry.
The gall mite is part of a suite of complementary broom agents comprising a leaf beetle (Gonioctena olivacea), seed beetle (Bruchidius villosus), psyllid (Arytainilla spartiophila),shoot moth (Agonopterix assimilella) and twig miner (Leucoptera spartifoliella). As well as being possibly the most damaging, the mites appear to be the most tolerant of shaded conditions. “This will be especially helpful in environments, such as regenerating forests, where broom can reach high densities in the understory,” said Quentin.
A video about ECAN’s project to deploy the gall mite in the Clarence River is available here. Trials to measure the impact of broom biocontrol agents in North Canterbury are funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme.