Tradescantia Beetles on the Up
Beetles brought to New Zealand from Brazil to control tradescantia (Tradescantia fluminensis) have found a new home on Mt Victoria in Wellington, after being released there by Conservation Minister Maggie Barry and Wellington Mayor Celia Wade-Brown in March.
Tradescantia is one of the ‘dirty dozen’ weeds that are the current focus of a new campaign called the “War on Weeds” to reduce the spread of unwanted plant pests. Tradescantia has been growing rampantly in native bush remnants and people’s gardens for decades now. But the three beetle species introduced to help stop the spread of this weed, the tradescantia tip beetle (Neolema abbreviata), tradescantia leaf beetle (Neolema ogloblini) and tradescantia stem beetle (Lema basicostata), are establishing well and starting to knock back the plant at release sites.
“Tradescantia is a significant environmental weed that prevents natural regeneration of native plants and changes the processes on the forest floor that sustain biodiversity,” said Quentin Paynter, who helped to discover these beetles.
Since the first release of the leaf beetle in 2011 at Mt Smart in Auckland, the beetles have been gradually increasing in number. “The leaf beetles got off to a slow start at Mt Smart, possibly due to the less than ideal weather conditions around the time of their release in late autumn,” said Quentin, who has been following their progress. Despite their slow start at this site, the damage to the tradescantia is becoming obvious now, with large patches of dead plant material visible (see photo in the gallery below).
At other release sites the beetles have multiplied more quickly and the damage to the tradescantia has been more visible as a result. Northland and Taranaki Regional Councils have been able to begin harvesting beetles from initial release sites to spread around their regions. The beetles are also proving to be very popular, with some groups now rearing their own on potted plants to boost numbers available for release. The Wellington Botanic Gardens, for example, are breeding up all three species of the beetles with the intention of releasing them in Wellington’s parks and reserves. The first such release was made by the Minister and Mayor at Mt Victoria, and it is hoped there will be many more to come.
Furthermore, a stem beetle site in Marlborough visited this autumn again showed that this insect alone can have a rapid impact on tradescantia. “At Waikakaho I estimate around 10 m2 of the weed had collapsed and disappeared at the release point, after only 3 years, with heavy damage around the edge out for another 20 m,” described Lindsay Smith. The beetles appeared to have not spread much beyond that though as yet.
“Now that we are confident that the beetles will establish on tradescantia throughout New Zealand, we have started to ask more complex questions that will hopefully demonstrate the benefits of these biocontrol agents,” Quentin said. This autumn a joint project has been set up on the Hikurangi floodplain (at the head of the Kaipara Harbour catchment) called ‘Living Water’. Living Water is a 10-year partnership between Fonterra and the Department of Conservation working with farmers, iwi, conservation groups, schools and other agencies to improve the ecosystem health of five key catchments in significant dairying regions throughout the country. Bev Clarkson, a wetlands expert at Landcare Research involved in the Living Water project, is passionate about seeing biodiversity restored in the tōtara and kahikatea remnants that form part of the floodplain habitat. “The Hikurangi floodplain is considered a ‘biodiversity hotspot’ and although it is fragmented, it still supports important forest habitat and wetlands containing threatened plant species,” said Bev. “The aim is to improve the biodiversity values of the forests found on the Hikurangi floodplain and to achieve this, we need to be able to control tradescantia, which has been identified as a fundamental problem,” said Bev. “The ability of the forest to replace itself is severely compromised by the presence of tradescantia, which is widespread and forms dense ‘mats’ (up to 1 m deep in places) that prevent native seedling regeneration,” she explained. “The Living Water project considers the biocontrol of tradescantia as a key restoration opportunity and is very supportive of the initiative,” said Bev.
So Bev and Quentin have teamed up to establish a research trial at four of the forest remnants to determine how efficiently the beetles control tradescantia, how far they move into the forest fragments over time, and to assess habitat recovery in terms of biodiversity values such as native insect diversity. “We have collected baseline data on insect diversity using malaise and pitfall traps and will repeat the same methods in 2−3 years’ time to see if there has been any change as a result of the tradescantia beetles being present,” said Bev. They are also comparing the effectiveness of the beetles with more traditional control methods such as manual weeding. Weeding has been done in the past by volunteers in an attempt to reduce the spread of the weed, and to replicate the natural flooding that used to occur across the floodplain prior to man-made changes to the hydrology of the region. There are promising signs already that the beetles are settling in well and we will share results from this trial in future issues.
Biocontrol of tradescantia has been funded by the National Biocontrol Collective and the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme. The Hikurangi biodiversity restoration project is funded by the Biological Heritage National Science Challenge, MBIE (Restoring Wetlands and Beating Weeds programmes) and the Fonterra/DOC Living Water Programme. Information about the War on Weeds is available at: http://www.doc.govt.nz/our-work/war-on-weeds/