Four classical biocontrol agents targeting tradescantia (Tradescantia fluminensis) have been present in New Zealand’s environment for up to 14 years. Three chrysomelid beetles – the tradescantia leaf beetle (Neolema ogloblini), stem beetle (Lema basicostata), and tip beetle (N. abbreviata) – were first released between 2011 and 2013, and the smut fungus (Kordyana brasiliensis) was first released in 2018. All four agents are now well established, and the leaf beetle and smut fungus, in particular, have spread from their original release sites, where their impacts are becoming increasingly apparent. Consequently, it’s a good time to take stock of how effective they have been at reducing tradescantia’s harmful impact.
Image: one tradescantia biocontrol agent, Neolema ogloblini.
The effort to estimate the impacts of the tradescantia agents began before their release in New Zealand. Between 2006 and 2010, during the initial stages of the tradescantia biocontrol programme, native range surveys were conducted in southern Brazil to identify candidate biocontrol agents. Coinciding with these surveys, Simon Fowler and colleagues from Manaaki Whenua – Landcare Research, now a group of the Bioeconomy Science Institute, collected data in both Brazil and New Zealand to compare tradescantia biomass between the two countries. The contrasting survey results indicated why tradescantia is such a successful invader in New Zealand: the average tradescantia dry biomass here was almost three times greater than that recorded in Brazil (455 g/m2 compared to 164 g/m2, respectively).
These results made sense: there was more tradescantia biomass but less damage in New Zealand than in Brazil, and we hypothesised that this was because New Zealand lacked the specialised enemies keeping tradescantia in check. This research confirmed that the researchers were looking in the right spot for candidate tradescantia biocontrol agents. It also provided baseline data that could subsequently be compared with biomass data collected in New Zealand after biocontrol agents were released.
Returning to the present, a post-release monitoring study has recently been conducted to determine how effective the agents have been at reducing tradescantia in New Zealand. In collaboration with regional councils and landowners, 10 permanent quadrats were set up per site at 24 release sites so that tradescantia biomass could be estimated non-destructively before and after the establishment of biocontrol agents. Quadrats were set up when the agents were released, at which time initial biomass data were collected, and the sites were revisited between 2018 and 2025 to remeasure biomass. We used biomass as a measure for comparison because it is associated with the cover and average height of the tradescantia mat above soil level. Previous ecological studies indicated that the target for biocontrol success was to reduce tradescantia biomass to below 200 g/m2 (dry weight), which is the threshold allowing native plants to regenerate through the mat of tradescantia foliage.
Image: left - tradescantia in north Waikato before agent release, right - same site in north Waikato after agent release.
The agents did not establish at five sites, making them impromptu controls for comparison with our findings from the other sites. Although the initial intention was to compare the impacts of individual agents, this was hard to assess because agent self-dispersal was too good. Most sites where only one species was released had been colonised by one of the other agents. Chris McGrannachan, a researcher at the Bioeconomy Science Institute (BSI) who has been leading the monitoring work, noted that “By the time we revisited the 11 monitoring sites where agents established, four had leaf beetles only, one had stem beetles only, four had leaf beetles and the smut fungus, one had both leaf beetles and stem beetles, and one had three agents present – stem beetles, leaf beetles, and the smut fungus. Unfortunately, we couldn’t assess the impact of the tip beetle because the quadrats at the only survey site where it established were washed away in a flood,” he added. Consequently, for the analysis, presence or absence of biocontrol agents, regardless of species, was compared.
Chris found that tradescantia biomass at sites with biocontrol agents significantly declined following establishment of the agents. Prior to their release the average dry weight biomass at these sites was close to 500 g/m2. After agent establishment the average biomass was slightly above 100 g/m2, which is well below the 200 g/m2 threshold needed for native regeneration. In contrast, at the five sites where agents had not established there was no significant reduction in biomass, with tradescantia remaining above 400 g/m2.
The post-agent release biomass data were also compared to the data collected from Brazil and from New Zealand prior to agent release. Biomass in New Zealand after agent release was considerably lower than before agent release and had decreased to levels similar to those recorded from Brazil. These results together demonstrate that the biocontrol agents are effectively reducing tradescantia biomass at sites where they have established.
“Although the analysis combined biocontrol agents, it is clear to us that three agents appear to be capable of reducing tradescantia biomass to below threshold levels,” said Quentin Paynter, a senior researcher with the BSI who has been involved in the tradescantia work since early survey work in Brazil. “Biomass has declined below threshold at all sites with the leaf beetle only, and we have also observed both the stem beetle and the smut fungus clear tradescantia patches on their own,” he added, noting that the beetles seem to prefer warmer, drier sites, and the fungus copes best in cooler, damper sites, so the agent impacts appear to complement one another.
Putting these results together reveals a heartening outcome for tradescantia biocontrol in New Zealand. The agents are capable of reducing tradescantia biomass at sites where they have established to levels observed in the native range, and to the point where native vegetation is able to regenerate. At some sites the results have been spectacular. A kahikatea forest remnant site in north Waikato has seen a near complete disappearance of tradescantia after the leaf beetle, stem beetle, and smut fungus all established there between 2019 and 2021. This site is now seeing recovery of native ferns and tree seedlings, an impossible outcome had the tradescantia remained dominant. Our hope is to continue monitoring changes in tradescantia biomass in New Zealand over the long term and to expand data collection to more sites throughout the country.
This work was funded by the Ministry of Business, Innovation and Employment’s Strategic Science Investment Fund in the Bioeconomy Science Institute’s Beating Weeds programme.
