Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Tradescantia Beetles Make Big Impression

Welcome Bay stem beetle release site at release time and 3 years later. Tradescantia has been substantially reduced. Image - Andrew Blayney

Welcome Bay stem beetle release site at release time and 3 years later. Tradescantia has been substantially reduced. Image - Andrew Blayney

They might be small, but the beetles released to control tradescantia (Tradescantia fluminensis) are already starting to make a big impression on their host plant here in New Zealand.

Tradescantia, which is native to south-east Brazil, is so widespread in New Zealand that manual or chemical control on a large scale is simply not feasible. The plant smothers the ground, preventing native tree regeneration, and is a real nuisance for home gardeners, as well as causing allergic responses in dogs. Even though it is still early days, the little Brazilian beetles appear to be establishing readily and quickly doing damage, described as ‘spectacular’ in some places!

Over the past few years, three beetle species have been released widely throughout the North Island and at some sites in the South Island, where tradescantia is less of a problem. Releases of the tradescantia leaf beetle (Neolema ogloblini), aka ‘Shiny’, got underway first in 2011, followed by the tradescantia stem beetle (Lema basicostata), aka ‘Knobbly’, in 2012, and finally ‘Stripy’, the tradescantia tip beetle (Neolema abbreviata) in 2013. Until recently the advice has been to release only a single species at each site. “There are several reasons for this approach, even though the beetles will happily coexist on the same plants and eventually we want them all working together,” explained Simon Fowler, who has been leading the project. “Initially we wanted to ensure that the beetles had the best possible opportunity to establish and so it was best to keep them apart. There were also technical reasons such as the staggered availability of beetles for release and the ease of monitoring their individual performance in terms of damage and survival,” Simon added.

Leaf beetle damage at Kerikeri.In March, Landcare Research board members and senior leadership were treated to a tour of weed biocontrol release sites in Northland. As well as seeing successful biocontrol of ragwort (Jacobaea vulgaris) and mist flower (Ageratina riparia), and participating in the first release of the lantana leaf rust (Prospodium tuberculatum) in New Zealand, they saw first-hand early signs of success against tradescantia, which is considered to be one of our worst environmental weeds. After just 4 years the tradescantia leaf beetle population has exploded at one of the earliest release sites in Kerikeri, with thousands of beetles clearly visible and doing obvious damage. As their name suggests, the larvae of the leaf beetles graze on the foliage, skeletonising the leaves, whereas the adults create notches on the side of the leaves. “Higher levels of damage than we expected this early on were clearly visible and the beetles were so numerous that it has already been possible to collect and distribute some to other sites where the weed is also a problem,” said Simon. “The beetles often drop to the ground when they are disturbed so we use a ‘pooter’ to suck them up or just catch them by hand,” explained Jenny Dymock, a local scientist who has been assisting the Northland Regional Council with the release, monitoring and redistribution of the beetles. “We have a long list of people waiting patiently to get some of the beetles and we will prioritise sites that are going to be safe from things like herbicide use, hand weeding and flooding,” Jenny said.

At least four tradescantia beetle release sites visited by Landcare Research staff this year have been sprayed, which is clearly not ideal, but fortunately in each case the beetles have survived on remaining foliage. Tip beetles also appear to have survived, but only just, following a major flood last winter near Kaeo in Northland. Successful biocontrol of tradescantia is likely to prove more difficult at sites that are regularly flooded, washing the beetles and their host plant away. “For this reason we also sought and gained Environmental Protection Authority approval to release a yellow leaf spot fungus Kordyana sp., which may prove better in these situations and should also complement the impact of the beetles,” said Lindsay Smith, who has devoted a significant amount of time to rearing the beetles and ensuring that they were disease-free before they were released. “We are keeping the fungus ‘up our sleeve’ until we have quantified the effect of the beetles alone and can decide whether we need to release it here in New Zealand,” Lindsay added. In the meantime the fungus has been taken up by Louise Morin from CSIRO, who is looking to see if it might be suitable for controlling tradescantia in Australia. We have also recently supplied Australian researchers with leaf beetles and South African researchers with stem beetles for studies to see if they might be suitable to release in their countries. Closer to home, the Wellington Botanic Gardens have set up their own tradescantia beetle rearing programme and a display to raise public awareness of tradescantia biocontrol.

Also showing early promise in the field is the stem beetle. Stem beetles released at Welcome Bay near Tauranga have already caused significant damage to tradescantia, according to Andrew Blayney from the Bay of Plenty Regional Council. “Although we don’t often see the beetles or their larvae, after only 3 years they have reduced the coverage of the plant to bare-ground in places at the release site,” said Andrew enthusiastically. Stem beetle adults are harder to see in the field than leaf beetles as they behave more cryptically. The larvae are also harder to see, being inside the stems, but there is no mistaking the damaged stems. Andrew is hoping to transfer some of the stem beetles to other sites next year and he also has a long list of gardeners keen to give the beetles a try!

Since we have always anticipated big things from the tradescantia beetles, a programme to monitor their impacts was put in place at the outset. Landcare Research, along with some interested councils, have set up plots to make detailed measurements, while others are making simpler measurements of beetle damage and impact on the weed as part of the new assessment protocol adopted by the National Biocontrol Collective. We also hope to team up with university staff and get some students involved in more detailed monitoring projects. “Some of the things we are interested in investigating include whether there are ecological benefits that can be directly attributed to the biocontrol of tradescantia, such as implications for native plant regeneration, improved ecosystem services in the form of nutrient cycling, increases in native invertebrate biodiversity and possibly other biota as well,” said Simon.

This project is funded by the Department of Conservation, National Biocontrol Collective, and the Ministry of Employment, Business and Innovation as part of Landcare Research’s Beating Weeds Programme.