First release of new agents
We are pleased to report the first field releases of two insect biocontrol agents – one for giant reed (Arundo donax) and another for field horsetail (Equisetum arvense). The field release of a pathogen against tradescantia (Tradescantia fluminensis) is also imminent.
Field horsetail weevil
In early November the Horsetail Control Group (HCG) released the first adult horsetail weevils (Grypus equiseti) at two sites near Bulls in the Manawatū. The first release was made on the edge of a wetland on a Parewanui dairy farm, where Alistair Robertson, chair of the group, was given the privilege of opening the box to give the weevils their first taste of freedom. Alistair Cole (NZ Landcare Trust), Craig Davey (Horizons Regional Council) and Lindsay Smith (Manaaki Whenua − Landcare Research [MWLR]) were also there to oversee the release. “We were able to add further weevils to the same site in late December,” said Lindsay Smith, who has been rearing the weevils. The new adults emerge slowly in dribs and drabs, possibly as a survival strategy, so accumulating enough for a release is a slow process.
Weevils were also released in November at a nearby site along a fenced riparian strip, with a further top up in January. “To study the impact of the weevil on field horsetail (Equisetum arvense) plants, the releases were made onto one of a paired set of plots at each of the release sites. As the weevil becomes established, we will be able to make comparisons between the ‘weevil infested’ and ‘weevil not infested’ plots and quantify the amount of damage that has occurred,” explained Lindsay.
Field horsetail is a fern-like plant that arrived from Eurasia around 100 years ago. It is generally a problem in wetter parts of New Zealand, such as the Rangitīkei area, and is difficult to control using herbicide because it has a deep root system. Not only that: it can spread vegetatively via stolons and tubers, which are often moved around on earthmoving equipment or in gravel. Successful biocontrol of the plant would help the agricultural sector as the plant is toxic to stock and displaces valuable pasture. Field horsetail also prevents the recruitment of native seedlings and grows particularly well on riparian margins, blocking waterways and impeding watercourses.
Giant reed gall wasp
It was serendipity that led to the release of a gall-forming wasp (Tetramesa romana) against giant reed (Arundo donax). The gall wasp was originally destined for a more tropical lifestyle in Rarotonga, but the plan changed when it became apparent that giant reed in Rarotonga had mostly been mistaken for a similar-looking plant. Rather than cull the gall wasp colony, which was thriving in our containment facility, a decision was made to apply to release them here, since giant reed is emerging as a new problem.
Although not widespread in New Zealand yet, giant reed is certainly on the radar of the Northland Regional Council, and they fronted the application to release the wasp, together with a scale insect (Rhizaspidiotus donacis). “Both of these agents have been released in the USA and Mexico, where the weed is a huge problem,” said Chris Winks, who has been working on the project. “It’s a bit of a ‘David vs Goliath’ battle for the tiny wasps, but they can tolerate a wide range of conditions and are expected to do well in the warm and humid conditions found in Northland,” Chris added.
The first release of the giant reed gall wasp was made in Kohukohu, in Northland, in early December. Local entomologist Jenny Dymock was on hand to help MWLR staff attach galled stems onto plants in the field. A further release was made in Northland just before Christmas, and two shipments of the wasp were released in the Manawatū−Wanganui Region in January. “Two releases are also planned for the Auckland region this summer, which is the best time to release them at their most active,” said Chris.
If the gall wasp releases take successfully, small swellings should begin to appear on giant reed after a few weeks. Since the galls are quite obvious, monitoring to check for signs of establishment can begin this autumn. Terminal galls reduce the height and overall biomass of the plants, and induce the plants to produce more side shoots, which are highly suitable
for the scale insect to attack, further reducing the vigour of the plant. Releases of the scale insect are planned to begin next spring. Successful biocontrol of giant reed is expected to deliver environmental gains for vulnerable habitats such as wetlands and riverbanks, which are not suited to herbicide use.
Tradescantia yellow leaf spot fungus
The yellow leaf spot fungus (Kordyana brasiliensis) was discovered in south-east Brazil when potential biocontrol agents were being sought for New Zealand. A decision was made to release three beetle biocontrol agents in New Zealand first and see if the yellow leaf spot would be needed. While the beetles were being established in New Zealand, CSIRO (Australia) became interested in the biocontrol of tradescantia and imported the fungus into their containment facility in Canberra. So when we decided that it would be prudent to release the fungus here, we were able to get a shipment from Australia and avoid the pitfalls of shipping a delicate living organism long distance.
“Testing has shown that the fungus is host specific and that it thrives in damp conditions,” explained Chantal Probst, who has been leading the project in Auckland. Spores germinate on the surface of tradescantia leaves and invade leaves through stomata. Obvious yellow spots develop on the upper leaf surface, expand, and the leaf ultimately withers. Assuming there is sufficient humidity, new wind-borne basidiospores are released around 2 weeks later from the affected area, assisting its spread to other plants.
It appears that the ability of the tradescantia beetles to build up high numbers and damage tradescantia can be disrupted by sporadic flooding in both Brazil and New Zealand. “During our visits to Brazil, it was apparent that the fungus was more prevalent close to waterways, suggesting that the fungus quickly moves back into areas that have been recently flooded,” said Simon
Fowler, the overall project leader. However, an assessment of the data collected in Brazil has shown there is no evidence that the beetles avoid foliage on which the fungus is present, so we hope it will be a perfect partner for them. Permission to take the fungus out of containment was received late last year, and since then Chantal has been working on bulking up the number of infected plants. All going well the first field release will take place at the end of February in the Auckland region.
The field horsetail project is funded by MPI’s Sustainable Farming Fund, with smaller contributions from a range of other organisations, and we are grateful to CABI (UK) for supplying the weevils. The giant reed project has been supported by the Ministry for Foreign Affairs and Trade, the Northland, Auckland, Hawke’s Bay, Horizons and Greater Wellington regional councils. We are grateful to John Goolsby (USDA) for supplying the giant reed agents. Finally, the tradescantia project is funded by the National Biocontrol Collective, and we thank Robert Barreto and Davi Macedo (University of Viçosa, Brazil) and Louise Morin (CSIRO) for providing the tradescantia fungus.
CONTACT
Chris Winks – winksc@landcareresearch.co.nz
Lindsay Smith – smithl@landcareresearch.co.nz
Chantal Probst – probstc@landcareresearch.co.nz