Cape ivy biological control
Cape ivy (Delairea odorata) is a scrambling, semi-succulent vine native to southern Africa that has become a significant environmental weed of moist forests across Victoria, South Australia, New South Wales and Tasmania. It was introduced into Australia as a cultivated ornamental garden plant in the temperate region, and is still commonly found growing in suburban gardens. There is strong evidence from other invaded locations (e.g. west coast of USA) that Cape ivy invasion reduces native vegetation biodiversity and alters forest structure by damaging branches of native shrubs and trees.
Our research aimed to develop a biological control (biocontrol) solution for sustainable Cape ivy management across Australian forests, with a focus on two candidate insect agents: a gall-forming fly (Parafreutreta regalis) and a stem-boring moth (Digitivalva delaireae). We conducted a range of host-specificity tests with Digitivalva delaireae; these tests concentrated on non-target plant species closely related to Cape ivy, including Australian native plants, to evaluate risks the candidate agent could pose if introduced into Australia.
This research on Cape ivy was part of a broader project seeking biological solutions for the sustainable management of environmental weeds of temperate forests, including wandering trad (Tradescantia fluminensis) and angled onion (Allium triquetrum), co-funded by CSIRO and the Commonwealth Department of Industry, Innovation and Science and supported by the Community Weeds Alliance of the Dandenongs.
Cape Ivy overgrowing native vegetation
Background
What is the weed problem?
Cape ivy (Delairea odorata) is a scrambling, semi-succulent vine native to southern Africa that has become a significant environmental weed of moist, temperate forests across Victoria, South Australia, New South Wales and Tasmania. Cape ivy can form dense mats over trees, shrubs and groundcover vegetation, suppressing the growth and regeneration of native plant species and reducing biodiversity. High biomass causes structural damage to native tree branches and may enhance tree fall during storm events. Its invasion threatens a number of endangered vegetation communities from dry coastal to riparian and wet sclerophyll forests across eastern Australia.
Cape ivy vines grow rapidly until summer; leaves start to senescence by midsummer, while at the same time new shoots begin to grow from stems growing across the soil surface. Cape ivy reproduces mainly via fragments of stems and stolons, which can set root and grow in moist and low light conditions. Stem sections can persist for months before setting root, and may be dispersed away from the parent plant during soil disturbance and flood events
Cape ivy Infestation
How is the weed currently managed?
Hand-pulling of younger plants and smaller infestations seem to be the most effective means of controlling Cape Ivy at early stages of invasion, but ripping up the root systems often causes disturbance to the soil and damage to native plants. Any stem material left behind may enable re-invasion.
Applying herbicides to cut stems (pasting) and mature foliage (spraying) may be suitable for larger infestations, yet this may result in non-target damage to native vegetation growing beneath Cape ivy foliage. In most cases, however, the large scale distribution and remoteness of Cape ivy infestations make physical removal and herbicide application unviable and expensive control options in the long term.
Additional information on current control strategies can be found here:
http://weeds.dpi.nsw.gov.au/Weeds/Details/30
http://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/invasive_cape_ivy
What can biocontrol offer to the weed’s management?
Classical biocontrol is the only recognised sustainable solution to the suppression (but not eradication) of environmental weeds to reduce their impacts at a landscape scale in Australia. Use of mechanical or chemical control for Cape ivy in sensitive forest settings is unviable over large spatial scales, because Cape ivy can routinely regenerate from stem and root fragments. Furthermore, herbicide application often affects the foliage of non-target native plants that Cap ivy is growing over, and manually removing Cape ivy vines can physically damage native tree branches. A biocontrol approach may offer a longer term, sustainable solution to controlling Cape ivy at large spatial scales, especially in sensitive ecological communities with lower risks to native vegetation. By reducing the growth of climbing stems, biocontrol agents would slow the rate of Cape ivy invasion into the native canopy, thereby reducing shading and structural damage to native shrubs and trees.
Further information about the biocontrol program run in the USA can be found here:
https://www.aphis.usda.gov/plant_health/ea/downloads/2016/parafreutreta-regalis-cape-ivy.pdf
https://www.ars.usda.gov/ARSUserFiles/20300535/reports/2002_annual.pdf
Historical context – the USA biocontrol program for Cape ivy
Cape ivy has been the target of a biological control program in the USA, with comprehensive natural enemy surveys performed over several years in the native range of South Africa. Risk assessments performed by the USDA on several potential insect agents identified the shoot tip-galling fly Parafreutreta regalis and the leaf and stem-boring moth Digitivalva delaireae as the most promising candidate agents for Cape ivy.
Parafreutreta regalis is a gall-forming member of the fruit fly family. In its native range in South Africa, it has only been observed on Cape ivy. It was introduced to California as a biocontrol agent for Cape ivy in 2017 by the USDA. Studies conducted prior to this release confirmed that P. regalis is highly host-specific to Cape ivy (within the USA context). Pre-release efficacy studies showed that even low levels of P. regalis attack significantly reduced the height and biomass of Cape ivy. Vines with galls were visibly stunted compared to ungalled stems, which were longer and had more nodes and larger leaves than galled stems. See additional information here:
Digitivalva delaireae is a leaf and stem-mining moth. Under laboratory conditions, it has been shown to significantly reduce Cape ivy stem growth and invasion potential. Results from host-specificity tests have demonstrated that it is sufficiently host specific to Cape ivy for introduction into the USA. An application to release the moth into western USA is currently being assessed by regulators. Further information about the release program can be found here:
https://www.ars.usda.gov/research/publications/publication/?seqNo115=312190
https://www.ars.usda.gov/research/publications/publication/?seqNo115=295143
Our research
As part of this project, an information dossier was prepared to nominate Cape ivy as a target for biocontrol in Australia and submitted to the national Environment and Invasives Committee. The nomination was endorsed by the committee in May 2018. Preliminary host-specificity testing of both P. regalis and D. delaireae was underway in South Africa at the University of KwaZulu-Natal on several closely-related plant species of importance to Australia (including Gynura, Curio and Cineraria species). D. delaireae was imported in mid-2019 to a CSIRO quarantine facility in Australia for comprehensive host-specificity testing.
Update on research coming soon.
Acknowledgement
Dr Ben Gooden (CSIRO) led the research with contributions from Isabel Zeil-Rolfe, Caroline Delaisse, Raghu Sathyamurthy, Michelle Rafter and Andy Sheppard.
Research in South Africa was led by Dr Terry Olckers and Dr Daniella Egli (University of KwaZulu-Natal). We thank Dr Alexander N. Schmidt-Lebuhn (National Research Collections Australia CSIRO) for resolving phylogenetic relationships amongst Senecio species to inform host-specificity testing.