Image: close-up of flowers of Honolulu rose.
Honolulu rose has two forms: one that reproduces sexually and one that reproduces vegetatively, with the latter form being invasive. Due to its ability to self-propagate via underground root suckers, Honolulu rose can rapidly expand and form dense thickets. Its use as an ornamental plant, careless disposal of waste material, and transportation of soil and gravel containing root suckers are other means through which the weed is dispersed.
The formation of dense thickets can exclude native vegetation and reduce biodiversity, and can also outcompete commercially important crop species, such as banana, taro, and coconut, which leads to reduced crop yields and detrimental economic impacts. In Samoa, stems have been reported to number as many as 11 per square metre in shaded areas and 30 per square metre in newly infested open areas. Honolulu rose also has the potential to damage infrastructure, with reports that it can penetrate the bitumen of roads. These detrimental impacts of Honolulu rose are likely to be exacerbated in the wake of climate change, meaning effective management will be crucial.
Current mechanical and chemical control methods are difficult and labour intensive, and at best provide only temporary relief. Biological control is likely to be the only sustainable and safe management tool for effectively controlling Honolulu rose. At a workshop in Apia in November 2023 the Pacific Invasives Learning Network determined the top 10 weeds in the Pacific region for which biocontrol needs to be developed. “Honolulu rose ranked number seven, and so a feasibility study was deemed a priority to undertake under the new Restoring Island Resilience Programme, which is supporting such work in the Pacific,” said Lynley Hayes, who facilitated the workshop.
Image: infestation of Honolulu rose in Niue.
Chris McGrannachan, with input from Luise Schulte and Murray Dawson, conducted the feasibility study. “Honolulu rose has been targeted for biocontrol in Thailand, with a leaf beetle (Phyllocharis undulata) released there to control it. However, the beetle attacks Volkameria inermis, a plant in the Lamiaceae family which is native to the Pacific, so it is not sufficiently host specific for consideration as a biocontrol agent for this region,” said Chris. A total of 163 arthropods and 20 fungal pathogens were found to be associated with Honolulu rose, predominantly based on surveys conducted in China, Thailand, and Vietnam. “However, the majority of these species, particularly the arthropods, were unidentified and their host specificity is also unknown, meaning further surveys in the native range are needed to determine whether any have potential as biocontrol agent candidates,” said Chris. Of the fungal pathogens, three species in the Mycosphaerellaceae, which cause leaf spots (Cercospora bakeri, Cercospora volkameriae and Passalora clerodendri), are of interest and three rust fungi (Aecidium clerodendri, Hemileia solaninum and Coleosporium clerodendri) are also worth investigating.
Although the invasive form of Honolulu rose reproduces vegetatively and is therefore unlikely to have a high level of genetic variability among introduced populations, a recommendation from the feasibility study is that it would still be prudent to conduct genetic analyses for this species. “Such a study would help to determine how genetically different populations are within the Pacific region and the native range and indicate which areas of the native range most closely match introduced populations. This would help guide where native range survey efforts would be best focused,” said Chris.
Although Honolulu rose is used in traditional medicine in Asia and the Caribbean, this does not appear to be the case in the Pacific, although the flowers are used to make necklaces (ula) in places such as American Samoa. In conclusion, Honolulu rose looks to be a promising target to further explore for biocontrol when funds permit.
The Restoring Island Resilience programme is funded by New Zealand’s Ministry of Foreign Affairs and Trade and administered by the Secretariat of the Pacific Regional Environment Programme as part of the Pacific Regional Invasive Species Management Support Service.
