Two Steps Forward and One Back for Wild Ginger Project
Examples of stem-mining flies from wild ginger. Further work is needed to understand how subtle variation in morphology aligns with two or more cryptic species. Image: CABI
Despite the efforts of regional councils and volunteer community groups around the country, wild ginger (Hedychium gardnerianum) remains one of the biggest threats to regenerating native forest in the North Island. “So far over 5,000 ha of land are thought to be affected by wild ginger in Northland, including pine forests, which are a valuable economic resource for the region,” said Ashlee Lawrence, of the Northland Regional Council. “We have a dual problem because the weed not only invades open areas but is sufficiently shade tolerant to supress any regeneration beneath closed forest canopies, creating large zones where the entire understorey is ginger,” she said. The rhizomes of the weed make the ground impenetrable, so that native birds such as kiwi have a difficult time probing for food. The enormous scale of the infestation across the region makes widespread control unfeasible in forested areas. In 2016 the Northland Regional Council formed the Stop Wild Ginger Group (SWGG) to raise public awareness of the threat of wild ginger to northern forests, and to seek better control tools.
A project to develop biocontrol for wild ginger began back in 2008, funded by the National Biocontrol Collective and a consortium of sponsors from Hawai‘i. Initially the project went well, with CABI-UK, who were contracted to do the work, finding a number of natural enemies during surveys in India that seemed to have potential as biocontrol agents. Of particular interest was a shoot-mining fly (Merochlorops cf. dimorphus) and a large red-and-black weevil (Metaprodioctes trilineata) that feeds on almost all parts of the plant. However, with the host-range testing of these insects tantalisingly close to completion, the project essentially had to go on hold for several years. Authorities in India required new paperwork and agreements to be drawn up to allow the collaborative project work to continue while conforming with central Indian Government guidelines relating to the fair and equitable sharing of benefits arising from the utilisation of genetic resources.
However, in 2018 the paperwork was finally sorted and the way was clear for the project to resume under a 3-year collaborative agreement with the Indian Council of Agricultural Research. Around the same time the SWGG successfully applied for funds from the Ministry for Primary Industries’ Sustainable Farming Fund to support the development of biocontrol for wild ginger. So with strong new funding secured for the next 3 years and work able to get under way again in India, the disappointments of previous years appeared to be behind us. However, as so often happens in weed biocontrol projects, it wasn’t long before a fresh challenge arose.
“It turns out that the stem-mining fly is not a single species but likely two (or more) cryptic species which cannot be reliably identified by morphological features,” explained Djami Djeddour of CABI. More studies are needed, including molecular work, to get to the bottom of this. One of the flies has definitely been shown to have too wide a host range to be considered as a biocontrol agent. The other, which is probably new to science, is potentially more specific but it is not clear whether this one is able to attack kahili ginger from New Zealand and Hawai‘i, which has been shown to be a hybrid between H. garderianum and H. coronarium.
“The failure of the ‘new’ flies to successfully attack this hybrid material in tests could just be due to not having the right sex ratio,” explained Djami. Infested stems are collected in India, and once the flies emerge they are used for testing. However, the flies are difficult to sex and possibly emerge asynchronously (males first, followed by females). So the question of whether the more specific fly can attack the kahili ginger hybrid still needs to be resolved. “If the fly cannot attack it then efforts will instead focus on other agents such as hispine beetles and several defoliating moths,” explained Ashlee of the SWGG. If it turns out the stem-mining fly can attack our wild ginger, then all the host testing will need to be repeated with a culture that is known to consist of only this species. Meanwhile work is also continuing shortly with the weevil.
For more information about the Stop Wild Ginger Group, see www.stopwildginger.co.nz. Co-funding for this project is also being provided by forestry companies and territorial authorities in Northland, Manaaki Whenua – Landcare Research, and the National Biocontrol Collective.
CONTACT
Hugh Gourlay – gourlayh@landcareresearch.co.nz or from May until September
Arnaud Cartier – cartiera@landcareresearch.co.nz