Turning up the Heat on Ginger
Ginger infesting plantation forestry in Northland. Photo credit: Northland Regional Council.
Wild ginger (Hedychium spp.) is highly invasive in New Zealand and has already made an especially bad name for itself in Northland. Molecular studies suggest that the most invasive form of wild ginger here is a hybrid between kahili ginger (Hedychium gardnerianum) and most likely white ginger (H. coronarium), both of which originate from India. Hybrid vigour may be giving this ginger a competitive advantage.
Quentin Paynter recently reviewed the impacts of wild ginger. In New Zealand this plant is able to invade forests that have experienced little or no human disturbance. “Apparently pure Hedychium gardnerianum rarely grows under low-light conditions in India, but it commonly does here, allowing it to suppress native forest regeneration and alter the food webs on the forest floor,” said Quent. “This is also happening in Hawai‘i and Reunion Island,” he went on to say, adding that “the lack of natural enemies is also likely to be allowing the plant to colonise habitats where it would not be able to thrive in India.” Studies in the Azores have found that where wild ginger is invading, there are fewer insects overall and this has negative implications for native birds and other insectivorous animals. Studies in New Zealand found that numbers of mites, amphipods, spiders, flies and bugs were lower where wild ginger was present, although overall invertebrate diversity was not affected.
Ginger infestations in Northland are now so bad that Ashlee Lawrence, from the Northland Regional Council, has started a public campaign aimed at raising public awareness about this plant and support for biocontrol. A group of concerned stakeholders, led by Ashlee, has formed the Stop Wild Ginger Group and developed a website (www.stopwildginger.co.nz/), which shows the damage the plant can do to native ecosystems and the extent of infestations in Northland and Auckland. The public are encouraged to add other known unmarked infestations to the maps. “We have noticed that in areas where the wild ginger has invaded, there are few opportunities for native plant recruitment,” Ashlee said. “This is very disheartening. In a few decades we will start to see major forest collapse along our coastlines due to the wild ginger. However, many residents are unaware of the scale of infestation as it is hidden beneath tall canopy species, or in the middle of dense state and production forests. The Stop Wild Ginger website highlights the problem and brings the issue into the public eye.”
Sadly, wild ginger is well beyond the point where eradication is feasible, and the cost of widespread herbicide use would be astronomical (tens of millions of dollars), let alone environmentally harmful. This leaves biocontrol as one of the only realistic options for this invader. Researchers from CABI in the UK have been looking for potential agents in Sikkim, India, for a number of years. This has required the potentially tricky balancing act of finding agents that are sufficiently specific but also able to attack a hybrid species that may not even exist naturally in the wild. Fortunately such candidates seem to exist, and the prospect of a successful biocontrol programme being developed for New Zealand is promising.
The two best-studied agents both appear to have good potential. One is a large weevil (Metaprodioctes cf. trilineatus) that feeds on all parts of the plant, including the rhizomes. The weevil also developed on white ginger and an ornamental (Cautleya spicata) in host range studies. This wouldn’t necessarily be a show stopper for New Zealand (unlike Hawai‘i, where white ginger is highly valued), because all Hedychium species are considered to be weeds and there are no native members of the Zingiberaceae family. Further testing is needed to determine the level of risk to other closely related ornamental Zingiberaceae and to edible ginger (Zingiber spp.).
Host-range testing of the shoot-mining fly (Merochlorops cf. dimorphus), whose larva mines the stem, is tantalisingly close to completion, and this fly is highly likely to be sufficiently host-specific for in New Zealand. “Work on the fly and weevil had been on hold since October 2016, when permits and agreements with Indian authorities were unexpectedly revoked. But these issues appear to have been resolved and the project is set to resume in September 2017,” said Quent. With a bit of luck the weevil and fly testing can be wrapped up fairly quickly. Other agents, such as a range of hispine beetles and several defoliating moths, will also be studied as funds permit.
This report on the impacts of wild ginger was funded by an Envirolink grant (1726-NLRC196) to Northland Regional Council. The biocontrol work is funded by the National Biocontrol Collective.